DAIRY FARM DIRTY WATER - SEEKING THE
BEST SOLUTIONS TO AVOID POLLUTION
(DW-STOP)
DEFRA PROJECT CODE: LK0650 / WA0525
FINAL REPORT
FOURTH (FINAL) DRAFT
5 DECEMBER 2005
Project Manager: Trevor Cumby1,
Co-authors: Andrew Barker5, Colin Burton1, David Chadwick2, Marc
Dresser3, Gari Fernandez5, John Gregory4, Peter Leeds-Harrison3, Ian Muir4,
Elia Nigro1, Sorche O’Keefe3, Ken Smith4 and Joe Wood5
1
Silsoe Research Institute
Wrest Park, Silsoe, Bedford,
MK45 4HS
2
Institute of Grassland & Environmental Research
North Wyke, Okehampton, Devon
EX20 2SB.
3
Cranfield University at Silsoe
Silsoe, Bedford, MK45 4DT
4
ADAS UK Ltd
Woodthorne, Wergs Road, Wolverhampton
WV6 8TQ
5
University of Birmingham
Edgbaston, Birmingham
B15 2TT
December 2005
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EXECUTIVE SUMMARY
Introduction and objectives. Most UK dairy farms produce large volumes of liquid wastes from dairy
parlours and collecting yards. Usually, irrigation to land is the best way to manage this “dirty water”
(DW), but the associated pollution risk is considerable, e.g., the total pollution load from 200 dairy
cows is equivalent to the sewage from 3600 people and about 25% of this is in the DW. Hence, the
objectives of this work were to identify, develop and assess the best practical means at minimal cost to
reduce the risks of water pollution and pathogen transfer caused by DW irrigation to land, and to
illustrate this by specifying two full-scale treatment systems.
Methods. Four pilot-scale treatment strategies: down-flow reed beds (DRB), percolating soil plots
(PSP) , overland flow plots (OFP) and settlement plus intensive aeration (IAP) were each developed to
treat 500 l/day, and all were evaluated on a 440-cow commercial dairy farm in Sussex. A
comprehensive set of performance indicators was used during two trial periods of 50 and 46 weeks
respectively. The impacts of land spreading both untreated and treated DW were assessed using three
in-field lysimeters, whilst complementary laboratory-scale studies provided further data for
subsequent mathematical modelling of the treatment and land-spreading processes.
Results. The farm site provided a rigorous test, with 5-day Biochemical Oxygen Demand (BOD5) and
Total Solids (TS) values ranging from 600 to 12000 mg/l and from 300 to 18000 mg/l respectively,
with substantial seasonal variations, thus resembling the range of DW characteristics previously
observed on other UK dairy farms. During Trial 1, all four systems treated this effluent effectively
with average reductions of over 80% in BOD5 and 40% in TS. Comparable results were found with:
chemical oxygen demand (COD), total and ammoniacal nitrogen, nitrates, nitrites, phosphorus (P) and
thermotolerant coliforms. The results defined the boundaries of efficient and reliable operation so that
changes implemented in Trial 2 improved performance in several ways. Trial 2 provided the data
required for development and validation of mathematical models to specify full-scale OFP, IAP and
DRB treatment systems, and to compare their costs and effectiveness. The lysimeter and laboratoryscale data facilitated a DW-SOIL model, to help avoid the inefficiencies of treating DW beyond
environmental needs and to prevent P accumulation and leaching in soils that receive DW.
Outputs and benefits. Through the models, the project reached its goal of providing the means to
define sufficient but not excessive DW treatment prior to land spreading. This was illustrated using
two full-scale case studies based on real farm data. The models could also meet other commercial
requirements, e.g. treatment of dairy farm DW before certain forms of re-use; discharge/re-use of
vegetable wash water and leachate from landfill sites; and alternative means of sewage treatment for
isolated rural communities. The project has also provided a data set that uniquely defines the
properties of DW and their seasonal variations on a large commercial dairy farm in the UK. Further
studies using this data could reveal more about the impacts of herd management and weather factors
on DW production, with implications for the design and management of DW systems.
Future scientific impact. The observations and occasional problems encountered during completion of
DW-STOP have indicated why certain assumptions and practices concerning the production and
management of DW should be re-assessed in the interests of safe and efficient dairy production, and of
practical environmental protection. For example, this includes possible interactions between certain
soils and sodium in DW, resulting from the use of dairy cleaning chemicals.
Future commercial and strategic impact. Effective exploitation of the results from DW-STOP by UK
dairy farmers requires a well-planned communications strategy including a full-scale demonstration
project to highlight the benefits and reduced costs, also to show milk retailers and consumers that milk
production can benefit the environment through efficient recycling. This would complement further
dissemination through the publication of a generic booklet on DW Management, compatible with
Defra’s current “Managing Livestock Manures” series. The project impinged on Defra’s “Agricultural
Waste Regulations Consultation” in March 2005, by identifying DW treatment technologies as part of
a farm manure management strategy to protect the environment. It would be unfortunate if significant
regulatory barriers prevented or seriously undermined future DW treatment developments.
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CONTENTS
CHAPTER 1 INTRODUCTION ......................................................................................................... 9
1.1
INTRODUCTION TO THE DIRTY WATER PROBLEM – INDUSTRY RELEVANCE (TC,
CoB, KS)....................................................................................................................................... 9
1.1.1 Current environmental legislation and Dairy Farm Dirty Water ............................................. 9
1.1.2 The current DW problem in the UK ..................................................................................... 9
1.1.3 Future trends and consequences............................................................................................. 10
1.1.3.1
The Agricultural Waste Regulations....................................................................... 10
1.1.3.2
Local planning and visual impact ........................................................................... 10
1.1.3.3
Re-use of DW ......................................................................................................... 11
1.1.3.4
Future needs............................................................................................................ 11
1.2 INTRODUCTION TO THE DW PROBLEM – SCIENTIFIC RELEVANCE (TC, CoB, KS)
11
1.2.1 Definitions of DW arising from Regulations and Codes of Practice .................................. 11
1.2.2 Research data ......................................................................................................................... 12
1.2.2.1
Characteristics of DW............................................................................................. 12
1.2.2.2
Treatment of DW .................................................................................................... 13
1.3 INTENSIVE AERATION SYSTEMS ....................................................................................... 14
1.3.1 Treatment strategies ............................................................................................................... 14
1.3.2 Key biological and physical processes................................................................................... 14
1.4 REED BEDS ............................................................................................................................... 14
1.4.1 Horizontal and vertical flow reed beds .................................................................................. 14
1.4.2 Use of DRBs in agriculture.................................................................................................... 15
1.5 SOIL BASED DW TREATMENT SYSTEMS ....................................................................... 15
1.5.1 Previous use of soil based water treatment systems .............................................................. 15
1.5.2 Mechanisms of potential pollutant removal from DW by percolating soil plots ................ 17
1.5.3 Mechanisms of potential pollutant removal from DW by overland flow plots .................. 17
1.5.3.1
Reducing the BOD5 concentration of DW .............................................................. 17
1.5.3.2
Reducing the solids content of DW ........................................................................ 18
1.5.3.3
Mechanisms of nitrogen removal from DW ........................................................... 18
1.5.3.4
Mechanisms of phosphorus removal from DW ...................................................... 19
1.5.3.5
Creating the right environmental conditions for treatment ..................................... 19
1.6 PROJECT OBJECTIVES AND APPROACHES (TC, CoB, KS).............................................. 20
1.6.1 Project objectives ................................................................................................................... 20
1.6.2 Outline of approaches ............................................................................................................ 20
CHAPTER 2 EXPERIMENTAL METHODS ................................................................................. 21
2.1
SITE FACTORS, DESIGN AND INTEGRATION OF SYSTEMS (TC, IM, JG).................... 21
2.1.1 Pallinghurst Farm................................................................................................................... 21
2.1.1.1
Site locations and layout ......................................................................................... 21
2.1.1.2
Site survey............................................................................................................... 22
2.1.2 Installation of treatment processes and lysimeters................................................................. 23
2.1.2.1
Treatment processes................................................................................................ 23
2.1.2.2
Diamond lysimeters ................................................................................................ 24
2.1.2.3
Laboratory lysimeters ............................................................................................. 25
2.1.3 Sources of DW and supplies to Sites 1 and 2 ..................................................................... 25
2.1.3.1
Initial arrangements: Site 1 ..................................................................................... 25
2.1.3.2
Initial arrangements: Site 2 ..................................................................................... 28
2.1.3.3
Revised arrangements ............................................................................................. 29
2.1.4 Trial 1..................................................................................................................................... 30
2.1.4.1
Trial 1 – experiment design and overview.............................................................. 30
2.1.4.2
Trial 1 – process monitoring and sampling strategy ............................................... 31
2.1.5 Trial 2..................................................................................................................................... 33
2.1.5.1
Trial 2 – experiment design and changes implemented following Trial 1.............. 33
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2.1.5.2
Trial 2 – overview................................................................................................... 35
INTENSIVE AERATION PLANT (CoB, TC, EN) ................................................................... 36
2.2.1 Description of process and equipment ................................................................................... 36
2.2.1.1
Plant overview: location, configuration and operation ........................................... 36
2.2.1.2
Primary sedimentation ............................................................................................ 37
2.2.1.3
Aerobic treatment by continuous stirred tank reactor (CSTR) ............................... 38
2.2.1.4
Secondary sedimentation ........................................................................................ 40
2.2.1.5
Aerobic treatment by high rate trickling filter (HRTF) .......................................... 40
2.2.1.6
Tertiary sedimentation ............................................................................................ 41
2.2.1.7
Control cabin........................................................................................................... 42
2.2.1.8
Changes implemented for Trial 2 ........................................................................... 42
2.2.2 Plant operation ....................................................................................................................... 44
2.2.2.1
General plant operation........................................................................................... 44
2.2.2.2
Two-hourly control sequence ................................................................................. 44
2.2.2.3
Explanation of individual control sequences - an example..................................... 46
2.2.2.4
Data logging procedure........................................................................................... 46
2.2.2.5
Sampling procedure ................................................................................................ 47
2.3 REED BEDS (AnB, GF, JW) ..................................................................................................... 47
2.3.1 Description of process and equipment ................................................................................... 47
2.3.1.1
Plant overview: location, configuration and operation. ......................................... 47
2.3.1.2
Reed bed design ...................................................................................................... 49
2.3.1.3
Tanks and auxiliary items. ...................................................................................... 49
2.3.2 Plant Operation ...................................................................................................................... 50
2.3.2.1
General plant operation........................................................................................... 50
2.3.2.2
Control sequence..................................................................................................... 51
2.3.2.3
Sampling procedure. ............................................................................................... 52
2.3.2.4
BOD5 analysis at the University of Birmingham.................................................... 53
2.4 PERCOLATION SOIL PLOT SYSTEM (DCh) ........................................................................ 53
2.4.1 Design and construction......................................................................................................... 53
2.4.2 DW application system ....................................................................................................... 54
2.4.3 Effluent collection system...................................................................................................... 54
2.5 OVERLAND FLOW SYSTEM (PL-H, MD)............................................................................. 55
2.5.1 Design and operation of the Overland Flow Plot................................................................... 55
2.5.2 Design Modifications............................................................................................................. 57
2.5.2.1
Sodium and Sodicity ............................................................................................... 57
2.5.2.2
Remediation ............................................................................................................ 57
2.5.2.3
Operational changes following remediation ........................................................... 58
2.6 LYSIMETERS (field and lab-scale) (DCh) ................................................................................ 58
2.6.1 Plot lysimeters at Pallinghurst Farm ...................................................................................... 58
2.6.2 Laboratory-scale column lysimeters ...................................................................................... 59
2.6.2.1
Overall approaches.................................................................................................. 59
2.6.2.2
Excavating intact soil cores .................................................................................... 60
2.6.2.3
Water and DW ........................................................................................................ 61
2.6.2.4
Water Sample collection and analysis .................................................................... 62
2.6.2.5
Statistical analysis................................................................................................... 62
2.2
CHAPTER 3 RESULTS, MODELLING AND DISCUSSION ....................................................... 63
3.1
OVERVIEW COMPARISON (TC, KS, IM, JG) ....................................................................... 63
3.1.1 Characteristics of the DW supplied to the four treatment processes. .................................... 63
3.1.1.1
Comparisons with DW on other farms. .................................................................. 63
3.1.1.2
Seasonal variations in the DW at Pallinghurst Farm and between Sites 1 and 2.... 64
3.1.2 Comparison of system performance – Total Solids (TS) concentration ................................ 66
3.1.3 Comparison of system performance- 5-day Biochemical Oxygen Demand (BOD5) ............ 68
3.1.4 Comparison of system performance - Total Suspended Solids (TSS) concentrations........... 69
3.1.5 Comparison of system performance - Nitrogen, Phosphorus and Chemical Oxygen Demand
70
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3.1.6 Comparison of system performance - during the intensive monitoring periods: Total
thermotolerant coliforms and other properties....................................................................... 73
3.1.7 Comparison of visual appearance of untreated DW and treated effluent from each of the
four treatment processes ........................................................................................................ 73
3.2 INTENSIVE AERATION PLANT (CoB, TC, EN) ................................................................... 77
3.2.1 Experimental programme and operation of the IAP .............................................................. 77
3.2.1.1
Flows and volumes of DW treated.......................................................................... 77
3.2.1.2
Redox and temperature ........................................................................................... 77
3.2.2 Removal of specific components from DW........................................................................... 78
3.2.2.1
BOD5 and COD....................................................................................................... 78
3.2.2.2
Total suspended solids (TSS).................................................................................. 81
3.2.2.3
Total solids (TS) ..................................................................................................... 81
3.2.2.4
Phosphorus (P)........................................................................................................ 82
3.2.2.5
Nitrogen (TAN and TN) ......................................................................................... 82
3.2.3 Operational factors................................................................................................................. 83
3.2.3.1
Redox values........................................................................................................... 83
3.2.3.2
Redox cyclic trends................................................................................................. 83
3.2.3.3
Power consumption................................................................................................. 84
3.3 REED BEDS (AnB, GF, JW) ..................................................................................................... 85
3.3.1 Removal of total solids .......................................................................................................... 85
3.3.2 Removal of BOD5 .................................................................................................................. 87
3.3.2.1
Mode 1 operation .................................................................................................... 87
3.3.2.2
Mode 2 operation .................................................................................................... 88
3.3.2.3
Modes 3 and 4 operation......................................................................................... 89
3.3.3 Removal of Suspended Solids ............................................................................................... 90
3.3.4 Removal of nitrogen, phosphorus, ammonia and COD ......................................................... 91
3.3.4.1
Ammoniacal nitrogen and total nitrogen ................................................................ 91
3.3.4.2
Phosphorus.............................................................................................................. 93
3.3.4.3
Chemical oxygen demand....................................................................................... 93
3.3.5 Treatment efficiency based on additional sampling by University of Birmingham .............. 94
3.4 PERCOLATION SOIL PLOT SYSTEM (DCh) ........................................................................ 96
3.4.1 Removal of BOD5 and TS...................................................................................................... 96
3.4.2 Removal of COD, TN, TAN and P........................................................................................ 97
3.4.3 Changes in PSP performance................................................................................................. 97
3.4.3.1
Changes in hydraulic conductivity.......................................................................... 97
3.4.3.2
Nutrient accumulation............................................................................................. 98
3.4.3.3
Remedial action ...................................................................................................... 98
3.4.4 Consequences of soil replacement ......................................................................................... 99
3.4.5 Previous research on development of percolation soil systems and practicalities of using soils
of different textures.............................................................................................................. 100
3.5 OVERLAND FLOW SYSTEM (PL-H, MD)........................................................................... 101
3.5.1 Hydraulic loading................................................................................................................. 101
3.5.2 Removal of BOD5 ................................................................................................................ 101
3.5.2.1
One-week treatment periods ................................................................................. 101
3.5.2.2
Two-week treatment periods................................................................................. 103
3.5.3 Removal of suspended and total solids ................................................................................ 107
3.5.3.1
One-week treatment periods ................................................................................. 107
3.5.3.2
Two-week treatment periods................................................................................. 108
3.5.4 Removal of Nitrogen............................................................................................................ 108
3.5.4.1
Total Nitrogen (TN).............................................................................................. 108
3.5.4.2
Total Ammoniacal Nitrogen (TAN) ..................................................................... 109
3.5.5 Removal of Phosphorus ....................................................................................................... 111
3.5.5.1
Total Phosphorus .................................................................................................. 111
3.5.5.2
Sustainability of the plot ....................................................................................... 112
3.5.6 Modelling and scale-up........................................................................................................ 113
3.5.6.1
DW supply reservoir ............................................................................................. 113
3.5.6.2
Area required for treatment................................................................................... 113
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3.6
LYSIMETERS (FIELD AND LAB-SCALE) (DCh) ............................................................... 114
3.6.1 Plot lysimeters at Pallinghurst Farm .................................................................................... 114
3.6.1.1
Raw DW lysimeter: removal of BOD5, TS and TSS ......................................... 114
3.6.1.2
Raw DW lysimeter: removal of COD, TN, TAN and P .................................... 115
3.6.1.3
Raw DW lysimeter: effect of soil depth............................................................. 116
3.6.1.4
Treated DW lysimeter: removal of BOD5, TS and TSS .................................... 118
3.6.1.5
Treated DW lysimeter: removal of COD, TN, TAN and P ............................... 120
3.6.2 Laboratory-scale column lysimeters .................................................................................... 121
3.6.3 Modelling............................................................................................................................. 123
3.6.3.1
Overview of model ............................................................................................... 123
3.6.3.2
Hydrology sub-model ........................................................................................... 124
3.6.3.3
BOD5 decay sub-model......................................................................................... 124
3.6.4 Model validation .................................................................................................................. 125
3.6.4.1
Method .................................................................................................................. 125
3.6.4.2
Results of the model validation............................................................................. 126
CHAPTER 4 OPTIMISATION OF DW MANAGEMENT.......................................................... 129
4.1
INTEGRATION OF MATHEMATICAL MODELS (TC, CoB, PL-H, DCh, JW))................ 129
4.1.1 Basic concepts underlying the approaches to modelling ..................................................... 129
4.1.2 Development of the modules to produce the collated DW-STOP mathematical model (“DWMODEL”) ............................................................................................................................ 130
4.1.3 “What if?” questions and scenarios ..................................................................................... 131
4.2 CASE STUDY 1 - PALLINGHURST FARM (>400 COWS): EXISTING ARRANGEMENTS
(TC, CoB, PL-H, DCh, JW, KS, IM, JG)) ................................................................................ 132
4.2.1 Description of Pallinghurst Farm......................................................................................... 132
4.2.1.1
Location and general features ............................................................................... 132
4.2.1.2
Herd management: calving patterns, grazing and herd replacement .................... 132
4.2.1.3
Buildings and housing management for in-milk cows.......................................... 133
4.2.1.4
Management of slurry, DW and other farm operations ........................................ 133
4.2.2 Engineering appraisal of the existing systems for slurry and DW management at Pallinghurst
Farm (Case Study 1) ............................................................................................................ 133
4.2.2.1
Volumes and flows of DW ................................................................................... 133
4.2.2.2
Properties of the DW at Pallinghurst Farm........................................................... 135
4.2.2.3
Current problems to be solved .............................................................................. 136
4.3 CASE STUDY 1 - PALLINGHURST FARM (>400 COWS): SPECIFICATIONS AND
COSTS OF ALTERNATIVE DW SYSTEMS (TC, COB, PL-H, DCH, JW, KS, IM, JG) ..... 137
4.3.1 Application of DW-MODEL: overview of approaches ....................................................... 137
4.3.2 Application of DW-STOP Model 1: Field flow................................................................... 138
4.3.2.1
Methods ................................................................................................................ 138
4.3.2.2
Results................................................................................................................... 138
4.3.3 Application of DW-STOP Model 2: Overland Flow treatment system ............................... 140
4.3.3.1
Methods ................................................................................................................ 140
4.3.3.2
Results: design of a full-scale system ................................................................... 141
4.3.3.3
Results: estimated costs of a full-scale system ..................................................... 145
4.3.4 Application of DW-STOP Model 3: Intensive Aeration: Continuous Stirred Tank Reactor146
4.3.4.1
Methods ................................................................................................................ 146
4.3.4.2
Model validation: Model 3.................................................................................... 148
4.3.4.3
Results: design of a full-scale system ................................................................... 150
4.3.4.4
Results: estimated costs of a full-scale system ..................................................... 153
4.3.5 Application of DW-STOP Model 4: Reed beds for secondary / tertiary treatment ............. 155
4.3.5.1
Methods ................................................................................................................ 155
4.3.5.2
Model validation: Model 4.................................................................................... 156
4.3.5.3
Results: design and estimated costs of a full-scale DRB system .......................... 159
4.3.6 Application of DW-STOP Model 5: Intensive Aeration: High Rate Trickling Filter for
secondary / tertiary treatment............................................................................................... 161
4.3.6.1
Methods ................................................................................................................ 161
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4.3.6.2
Model validation: Model 5.................................................................................... 162
4.3.6.3
Results: design of a full-scale HRTF system ........................................................ 164
4.3.6.4
Results: estimated costs of a full-scale HRTF system .......................................... 165
4.3.7 Application of the DW-STOP Models: comparison of results ............................................ 166
4.3.7.1
Comparisons of benefits ....................................................................................... 166
4.3.7.2
Comparisons of costs ............................................................................................ 168
4.4 CASE STUDY 2 - NORTH BREAZLE FARM (~ 150 COWS) EXISTING
ARRANGEMENTS (TC, CoB, PL-H, DCh, JW, KS, IM, JG)) .............................................. 171
4.4.1 Description of North Breazle Farm...................................................................................... 171
4.4.1.1
Location and general features ............................................................................... 171
4.4.1.2
Herd management: calving patterns, grazing and herd replacement .................... 171
4.4.1.3
Buildings and housing management for in-milk cows.......................................... 171
4.4.1.4
Management of slurry, DW and other farm operations ........................................ 173
4.4.2 Engineering appraisal of the existing systems for slurry and DW management at North
Breazle Farm........................................................................................................................ 173
4.4.2.1
Volumes and flows of DW ................................................................................... 173
4.4.2.2
Properties of the DW at North Breazle Farm........................................................ 175
4.4.2.3
Current problems to be solved .............................................................................. 177
4.4.2.4
“What if?” questions and scenarios ...................................................................... 178
4.5 CASE STUDY 2 - NORTH BREAZLE FARM (~ 150 COWS)): SPECIFICATIONS AND
COSTS OF ALTERNATIVE DW SYSTEMS (TC, COB, PL-H, DCH, JW, KS, IM, JG) ..... 179
4.5.1 Application of the DW-STOP Models: overview of approaches......................................... 179
4.5.2 Application of DW-STOP Model 1: Field flow................................................................... 179
4.5.2.1
Methods ................................................................................................................ 179
4.5.2.2
Results................................................................................................................... 180
4.5.3 Application of DW-STOP Model 2: Overland Flow treatment system ............................... 181
4.5.3.1
Methods ................................................................................................................ 181
4.5.3.2
Results: design of a full-scale system ................................................................... 181
4.5.3.3
Results: estimated costs of a full-scale system ..................................................... 184
4.5.4 Application of DW-STOP Model 3: Intensive Aeration: Continuous Stirred Tank Reactor185
4.5.4.1
Results: design of a full-scale system ................................................................... 185
4.5.4.2
Results: estimated costs of a full-scale system ..................................................... 187
4.5.5 Application of DW-STOP Model 4: Reed beds for secondary / tertiary treatment ............. 189
4.5.5.1
Results: design and estimated costs of a full-scale system ................................... 189
4.5.6 Application of DW-STOP Model 5: Intensive Aeration: High Rate Trickling Filter for
secondary / tertiary treatment............................................................................................... 190
4.5.6.1
Results: design of a full-scale system ................................................................... 190
4.5.6.2
Results: estimated costs of a full-scale system ..................................................... 191
4.5.7 Application of the DW-STOP Models: comparison of results ............................................ 192
4.5.7.1
Comparisons of benefits ....................................................................................... 192
4.5.7.2
Comparisons of costs ............................................................................................ 193
4.6 GENERAL RECOMMENDATIONS AND KEY PRACTICAL FINDINGS (TC, CoB, PL-H,
DCh, JW, KS, IM, JG) .............................................................................................................. 196
4.6.1 General items required in most DW handling and treatment systems ................................. 196
4.6.2 Specific recommendations for systems using CSTR and/or HRTF processes..................... 196
4.6.3 Specific recommendations for Soil based treatments (OFP and PSP) ................................. 196
4.6.4 Specific recommendations for DRBs................................................................................... 196
CHAPTER 5 DW-STOP EXPLOITATION PLAN ....................................................................... 197
5.1
RATIONALE FOR COMMERCIAL EXPLOITATION ......................................................... 197
5.1.1 Environmental protection..................................................................................................... 197
5.1.2 Water recycling to secure future water supplies .................................................................. 197
5.1.3 Other applications (non-dairy)............................................................................................. 197
5.2 EXPLOITABLE OUTPUTS FROM DW-STOP...................................................................... 197
5.2.1 Mathematical models and case studies: the DW-STOP Data Set ....................................... 197
5.2.2 Case studies.......................................................................................................................... 198
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5.3
5.2.3 The DW-STOP data set ....................................................................................................... 198
EXPLOITATION ACTIVITIES............................................................................................... 198
5.3.1 Commercial exploitation of results ...................................................................................... 199
5.3.1.1
ARM Ltd............................................................................................................... 199
5.3.1.2
Pallinghurst Farm Partners.................................................................................... 199
5.3.1.3
Carier Pollution Control Ltd and others................................................................ 199
5.3.2 Technology transfer ............................................................................................................. 199
5.3.2.1
The Milk Development Council ........................................................................... 199
5.3.2.2
Demonstration projects: answering ten key questions.......................................... 199
5.3.2.3
Other means of technology transfer ...................................................................... 200
5.3.3 Strategic issues: interaction with the UK environmental regulatory authorities.................. 200
5.3.4 Further research ................................................................................................................... 201
5.3.4.1
Experimental and other data acquisition studies................................................... 201
5.3.4.2
Mathematical modelling ....................................................................................... 202
CHAPTER 6 CONCLUSIONS ........................................................................................................ 203
6.1
SCIENTIFIC FINDINGS ......................................................................................................... 203
6.1.1 Trial 1 - Overview................................................................................................................ 203
6.1.1.1
Overall Performance ............................................................................................. 203
6.1.1.2
Overall Performance of the PSP ........................................................................... 203
6.1.1.3
Overall Performance of the OFP........................................................................... 203
6.1.1.4
Overall Performance of the DRB.......................................................................... 203
6.1.1.5
Overall Performance of the IAP ........................................................................... 204
6.1.2 Trial 2 - Overview................................................................................................................ 204
6.1.3 Case studies.......................................................................................................................... 204
6.2 RECOMMENDATIONS FOR NEXT STEPS ......................................................................... 205
6.3 REFERENCES.......................................................................................................................... 206
APPENDICES ................................................................................................................................... 210
APPENDIX 1
Glossary ................................................................................................................ 210
APPENDIX 2
Response of behalf of DW-STOP to the Defra Consultation on the Draft Waste
Management (England and Wales) Regulations 2005 (Agricultural Waste Regulations
Consultation) March 2005 ....................................................................................................... 212
APPENDIX 3
MPhil thesis of Gari Fernandez, University of Birmingham ................................ 217
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CHAPTER 1 INTRODUCTION
Chapter authors: Trevor Cumby, Colin Burton, John Gregory, Ian Muir and Ken Smith
1.1 INTRODUCTION TO THE DIRTY WATER PROBLEM – INDUSTRY RELEVANCE
(TC, CoB, KS)
1.1.1 Current environmental legislation and Dairy Farm Dirty Water
In common with the rest of the UK Agriculture Industry, the dairy farming sector has been subject for
some years to the provisions of The Water Resources Act 1991. These are designed to prevent water
pollution and empower the Environment Agency and others to take legal proceedings against those
responsible for pollution incidents. In particular, under Section 85 of this Act it is an offence to cause
or knowingly permit a discharge of poisonous, noxious or polluting matter, or solid waste matter, into
any “controlled waters” without the proper authority. “Controlled waters” include groundwater,
lakes, ponds, rivers, streams, canals, field ditches and coastal waters. Temporarily dry watercourses
are also included. ‘Proper authority’ is usually consent to discharge from the Environment Agency
under Section 86 of the Water Resources Act 1991. Penalties for such offences include payments for
costs and damages, unlimited fines and/or imprisonment. Appendix 1 provides a glossary of relevant
terms and definitions
General practical advice is available to farmers in The Code of Good Agricultural Practice for the
Protection of Water (the Water Code) (Defra 2001), but in itself, following this Code will not provide
a defence against a charge of causing pollution. Hence, most farmers require detailed, specialised
technical information that will minimise the risks of causing pollution, (and thus of being prosecuted),
whilst also minimising the related capital and running costs. Farm Waste Management Planning is
probably the most common example of the application such detailed information. However, as the UK
dairy farming sector continues to intensify and specialise, this inevitably leads to conflicts between the
pressures of efficient milk production and the need to manage, and where possible, utilise the resulting
wastes and by-products. Currently, so-called “dirty water” (DW) is one of the most prominent
examples of this difficulty.
1.1.2 The current DW
problem in the UK
Most UK dairy farms produce large volumes of DW, which typically includes drainage and washings
from dairy parlours, collecting yards and related sources. Its disposal is a major problem, due largely
to the substantial volumes produced and to the great variability in its characteristics. Unlike faeces
and urine, which vary within the limits imposed by dairy cow diet and metabolism, and are often
collected and stored separately; DW often includes a wide assortment of other materials. These
include rainwater, wash water, dairy cleaning chemicals and disinfectants from biosecurity measures.
Naturally therefore, substantial farm-to-farm differences can arise due to management practices and
farm layout. Annual rainfall is also major factor, but moreover, the seasonal distribution of this
rainfall, together with the effects of herd calving patterns, can lead to substantial temporal variations in
the characteristics of the DW produced on a single site.
Clearly, direct discharge of DW to watercourses is totally unacceptable and so alternative,
environmentally sound measures are required. In principle, there are three possible alternative DW
management strategies available:
•
Spread DW on to land as it is produced. These systems have no storage and provide minimal
settlement. However, under certain circumstances it incurs a high risk of water pollution.
These circumstances include spreading DW on fine textured soils (especially in wet weather),
on soils that crack in dry weather, in fields with intensive under-drainage systems and in fields
with high risk of surface runoff into a watercourse (silty or fine sandy soils on sloping ground).
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•
Combined storage of DW and slurry. These systems are common where building design and
other factors preclude the separate collection of DW and slurry. In some instances, this strategy
•
is used to reduce slurry viscosity so that it can be mixed and pumped. However, the
characteristics of DW / slurry mixtures vary widely with time, and therefore handling
difficulties may remain, possibly requiring specialised equipment. In principle, the mixture
must be stored until it is spread on land, when field and weather conditions are suitable. As for
slurry storage, the Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations
1991 (HMSO, 1997) apply to stores of DW / slurry mixtures. Unless demonstrated otherwise,
these regulations require a minimum of 4 months storage for slurry, DW and rainfall likely to
fall directly on to the store. Therefore, provision of the necessary storage volumes can be
expensive.
•
Separate storage for slurry and DW. In order to reduce costs, some dairy farmers use weeping
wall stores or strainer boxes to increase their effective slurry holding capacity by allowing some
liquid to “drain-off”. This leachate is often managed as DW in combination with other effluents,
either with or without further storage. However, since the leachate is derived from slurry, it
contains very high concentrations of both soluble and suspended organic matter, which increase
DW concentration, and hence the pollution risk.
Given the above options, on many dairy farms, irrigation of DW to land, without significant
intermediate storage, is the only feasible and affordable disposal route. Moreover, the DW often
includes weeping wall leachate. Therefore, on heavy soils with under-drainage the resulting risks of
water pollution must be considered carefully. These risks arise from rapid by-pass movement through
the drainage systems, particularly when soil is at field capacity or even in very dry conditions in heavy
soils if they are strongly cracked. In wet conditions, surface run off presents another major risk.
1.1.3 Future trends and consequences
1.1.3.1
The Agricultural Waste Regulations
Besides the implications of dairy farm intensification and specialisation described above, the
management of DW will also be subject, in time, to the same progressive tightening of environmental
legislation that is affecting all sections of the UK economy. The most recent example of this concerns
the Waste Management (England and Wales) Regulations 2005, i.e. the so-called “Agricultural Waste
Regulations” (Defra, 2005), which was issued for public consultation early in 2005. Although, at the
time of writing, these draft Regulations have yet to be implemented, the proposals illustrate the ways
in which such measures could have far-reaching effects on the possible future use of certain
techniques within the management of DW on UK dairy farms.
Appropriately, these draft Regulations recognise that where “manure, slurry and effluent” are used on
the farm of origin in accordance with good agricultural practice, they are not regarded as wastes, and
are therefore not covered by the Regulations. However, this exclusion from the Regulations requires
that the material is not used in quantities that exceed the requirements of normal land use, and
specifies definite limits on application rates. Hence, this has the potential to impose constraints or
controls on management techniques that involve the transfer of DW between farms, as well as limits
on the annual amount of DW that could be applied to a given area of land. Further details are
discussed in Appendix 2.
1.1.3.2
Local planning and visual impact
Although storage of DW allows spreading to take place at times when the resulting risks of water
pollution are reduced, the provision of suitable storage structures may incur difficulties connected with
issues of Local planning policy and visual impact. For example, from Sussex to Devon, both Local
Planning and Area of Outstanding Natural Beauty (ANOB) Authorities appear to be taking
increasingly harder lines where open lagoons are proposed for slurry or DW
storage (Gregory,
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2005). Objections are primarily concerned with the visual impact of large lagoons and the scale of
associated earthworks (embankments, site levelling etc). These observations are concurrent with
changes in the Planning System in England and Wales (ENDS, 2004; ODPM, 2005; Lichfield et al,
2003).
If treated DW is suitable for year-round spreading on soils that are normally unsuitable for winter
spreading of raw DW, lagoons could be made smaller, cheaper and less visually intrusive.
1.1.3.3
Re-use of DW
If the use of potable mains water on dairy farms is subject to future restrictions or escalating costs, reuse of water will become increasingly important. It is conceivable that water re-use, based on the lowcost DW treatment technologies developed through DW-STOP, could lead to direct cost savings in
such circumstances. The outputs from DW-STOP might facilitate this as follows:
•
•
•
washing/flushing yard areas, buildings and some equipment,
use in disinfectant wheel dips/boot baths, and
crop irrigation.
1.1.3.4
Future needs
Therefore, although DW is less concentrated than slurry and is sometimes not regarded by farmers as a
major pollution threat, it remains a serious pollution risk if poorly managed. Moreover, there are
strong indications that, in future, DW management will become the subject of more stringent
regulatory controls. Hence, to protect the rural environment, and to ensure that it is best equipped to
meet the future challenges of tighter environmental regulation, the UK dairy farming industry needs
clear practical advice. In turn, this requires a robust basis of reliable research data and expertise to
define the most cost-effective ways to manage DW possibly leading to reductions in the use of potable
water for some purposes. This is the key factor underpinning the industrial relevance of this project
and alongside this, it is also important to review the project’s role in the context of the established
scientific and technical evidence relating to DW in the UK.
1.2 INTRODUCTION TO THE DW
KS)
1.2.1 Definitions of DW
PROBLEM – SCIENTIFIC RELEVANCE (TC, CoB,
arising from Regulations and Codes of Practice
As noted above, the Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations 1991
(HMSO, 1997) regard DW as a subset of the material referred to as “Slurry”, and define this as
follows:
“(a) excreta produced by livestock whilst in a yard or building; or
(b) a mixture consisting wholly or mainly of such excreta, bedding, rainwater and washings
from a building or yard used by livestock or any combination of these; of a consistency that
allows it to be pumped or discharged by gravity at any stage in the handling process.”
However, unlike the Regulations, the Water Code (Defra, 2001) does distinguish between slurry and
DW, describing DW as follows:
“Dirty water is a waste containing washings from milking parlours, farm dairies, cleaning
work and run-off from open concrete areas that are dirtied by manure or silage. Generally, it
contains less than 3% dry matter. Liquid that drains from manure and slurry stores and silage
effluent are often collected in dirty water handling systems. These materials are a lot more
polluting than yard run-off or cleaning water……………… The biochemical oxygen demand
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(or BOD) and the amount of plant nutrients in dirty water can vary widely, depending upon its
source.”
The details presented in Table 1 of the Water Code indicate for DW that: 1000 < BOD5 < 5000 mg/l,
(where BOD5 is the 5-day Biochemical Oxygen Demand) and Paragraph 112 includes the following
additional description:
“Dirty water is waste, generally less than 3% dry matter, made up of water contaminated by
manure, urine, crop seepage, milk, other dairy products or cleaning materials…..”
[Note: in this report, the term “Total Solids (TS)” is used in preference to “dry matter”, and is
expressed in units of mg/l. Hence, a dry matter content of 3% is approximately equivalent to a TS
concentration of 30000 mg/l.]
It is interesting to note that these definitions are some what wider than those that were published for
farmers’ guidance in the 1980’s, which defined DW as: 1000 < BOD5 < 2000 mg/l and 5000 < COD <
11000 mg/l. (MAFF, 1986). Further comparison can be made with the Defra GUIDELINES for
FARMERS in Nitrate Vulnerable Zones – England (Defra, 2002) which states the following:
“You should note that SLURRY is defined as excreta produced by livestock while in a yard or
building, including mixtures with bedding, rainwater and washings, that have a consistency
that allows them to be pumped or discharged by gravity at any stage of the handling process.
Very dilute wastes such as parlour washings on dairy farms and run-off from lightly fouled
yards are excluded as long as they do not include liquid effluent from silage or stored slurry,
for example effluent from a weeping wall store.”
In view of these varied descriptions and guidelines, it is not surprising that there are many differing
views of “what is regarded as DW” from within the UK dairy farming industry and from others with a
professional interest in the management of DW. Previous research evidence provides some guidance
as described in Section 1.2.2.1.
1.2.2 Research data
1.2.2.1
Characteristics of DW
Results from an earlier survey of DW on 20 commercial dairy farms in England and Wales revealed
TS concentrations ranging between 2000 and 40000 mg/l and BOD5 concentrations up to 30 000 mg/l
(Cumby et al, 1999; Brewer et al, 1999). The median values of 8000 and 5000 mg/l were typical of
many farms. The average herd size in the survey was 111 animals, and the average production of DW
was approximately 2300 m3/yr. Comparing the observed values with the corresponding definitions
from the Regulations and Codes of Practice listed above shows that whilst both of the prescribed
ranges for TS and BOD5 can be exceeded on some commercial farms, the median values from the
survey data were within the ranges (although only just in the case of BOD5).
Based on these reported values, typical DW is approximately 10 times more concentrated than raw
domestic sewage. Hence, when expressed as biochemical pollutant load (i.e. kg of Biochemical
Oxygen Demand produced per day), the DW from 200 dairy cows is equivalent to the sewage from
950 people! This is in addition to the excreta, which is equivalent to the sewage load from a further
2650 people.
Given the diversity of views and evidence to define the nature of DW, it was decided, within the
context of the DW-STOP project, to make careful comparisons between the properties of the DW
sampled at the experimental site and the previously reported findings. In this way, it was possible to
establish the relevance of the findings to other commercial dairy farms.
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1.2.2.2
Treatment of DW
As noted above, on many dairy farms, since discharge to a watercourse is unacceptable and storage is
too costly, irrigation to land is the only feasible disposal route for DW. The effects of spreading DW
on land have been investigated previously. These investigations have shown that movement of DW
through or over soil can reduce pollution risks to some extent, although some risks of water pollution
from the drained water or run off remain (Brookman et al., 1994; Williams and Nicholson, 1995).
Hence, in order to control and reduce such risks when DW is spread on land throughout the year, it is
useful to consider the possibilities for using on farm treatment processes for DW. In principle, these
could be used to remove most of the polluting power from DW before land spreading, leaving soil
activity to complete the process.
Many treatment processes have been proposed or tested during the last few years, and most have
demonstrated some success, although each also has its drawbacks. For example, small-scale studies in
Israel have examined to potential for using vertical-flow packed beds with passive aeration to treat
dairy wastewater, but found that although 66% of the BOD5 was removed, the system clogged after 21
days (Green et al 2004). Large constructed wetlands have been used in the United States to treat dairy
wastewater, achieving a 76% reduction in BOD5 with a minimum hydraulic retention time of 12 days
(Newman et al, 2000). In the UK, unpublished studies of DW treatment using chemical flocculants
have been reported, but the findings suggested that the high and variable concentrations of dairy farm
DW meant that such processes were largely unsuitable (Timmons and Flint, 2004).
Large multiple-stage pond systems have been studied in New Zealand including a two-stage system
design to act as “nutrient trap”, using a mixture of composted bark and zeolite (Bolan et al 2004).
Removal of BOD5 and Total Suspended Solids (TSS) in this system ranged from 46% to 71% and
from 68% to 98% respectively. The total retention time of the DW in the system was about 90 days.
Similarly, the National Institute of Water and Atmospheric research, (NIWA) in New Zealand, has
developed and demonstrated a full-scale “Advanced Pond System” (APS) to treat DW on large dairy
farms (Mountfort, 2004, NIWA, 2005). This is an extensive process involving four consecutive
stages, designed to suit the New Zealand climate, together with the capacity to store water:
•
•
•
•
Stage 1: fermentation ponds, typically 4 m deep to allow sedimentation and methane
fermentation. These ponds require sediment removal every five years.
Stage 2: large, shallow high-rate pond, with baffles and paddle wheels to aerate the DW.
Stage 3: Algal settling ponds, where algae settle out and are collected and returned to Stage 1
Stage 4: Large maturation pond, 1 m deep to enable disinfection from ultraviolet radiation in
sunlight, plus protozoa grazing and final sedimentation
Like the two-stage system, the APS has 90 days’ capacity and therefore, the total storage volume
amounts to about 75% of the 4 months’ storage capacity required under the UK’s Control of Pollution
(Silage, Slurry and Agricultural Fuel Oil) Regulations 1991 (HMSO, 1997).
Undoubtedly, the APS has been carefully developed for New Zealand conditions, where water
conservation is important. However, previous UK research show that within UK / Northern European
circumstances, more compact, and possibly cheaper approaches are possible, being only about 10%
the volume of the APS for a similar duty. This previous UK research includes the former MAFF
funded Project WA0501, which investigated the use of reed beds and aerobic treatment systems
(Nicholson et al, 1998; Burton et al, 1996). Similarly, project WA0518 examined soil based treatment
systems (Chadwick et al, 2000). Comparison of these results with the other international reports of
DW treatment suggests that objective comparison of these UK processes would help farmers to select
the most suitable approaches, and that some amalgamations of the separate approaches might lead to
overall improvements. The observations provide the background to the DW-STOP project, which was
initiated to meet the objectives and approaches described in Section 1.5.
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1.3
INTENSIVE AERATION SYSTEMS
1.3.1 Treatment strategies
A complete treatment option requires a high percentage reduction of the BOD5, TSS and TAN
concentrations in the DW. For example, if discharge to a watercourse is permitted and necessary,
local conditions might require treated effluent with consistent values below 0.02 kg/m3, 0.20 kg/m3
and 0.01 kg/m3 for BOD5, TSS and TAN respectively. Attainment of such values using a combination
of aerobic biological processes and physical separation techniques requires a multi-stage treatment
strategy.
Previous research (Nicholson et al, 1998) has shown that the characteristics of DW on commercial
dairy farms is inherently subject to substantial variability, and therefore, use of two biological
treatment processes in tandem will minimise the risk of system overload and consequent pollution .
Most of the treatment will occur in the first stage and the second stage meets the final target
specification. Naturally, where treatment targets are less rigorous, it may be possible to dispense with
the second stage.
1.3.2 Key biological and physical processes
Although biological treatment is effective in reducing BOD5 concentrations, which are largely
composed of soluble matter, they are less effective in removing insoluble suspended matter, which
consists in part of complex organic matter such as lignocellustic material that requires slow hydrolysis
reactions for decomposition. However, the insoluble nature of such material enables its removal by
physical means such as sedimentation. In summary, the relevant biological and physical effects
adopted in the intensive aeration techniques used in the DW-STOP project were as follows:
Biological processes:
•
removal of soluble organic matter as represented by BOD5 by degradation to carbon dioxide and
water;
•
some breakdown of complex organic material by limited hydrolysis;
•
some breakdown of organically bound nitrogen to ammoniacal nitrogen but this will be offset
by the concurrent growth of biomass;
•
removal of ammoniacal nitrogen via the nitrification-denitrification route with its final removal
as di-nitrogen (N2) gas;
•
reduction of numbers of some pathogens by generation of an aerobic environment.
Physical processes:
•
removal of coarse insoluble matter by screening;
•
removal of insoluble organic matter (including biomass) by sedimentation;
•
removal of most phosphorus as insoluble phosphates in the settled sludge;
•
removal of insoluble heavy metals in the settled sludge.
1.4
REED BEDS
1.4.1 Horizontal and vertical flow reed beds
Constructed reed beds have been used for more than twenty years to treat wastewaters such as local
domestic sewage, airport runway run-off and agricultural effluents (e.g. Newman et al, 2000). Reed
beds operating with subsurface flow may be divided into two categories – horizontal and vertical.
Vertical, or down flow reed beds (DRBs) have an unsaturated bed of gravel, which enables them to
have a much higher oxygenation ability than horizontal flow systems. Tidal flow systems are a variant
of the latter type, in which the bed is alternately filled with wastewater and then drained (Sun et al,
1999). The principle of tidal operation is that air is drawn into the bed as the water drains out,
whereupon it is used to oxidise the pollutants held within the matrix of the bed. The DRB matrix
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consists of gravel, in several layers of graded particle sizes ranging from 2 mm fine gravel at the top to
stones of 30-60 mm diameter at the bottom. The beds are planted with common reeds, Phragmites
Australis.
1.4.2 Use of DRBs in agriculture
The treatment of agricultural effluent represents a particular challenge due to the high BOD5 and
seasonal variations in pollutant loads. Tanner et al (1995a) studied the effect of loading rate and
planting on the treatment of dairy farm wastewaters, in particular the removal of nitrogen, phosphorus.
In another publication (Tanner et al 1995b) the removal of oxygen demand, suspended solids and
faecal coliforms were considered. Total BOD5 removal was in the range from 50 to 80 % from an
influent concentration of 300 g m-3, and was sensitive to the wetland loading and hydraulic residence
time. The study showed that the DRBs have considerable potential for removal of N, P, BOD5, TSS
and faecal coliforms (FC) from dairy waters, following pre-treatment in oxidation ponds.
Sun, et al (1998) studied the application of tidal flow DRBs to the treatment of high strength
agricultural wastewater with BOD5 in the range 400 – 1500 mg/l. The removal of organic matter in
terms of BOD5 averaged 74 %. They developed a mathematical model that allows for removal of
organic pollutants based on first order kinetics with respect to BOD5. The pollutants are assumed to
be removed from the wastewater by adsorption and aerobic decomposition. The good agreement
between the experimental data and model calculations gave confidence that the model could be used
for design purposes. Kern and Idler (1999) studied the treatment of a mixture of domestic and
agricultural wastewaters produced by a cheese dairy. Treatment efficiency ranged between 13 and 99
%. Seasonal variation in performance was investigated and it was found that treatment in the summer
displayed increased removal rates compared with winter.
Newman et al (2000) reported a study of a surface flow wetland to treat 2.65 m3/d of milk house
water. They were interested in the performance of the reed bed in a cold climate, as winter
temperatures were thought to impair nitrogen and phosphorus removal. They found that winter
conditions do affect the treatment efficiency. The retention of all pollutant variables was higher in the
reed bed during summer operation, with the exception of faecal coliform bacteria. However due to
BOD5 overloading the wastewater outflow did not meet the specified criteria.
Rousseau et al (2004) reviewed the application of constructed wetlands in Flanders, Belgium. Vertical
flow reed beds gave the best overall performance compared with free water surface (horizontal) beds.
However, combination of vertical-horizontal flow offered some advantages for total nitrogen removal.
Dunne et al (2005) studied the application of a large scale reed bed of 4800 m2 to the treatment of
dairy DW from a 42 cow unit. Before application to the reed bed, the farm DW underwent some pretreatment by sedimentation. Precipitation events were found to influence significantly the amount of
DW produced and hence the peak load of the installed treatment system.
1.5
SOIL BASED DW
TREATMENT SYSTEMS
1.5.1 Previous use of soil based water treatment systems
Several effluent treatment systems have been reported in the literature in which soil and vegetation
comprise the medium in which pollutants are immobilized or removed from the waste water, e.g.
Martinez and Hao, 1996; Brookman et al., 2000; Leeds-Harrison et al., 2000; Mazer et al., 2001;
Bowman et al., 2002; Tyrrel et al., 2002; Peu et al., 2004. These systems have been designed for the
treatment of dairy farm DW, pig slurry, landfill leachate and urban runoff. Indeed, the concept of
using land as a means of wastewater treatment and disposal is not new. Such systems are currently in
use in sewage treatment works in the UK as tertiary systems and the use of “sewage farms” to deal
with raw sewage was common in the 19th century.
Soil has an inherent ability to remove potential contaminants from effluent such as DW. Organic
material (the major component of BOD5) is removed by soil micro organisms, suspended solids are
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removed through filtration, phosphorus is adsorbed onto particle surfaces, whilst ammonium-N is
either fixed within clay particles or nitrified to nitrate. Hence, high nitrate concentrations can occur in
leachates, although some N is also lost in gaseous forms.
The use of grassed systems to treat runoff water has become an integral part of treatment systems in
urban and peri-urban situations in the USA and are increasingly used in the UK in sustainable urban
drainage systems (SUDS) (Mazer et al., 2001). For instance, over 100 bioswales (grassed ditches)
were constructed in King County, USA, between 1990 and 2000 to treat runoff associated with
residential, commercial, and light industrial development. These aimed to achieve up to 80% TSS
removal and have been shown to remove more than 80% of the total phosphorus.
In one form of soil based DW treatment, the water to be treated is applied to the soil and allowed to
travel downwards through the soil profile. The SOLEPUR process (Martinez and Hao, 1996) was a
percolating soil based treatment system for the treatment of pig slurry. It was constructed as three
units. The first unit was a hydrologically isolated soil plot with a depth of 800 mm. The second unit
was a denitrification basin reactor. The third unit was a grass field. The system consisted of both soil
flow stage of aerobic nitrification and a subsequent denitrification stage in a lagoon. The mean
removal of nitrogen, phosphorus, BOD5 and solids for this system was 99.9%. The system worked
satisfactorily for a period of six years. No significant amounts of phosphorus were found in the
drainage water from the first, soil based stage in any season, with 82% of the applied phosphorus
being retained by the soil. However, it was noted that the available phosphorus in the soil rose from
158 mg/kg to 1225 mg/kg. After applications ceased, soil tests suggested that there was migration of
phosphorus down the soil profile (Peu et al., 2004).
Brookman et al. (2000) also explored the use of blocks of soil to treat DW from dairy farms,
comparing the performance of disturbed and intact soil columns. Both disturbed and intact soil
columns achieved removal rates for ammonium and molybdate reactive phosphorus in excess of 99%.
The disturbed soil column also reduced BOD5 by over 99%. The intact soil column, however, reduced
the BOD5 by slightly less than the disturbed column, when the DW application rates were increased
from 14 mm week-1 to 42 mm week-1.
Bowman et al. (2002) investigated the feasibility of the disposal of leachate from landfill to a
recreational grassed area, effectively creating a percolation system. The leachate was applied to the
soil via sub-surface pipes, at a depth of 110 mm, in pulses, temporarily creating anaerobic conditions.
Three different application concentrations were tested: undiluted leachate; 50:50 mains supply water
to leachate; and 80:20 mains supply water to leachate. This equated to application rates of 6082, 3337
and 1216 kg N ha-1 yr-1 respectively. A control, which contained no leachate, was also included. The
soil water was sampled at a depth of 300 mm below the leachate distribution system. The experiment
ran for two years. The mean nitrogen remaining in the soil water after application of the 100, 50 and
20% leachate solutions was 9, 10 and 8% respectively (implying that concentration had no effect on
treatment) and there was very little difference between the first and the second year. However, the
amounts of applied nitrogen bound to the soil matrix and the amounts removed from the system in
gaseous losses were different in the first and second years. In the first year, gaseous losses accounted
for 55, 43 and 43% of the nitrogen applied in the 100, 50 and 20% leachate dilutions respectively. In
the second year, they accounted for 57, 60 and 61% respectively. Vegetative uptake accounted for a
very small percentage of the nitrogen lost from the system.
Work at Cranfield University using a recirculating overland flow soil based treatment system (Tyrrel
et al. 2002) was originally designed to treat landfill leachate although the target pollutants were BOD5
and ammoniacal nitrogen. In these experiments, 25 m2 areas of vegetated, soil-filled troughs were
irrigated with landfill leachate. The leachate was cycled for periods of up to 35 days. The reduction in
nitrogen was monitored on a daily basis. The reduction in the concentration of nitrogen ranged from
57% (18 days treatment, 17 litres m-2 day-1) to 99% (22 days treatment, 70 litres m-2 day-1). This
reduction typically followed a first order exponential decay, but the relationship describing this decay
was specific to each set of hydraulic and climatic conditions. In considering the nitrogen sinks in these
cases, it was suggested that gaseous nitrogen losses could account for the deficit between the nitrogen
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bound to the soil matrix, plus that taken up by the vegetation, and the nitrogen in the effluent at the
end of the treatment process.
The work with landfill leachate led onto experimental trials with dairy DW (Leeds-Harrison et al.
2000). These experiments measured the nitrogen, phosphorus and BOD5 removed from DW when this
was applied to a recirculating soil based overland flow system. It was found that the reduction in
BOD5 followed a first order decay, achieving an 80% reduction in concentration after 7 days and an
almost complete removal after 21 days. The reduction in ammoniacal nitrogen followed a similar
pattern. However, the removal of molybdate reactive phosphorus (MRP) from the DW was less
predictable and the amount of phosphorus in the effluent increased as well as decreased during the
treatment period. For example, 60% of the concentration of phosphorus was removed on the first
treatment day. After 21 days, 90% had been removed. TSS removal was also good, approximately
65% being removed in the first 7 days and almost all being removed after 21 days. The authors
suggested that this was achieved through a combination of mechanical filtering and. microbiological
activity.
Overall, the results from the recirculating soil based overland flow system suggested that it was
successful in treating the leachate from landfill and DW from dairy systems. During the DW-STOP
project, the aim was to create a model overland flow treatment system for dairy farm DW capable of
treating up to 6 m3 / week, to investigate the efficiency of pollutant removal over a long operational
period (~ 2 years) and to consider the engineering, environmental and economic constraints to using
such a system at full scale.
1.5.2 Mechanisms of potential pollutant removal from DW
by percolating soil plots
Previous research (Chadwick et al, 2000) has shown that dairy farm DW
can be treated by
percolation-based soil treatment systems, i.e. percolating soil plots (PSPs) . Research at the plot and
field-scale has focussed on freely draining soils for obvious reasons. Treatment requires frequent
applications throughout the year and there is the risk of exceeding hydraulic conductivities and
infiltration rates on heavier soils, particularly during the wetter months of the year. (Ironically, soils
with a greater clay content have more capacity to reduce concentrations of some contaminants than
sandier soils, e.g. Chadwick and Pain, 1997).
The concept of using soil based treatment systems is simple. Excess DW that cannot be disposed of
safely on a farm is applied to a dedicated area of land from which all the effluent can be collected.
This area of land is managed solely for the purpose of treatment and is not compromised by pressures
for grazing or producing forage. Advantages of soil based treatment systems include:
•
•
•
low fixed costs,
simplicity (comprising an application and collection system) and
familiarity to farmers.
Potential disadvantages include:
•
•
taking land out of ‘production’, and
limits to the life-time of the system e.g. due to P accumulation. Hence the system may have to
be rotated around the farm to ‘fresh’ areas of land.
1.5.3 Mechanisms of potential pollutant removal from DW
1.5.3.1
by overland flow plots
Reducing the BOD5 concentration of DW
Since BOD5 is a measure of the dissolved oxygen used by microorganisms in the biochemical
oxidation of pollutants, it is greatly increased by the presence of fats, sugars and proteins, such as
those found in milk. These are an ideal nutrient source for microorganisms, which, in turn, increase
17 of 217
their activity (and their oxygen usage) (Brady and Weil, 1999). In a soil-based system of treatment, the
high BOD5 effluent flows over the soil surface bringing pollutants into contact with microorganisms,
in an environment in which oxygen is readily available. The organic compounds are readily broken
down, usually with the release of carbon dioxide to the atmosphere. In these systems the soil, or more
particularly the soil pores and soil surface, provide a support matrix for microbial activity. Treatment
can thus take place within the soil mass as well as at the soil surface.
1.5.3.2
Reducing the solids content of DW
The TSS present in DW can be removed by mechanical filtering, which is aided by vegetative cover.
As the DW passes over the grass plane, velocities of flow decrease and suspended particles may settle
out from the flow and become trapped in the grass and/or on the soil surface. These predominantly
solid particles will then be broken down by microbial activity in a similar way to the biodegradation of
manure applied to soil (Leeds-Harrison et al., 2000).
1.5.3.3
Mechanisms of nitrogen removal from DW
Animal wastes contain high levels of nitrogen in the form of urea, which leads to a complex series of
reactions and exchanges of matter (Fig. 1.3.1). Nitrogen from urea is released to the environment by
the activity of urease enzymes in the form of ammonia gas (NH3). These NH3 molecules are in
equilibrium with ammonium ions (NH4+) in the soil and the relative proportions of each depend
largely on temperature and pH. Ammonium ions are also released by enzymes, from decaying organic
matter. This is known as ammonification, or mineralization (Lavelle and Spain, 2001). Nitrification is
the process by which ammonium is converted to nitrite and then to nitrate via bacterial activity
(Deacon, 2004).
Atmospher
Atmosphere
NH3
Plant
Plant Uptake
Volatilisation
Volatilisation
Plant Uptake
Nitrification
Dirty
Wate
Water
Denitrification
Denitrification
Nitrification
Ammonificatio
Ammonification
NH 4
+
Adsorption
Adsorption
-
NO 2 -
-
NO
N 3-
-
NO
N 2-
NO
N 2O
N2
Desorption
Desorption
Soil
Soil
Fig. 1.3.1 The Nitrogen cycle with reference to DW (adapted from Brady and Weil,
1999). The desired nitrogen transformations are shown in red.
Following nitrification, denitrification can occur. This is the process by which nitrate is converted to
gaseous nitrogen by the activity of bacterial genre such as Pseudomonas, Bacillus, Micrococcus and
18 of 217
Achromobacter and is, therefore, lost from the system (Brady and Weil, 1999). In soil-based systems,
ammoniacal nitrogen can be bound to cation exchange surfaces (i.e. negatively charged particles) in
soil.
1.5.3.4
Mechanisms of phosphorus removal from DW
In the treatment of dairy farm DW, target pollution levels in the final effluent are usually concerned
with BOD5, TSS and ammoniacal nitrogen. More recently, phosphorous in final effluent has become a
concern and phosphorous stripping is now common in sewage treatment.
The phosphorus cycle, with reference to the overland flow plot and DW , is summarised in Fig. 1.3.2.
Particulate phosphorus, however, dose not fit easily into this diagram. Particulate phosphorus in the
applied DW is assumed to settle out of solution onto the soil surface, where it may be broken down
by enzymes and dissolved (Reddy, 2002). Phosphorous may be present in organic matter fractions in
the DW or in mineralised forms, which can be adsorbed onto soil particles.
Dirty Water
immobilisation
Soil solution
Soil Organisms
mineralization
adsorption
Readily soluble Ca-P
Very slowly soluble
Ca-P minerals
desorption
P retained by
clay and by Fe
and Al oxides
P occluded in Fe and
Al minerals
(Extremely insoluble)
Dominant form of inorganic P
in high pH and calcareous
soils.
Dominant form of inorganic P
in low pH, highly weathered
soils.
P in active
soil OM
P in slow and
passive soil OM
Organic P.
Fig. 1.3.2 The phosphorus cycle. Phosphorus will move into the less
labile pools with time (adapted from Brady and Weil, 1999).
Dissolved organic phosphorus is generally more mobile than soluble inorganic phosphates, probably
because it is not so readily adsorbed by organic matter, clays minerals and by Calcium Carbonate
(CaCO3) layers in the soil (Brady and Weil, 1999). This fact could prove problematic for the
effectiveness of the overland flow plot, regarding the removal of phosphorus from solution, however,
organic P is both held in microbial biomass and is mineralised through microbial action, allowing it to
move into less available pools.
1.5.3.5
Creating the right environmental conditions for treatment
In order for an overland flow DW system to work effectively it is clear that the system must provide
a support matrix and suitable environment on which aerobic bacteria can survive and provide
adsorption sites for phosphorus and ammonium. Clay soils can provide this if only intermittently
wetted and well vegetated. Vegetation not only provides the filtering effect already mentioned with
respect to TSS but also dries the soil via transpiration.
19 of 217
1.6
PROJECT OBJECTIVES AND APPROACHES (TC, CoB, KS)
1.6.1 Project objectives
The overall objective of this work was to identify, develop and assess the best practical means at
minimal cost to reduce the risks of water pollution and pathogen transfer caused by contaminated
drain flow, runoff or diffuse flow following DW irrigation to land. The specific scientific objectives
included: (a) pilot scale process comparisons, (b) assessing the effects of treatment on drain flow and
run-off from field sites and (c) process optimisation.
Due to the many factors that can affect its properties, it is extremely difficult to define a single optimal
strategy for cost effective DW management. Therefore, the key deliverable of the DW-STOP project
from the perspective of the UK dairy farming industry was to demonstrate efficient DW management,
and the associated costs. This was to be accomplished by producing process specifications for two
full-scale treatment systems, based on experimental results and mathematical modelling. Hence, the
project aimed to take the first step in bridging the gap between pilot scale research and full-scale
demonstration of results. This aligned with the research priorities of the Milk Development Council
(MDC) and Defra.
1.6.2 Outline of approaches
To meet the scientific and industrial objective of the project, four pilot-scale treatment strategies:
down-flow reed beds (DRB), percolating soil plots (PSP) , overland flow plots (OFP) and settlement
plus intensive aeration (IAP) were developed and evaluated on a 440-cow commercial dairy farm in
Sussex. Each was designed to treat 500 l/day (i.e. about the volume of DW from 8 to 10 cows). The
capacity of the systems to remove organic matter, i.e. BOD5 and TS, was assessed at weekly intervals
during two trial periods of 50 and 46 weeks respectively. Additional intensive monitoring periods also
included nitrogen and phosphorus compounds plus thermotolerant coliforms. The first trial revealed
the performance of each system working independently, whilst the second enabled the systems to be
used in combination to increase efficiency.
Since soil has the inherent ability to remove potential contaminants from livestock effluents, there is
no reason to DW more than is necessary, if the final application to land can result in additional
removal of diffuse pollutants. Hence, the impacts of land spreading both untreated and treated DW
were assessed using three in-field lysimeters. These provided an assessment of the ability of soil at the
Farm to remove potential pollutants.
Laboratory-scale studies supported the pilot-scale investigations and provided important data for
subsequent mathematical modelling of the treatment processes by SRI, IGER, CUS and UoB. These
laboratory studies comprised:
•
•
•
Investigation of the ability of five different soil types to remove potential pollutants after land
spreading (IGER, Devon). Linked with this, IGER was also responsible for the development of
a soil model to predict the ability of soil to remove BOD5 after land spreading. This model was
designed to operate in Excel and was targeted at researchers and advisors, not farmers.
Phosphorus accumulation and leaching in OFP systems (CUS, Bedfordshire); and
Flow properties and pollutant removal in packed DRBs (UoB, Birmingham).
20 of 217
CHAPTER 2 EXPERIMENTAL METHODS
Chapter authors: Trevor Cumby, Andrew Barker, Colin Burton, David Chadwick, Marc Dresser, Gari
Fernandez, John Gregory, Peter Leeds-Harrison, Ian Muir, Elia Nigro, Ken Smith and Joe Wood
2.1
SITE FACTORS, DESIGN AND INTEGRATION OF SYSTEMS (TC, IM, JG)
2.1.1 Pallinghurst Farm
2.1.1.1
Site locations and layout
All of the pilot scale experimental work was completed at Pallinghurst Farm, near Horsham in West
Sussex (Fig. 2.1.1).
Fig.2.1.1 General location of the experimental site, indicated by the red
circle
The detailed location of the experimental site is shown in (Fig. 2.1.2). The local arrangements at
Pallinghurst Farm required that the experiments were set up on two sites: Site 1 was located next to an
existing slurry lagoon, on an area of concrete, and Site 2 was approximately 300 m from Site 1 in a
meadow, known as “The Moor”, which was largely surrounded by trees (Fig. 2.1.2).
21 of 217
Fig.2.1.2 Detailed locations of the experimental sites: Site 1 is indicated
by the arrow and Site 2 is coloured pink
2.1.1.2
Site survey
Before any installations were completed at Site 2, a topographic survey of whole of The Moor was
undertaken by the CUS team (Fig. 2.1.3).
Fig. 2.1.3 Contour map of “The Moor” Pallinghurst Farm
The survey showed that the average gradients were approximately 1%, with the steepest gradient
(about 2%) in the lower southwest limb of the field, which is where Site 2 was located. Samples of the
indigenous soil were taken at Site 2, and the results were taken into account in preparing the two soilbased treatment systems. Since the soil at Site 2 was found to contain high concentrations of
22 of 217
phosphorus, both of the soil-based treatment systems and the lysimeters installed there were built
using imported local soils containing less phosphorus.
2.1.2 Installation of treatment processes and lysimeters
2.1.2.1
Treatment processes
The IAP and DRB systems were located at Site 1 (Fig. 2.1.4), whilst the OFP and PSP systems plus
the three diamond lysimeters were located at Site 2, (Fig. 2.1.5).
Fig. 2.1.4 General view of Site 1 at Pallinghurst Farm, shortly after installation. The Reed bed
treatment system is in the foreground, in front of the farm’s slurry lagoon and three-tank DW
settlement system. The IAP is located on the raised concrete area behind the railings.
Fig. 2.1.5 General view of Site 2 at “The Moor” Pallinghurst Farm. The PSP system is in the right
foreground, with the OFP system in the centre background. The larger tank near the centre of the
area stored and supplied untreated DW, whilst the smaller tank on the left was for treated effluent.
23 of 217
2.1.2.2
Diamond lysimeters
Following the site survey of The Moor, including Site 2 (Fig. 2.1.3), three diamond lysimeters were
constructed there. All three were approximately 5 m * 5 m in plan area, 0.3 m in depth, with a slope
of about 2%, with one apex the highest point (hence giving rise to the “diamond” description with
respect to the direction of the slope). Each one was hydrologically isolated from the underlying soil
using 800 gauge polythene membrane. One diamond lysimeter was adjacent to the PSP and OFP
treatment plots, (Fig. 2.1.6) and received treated DW.
Fig. 2.1.6 Diamond lysimeter at Site 2 for treated DW
The other two diamond lysimeters were located towards the Southern end of “The Moor”, in an
appropriate position for application of untreated DW to one, leaving the third as the untreated control
plot (Fig. 2.1.7). Automatically operated pumps, tanks and valves were used to supply DW to the
diamond lysimeters. Further details are provided in Section 2.6.
Fig. 2.1.7 Diamond lysimeters at Site 2. The installation in the
foreground was for untreated DW and the one in the background was a
“control”, which did not receive any applications apart from natural
rainfall
24 of 217
2.1.2.3
Laboratory lysimeters
The field scale lysimeters were complemented with laboratory-scale lysimeter studies at IGER, North
Wyke, Devon. Forty-five soil cores were set-up in total, each of 20 cm diameter, and 50 cm depth, and
were located in a plastic tunnel on benches enabling the collection of drainage water plot (Fig. 2.1.8).
Fig. 2.1.8 Laboratory scale lysimeters (soil cores) at IGER, North Wyke,
Devon
This apparatus enabled three replicates of each of three treatments for each of five soil types. The
treatments comprised: untreated DW, treated DW (both from Pallinghurst Farm) and clean water (as a
control). The five soil types were:
•
•
•
•
•
Loamy sand (Bicton) – Bridgnorth series
Coarse sand loam (De Bathe Cross) – Crediton series
Clay loam (IGER North Wyke) – Halstow series
Silty clay loam (Dorset) – Andover series
Silty clay (Duchy) – Denbigh series
Further details are described in Section 2.6
2.1.3 Sources of DW
2.1.3.1
and supplies to Sites 1 and 2
Initial arrangements: Site 1
In view of the comparative nature of the Trials, it was important to provide the means for a fair
comparison of system performance. This included the provision of similar DW to all of the processes,
and this required the near-simultaneous supply of untreated DW to both Sites 1 and 2, which were
each equipped with a tank to store and thus supply DW, on demand, to the treatment processes, as well
as tanks for treated DW. Untreated DW was obtained from two sources:
•
•
a strainer box attached to Pallinghurst Farm’s slurry lagoon (Fig. 2.1.9), and
run-off from the concrete yard areas between the lagoon and the silage store (Fig. 2.1.10).
25 of 217
Fig. 2.1.9 Source of DW: Strainer box and part of lagoon at Pallinghurst
Farm
Fig. 2.1.10 Source of DW: Runoff from yard area at Pallinghurst Farm
Effluent from both sources was collected in a “3-tank system” at Site 1 (Fig. 2.1.4), which was next to
a positive displacement pump connected to a “ring main” DW distribution system. This was reconnected as shown in Fig. 2.1.11 and extended to enable untreated DW to be conveyed to Site 2
using the existing positive displacement pump.
26 of 217
N
100 m
Rain gun plot
Raw DW
Standpipe
Area currently covered
by Briggs irrigators
Proposed
disconnection
Treated DW
Standpipe
Raw DW
Treated
DW
Percolation plot
Overland flow plot
Diamond lysimeters
Dairy unit buildings
Fig.2.1.11 Aerial view of the experimental sites. The Pallinghurst Farm DW
distribution pipeline is shown in yellow
A vertical steel cylindrical tank was installed at Site 1 for untreated DW (Fig. 2.1.12). It had a capacity
of approximately 10 m3 and was fitted with an electrically powered stirrer, which ran intermittently
under timer control, to reduce energy consumption. Further details are provided in Section 2.2.2.
Nearby, a pyramid-bottom tank of approximately 3 m3 capacity provided a common reception point
for the treated effluent from both the IAP and the DRB systems (Fig. 2.1.13). This supplied a second
positive displacement pump that was connected to the other section of the disconnected ring main, and
so this conveyed treated DW from this tank to Site 2.
Fig.2.1.12 Steel tank at Site 1, with stirrer, to supply untreated DW to the
IAP and Reed bed systems, capacity: 10 m3. Part of the Pallinghurst
Farm slurry lagoon is visible to the right of the tank, behind the weeping
wall and concrete strainer box
27 of 217
Fig. 2.1.13 Installation of the Reed bed system at Site 1, including, on the
left, the 3 m3 capacity pyramid-bottom tank for treated DW from both
the Reed bed system and the IAP
2.1.3.2
Initial arrangements: Site 2
The tanks for untreated and treated DW at Site 2 were placed together as shown in Fig 2.1.5. The
larger of these tanks had an effective capacity of 8.5 m3 and provided a supply point for untreated DW
to each of the adjacent treated systems, and to the “untreated DW” diamond lysimeter. A 1 kW
submersible pump was used to mix the contents of this tank before these were transferred to the OFP
system. A limited electrical supply to Site 2 prevented use of a larger mixer.
The second tank held 5.7 m3 and received treated effluent from the soil-based treatment systems at Site
2, plus the discharge flow of treated DW from the pyramid-bottom tank at Site 1. This smaller tank at
Site 2 provided a source of liquid treated for the “treated DW” lysimeter. Since the volumetric
requirements of this lysimeter were much less than the combined output of the four treatment
processes, surplus treated DW was applied, by static sprinkler, to anther part of The Moor that was
more low-lying than Site 2 (Fig. 2.1.14).
Fig. 2.1.14 Static sprinkler, in foreground, to apply surplus treated DW to
a lower-lying part of The Moor. Site 2 is in the background
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2.1.3.3
Revised arrangements
It had been anticipated that an abundant supply of DW at Site 1 from the strainer box and yard run-off
via the “3-tank system” would be sufficient to fill both supply tanks simultaneously. However, when
the lagoon was emptied in the spring of 2003 and 2004, according to normal farm practice (Fig.
2.1.15), the strainer box system was unable to supply more than a tiny fraction of the total flow
needed.
Fig. 2.1.15 Normal farm practice: emptying the lagoon at Site 1
Shortage of DW was rectified by the installation of an additional 18 m3 storage tank for untreated
DW at Site 1, (Fig. 2.1.16) plus a submersible pump and a 300 m surface pipeline to supply this with
wash water from the 3-chamber effluent settling tanks near the dairy parlour (Fig. 2.1.17)). Hence, this
supplied effluent when DW supplies from yard run-off and the strainer box were not available.
Fig. 2.1.16 Installation of an additional 18 m3 storage tank for untreated
DW at Site 1
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Fig. 2.1.17 Source of DW: 3-chamber effluent settling tanks near the
dairy parlour
2.1.4 Trial 1
2.1.4.1
Trial 1 – experiment design and overview
Trial 1 was designed to provide an unbiased comparison of the performance of the four key DW
treatment processes, with each process being set-up according to the best available knowledge from
previous research and experience. The trial began, as planned, on 1/5/03 and ended on 27/4/04.
During this period, the four treatment systems were generally operated in parallel as shown in Fig.
2.1.18. The only significant exceptions to this arrangement occurred between 1/5/03 and 3/9/03, and
again between 17/2/04 and 27/4/04, when the DRB was supplied with pre-treated DW, supplied from
the IAP. Further details of these exceptions, and other key events, are listed in Table 2.1.1
Fig. 2.1.18 Schematic layout of comparative assessment experiments: Trial 1
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Table 2.1.1 Diary of key events: DW-STOP Trial 1
Date
1/5/03
3-9/6/03
15/7/03
15-21/7/03
12/8/03
12-26/8/03
2-8/9/03
3/9/03
Event
Start of Trial 1. DRB system began operation by treating
effluent that had been pre-treated by the IAP.
First intensive monitoring period
Additional 18 000 litre tank commissioned at Site 1
Second intensive monitoring period
IAP flow reduced from 550 l/day to 275 l/day due to high 5day biochemical oxygen demand (BOD5) and total solids (TS)
concentrations in the untreated DW.
The IAP aeration
control system was also switched to continuous operation.
OFP system shut down for remedial action
Third intensive monitoring period
Reed-bed system re-configured to treat a flow of
approximately 30 l/day of untreated DW
Start of photographic records of untreated and treated DW
samples
Applications of untreated DW to the OFP and PSP systems
stopped due to water-logging.
IAP input flow restored to 550 l/day
16/9/03
30/9/03
3/11/03
12-28/11/03
Tangential Flow Separator (TFS) used at Site 1 under the
supervision of Mr Alban Timmons
18/11/03
25/11/03
16/12/03
Soil replaced in the PSP system
Normal operation restored in the PSP system
All systems shut down during Christmas holidays. Extra
remedial action taken in connection with the OFP system
included the addition of a gypsum trench and division of the
plot into two parallel treatment areas.
All four systems re-started.
6/1/04
17/2/04
Supply of untreated DW to the DRB was ended and the
system was re-configured to treat approximately 450 l/day
from the HRTF stage of the IAP.
9-15/3/04
13/4/04
20/4/04
Fourth intensive monitoring period
IAP shut down due to faulty float switch
IAP repaired and restarted
27/4/04
“The Haymakers” discussion group visited Pallinghurst Farm,
including the two DW-STOP project sites
End of Trial 1 - all four systems shut down as planned.
27/4/04
2.1.4.2
Comment
Started on schedule
Completed on schedule
Thanks to financial support
from Defra
Completed on schedule
Changes were necessary
because the untreated DW
concentrations
exceeded
design levels at this time.
Necessary
to
improve
infiltration
Completed on schedule
As requested by some
Project Partners
See Sections 3.4 and 3.5
Untreated
DW
concentration had returned
to design levels
Not part of DW-STOP, but
the risk of chemical
contamination of the DW
used by DW-STOP required
close liaison with the TFS
team
See Section 3.4
All
systems
re-started
satisfactorily.
This change was necessary
because the gravel of the
DRBs had blocked several
times with solid debris from
untreated DW
Completed on schedule
Repair delayed by delivery
of new switch
Trial 1 – process monitoring and sampling strategy
Throughout the whole of Trial 1 (apart from a Christmas break between 16/12/03 and 6/1/04, DW
sampling and data collection were undertaken weekly, with additional daily sampling on weekdays
during four intensive monitoring periods. Each intensive monitoring period began on a Tuesday, thus
31 of 217
coinciding with the regular re-supply of untreated DW to the four treatment systems. All four
treatment systems were operational throughout all of these intensive monitoring periods.
The process monitoring and field application monitoring strategies are summarised in Tables 2.1.2 and
2.1.3
To fulfil these requirements, data concerning the day-to-day operation of the treatment
processes were recorded in weekly summary reports.
All samples were analysed by Direct
Laboratories Ltd, Wolverhampton (formerly ADAS Laboratories), or their nominated sub-contractors.
In addition, from September 2003 onwards, each of the separate DW samples, both untreated and
treated, were sub-sampled and photographed together, shortly after sampling. This was completed in
front of a light source, including an opaque band to indicate sample turbidity (through loss of
contrast). Example photographs are shown in Section 2.6.
Table 2.1.2 DW-STOP Sampling and Analysis Strategy: Process monitoring
Measured property
sample
frequency
*
Flow of DW
D
Flow of treated effluent
D
Volume of sludge produced
W; I
Herbage off-take
W; I
COD (in raw or treated effluent)
I
BOD5 (in raw or treated effluent)
Inputs
Outputs
System no.
System no.
1
2
3
4
U
U
U
U
1
2
3
4
U
U
U
U
U
U
U
U
U
U
U
U
U
U
W; I
U
U
U
U
U
Total solids (TS) (in raw or treated effluent)
W; I
U
U
U
U
U
Kjeldahl nitrogen (in raw or treated effluent)
I
U
U
U
U
U
Ammoniacal N (in raw or treated effluent)
I
U
U
U
U
U
Total phosphorus (in raw or treated effluent)
I
U
U
U
U
U
Nitrates and nitrite (by test sticks) (in raw and
treated effluent)
I
U
U
U
U
U
pH
I
U
U
U
U
U
Total Thermotolerant Coliforms (at 44 C)
I
U
U
U
U
U
Total solids (TS) (in sludge)
W
U
Total phosphorus (in sludge)
W
U
Accumulation of phosphorus
I
Capital costs
O
o
Land area required
U
U
U
U
U
U
U
U
U
Electricity consumed
W
U
U
U
U
Chemicals consumed
W
U
U
U
U
Labour required for operation/supervision
W
U
U
U
U
Operation diary (as assessment of reliability)
D
U
U
U
U
* Sampling frequency:
32 of 217
U
U
Measured property
sample
frequency
*
Inputs
Outputs
System no.
System no.
1
2
3
4
1
D= daily; W = weekly; I = intensive monitoring periods; O = once only for each system
Definitions of systems:
1: Reed beds; 2: Percolating flow; 3: Overland flow;
4: Settlement + intensive aeration
2
3
4
Table 2.1.3 DW-STOP Sampling and Analysis Strategy: Field application monitoring
Properties to be monitored for each designated area
Inputs
Outputs
Property
Method/location
Property
Method/location
DW
application and
precipitation rates
Calibration and running
hours of irrigator plus
collection trays
Drain flows
V-notch weirs or tipping
buckets
Rainfall
Rain gauge
Limited
number
BOD5 samples
of
Total solids (TS)
Kjeldahl nitrogen
Laboratory or on-site
analysis of samples from
shared tank of treated
effluent
Limited
number
BOD5 samples
of
Total solids (TS)
Kjeldahl nitrogen
Electrical conductivity
Electrical conductivity
Ammoniacal nitrogen
Ammoniacal nitrogen
Nitrates and nitrites
Nitrates and nitrites
Total phosphorus
Total phosphorus
Colorimetric
assessments
Colorimetric
assessments
Total
Thermotolerant
Coliforms (at 44oC)
Total
Thermotolerant
Coliforms (at 44oC)
Laboratory or on-site
analysis of samples from
drainage outfalls and
from captured run-off.
2.1.5 Trial 2
2.1.5.1
Trial 2 – experiment design and changes implemented following Trial 1
Trail 2 was designed to evaluate a number of changes and improvements implemented in the light of
the results from Trial 1. It began on 6/7/04, and it was completed on 31/5/05. Following extensive
consultation and discussion amongst the Project Partners, the design of Trial 2 followed the format
shown in Fig 2.1.19. Some key changes were implemented in Trial 2, including a number of options
for investigating alternative process configurations. Operational results and experience since the start
of Trial 1 in July 2004 enabled some key decisions to be made in connection with these options, as
detailed in Table 2.1.4.
33 of 217
Fig. 2.1.19 Schematic layout of comparative assessment experiments: Trial 2
Table 2.1.4 Use of combined treatments in Trial 2 – Key changes compared with Trial 1
Treatment
system
IAP
Materials
to be used
Untreated
DW (farm
site)
DRB
Pre-treated
DW from
the CSTR
stage of the
IAP
(Option 1)
ditto
Pre-treated
DW from
the
OFP
system
(Option 2)
Major Modifications implemented since
Trial 1
DW throughput increased from 450l/day to
750 l/day, with the ability to divide the flow
after the first biological treatment stage, a
Continuous Stirred Tank Reactor, (CSTR)
into three parts. The second biological
treatment stage, a High Rate Trickling Filter,
(HRTF) has received one of these fractions
(250 l/day), with 400 l/day of the remainder
being available for the DRB system
Improvements were implemented to control
flows and liquid levels in the system. Other
measures reduced the number and extent of
blockages. Gross daily throughput of 800
l/day comprised approximately 400 l/day
from the CSTR plus a recycle flow of about
400 l/day of treated effluent from the DRB
discharge flow, thus giving a net throughput
of 400 l/day.
This option has not been used since
substantial extra DW handling facilities
would be needed to direct the output from
the OFP system to the DRB.
34 of 217
Comments
The changes have achieved the
intended effects and the IAP has
operated effectively with this
increased throughput.
This configuration has been used
since the start of Trial 1, without
serious malfunctions, although
some occasional blockages have
occurred.
It was agreed by all Partners that
this approach would not be
evaluated by experiment. Instead,
the likely effects of this mode of
operation will be assessed within
the modelling activities of DWSTOP
Treatment
system
PSP system
Materials
to be used
Untreated
DW
OFP system
Untreated
DW
(Option 1)
ditto
Untreated
DW
(Option 2)
Untreated
DW
(Option 3)
ditto
2.1.5.2
Major Modifications implemented since
Trial 1
No mechanical medications were completed
following Trial 1, expect for reprogramming the control system to reduce
DW application rates to those of the
diamond lysimeters.
The PSP system
continued to operate in this mode since the
start of Trial 2.
The DW handling facilities were improved,
including transverse “blind French drains”
built across each of the two treatment areas,
to improve lateral distribution of DW.
Comments
The results from sampling the
outflows from the PSP system have
been collated and are being
analysed together with those from
the diamond lysimeters.
This
system
has
operated
satisfactorily in this way since the
start of Trial 2, with a weekly input
of 2400 l (i.e. equivalent to 343
l/day). Operation was switched to a
two-week cycle with effect from
30/11/04 (see Section 6.4).
Not used – see above
Although the “spare” output from the CSTR
stage of the IAP could be diverted to the
OFP system, this option has not been
implemented since the latter has maintained
good performance using raw DW.
This remains is a contingency
option if the OFP system is found
to be unsuitable for prolonged
operation using untreated DW.
Trial 2 – overview
Trial 2 began on schedule on 6/7/04, and four intensive monitoring periods were completed before the
Trial was completed on 31/5/05. However, in view of the large amount of scatter observed in the Trial
1 measurements of Total Thermotolerant Coliforms, it was decided, following careful scrutiny by the
Project Partners, to discontinue these assays after the first intensive monitoring period in Trial 2. This
saving provided resources for other more informative analyses, including TSS during Trial 2. Unlike
Trial 1, the treatment systems all continued normal operation throughout the Christmas period, as
shown in Table 2.1.5.
Table 2.1.5 Diary of key events: DW-STOP Trial 2
Date
6/7/04
Event
Start of Trial 2 – DW treatment resumed in IAP, DRBs and
OFP System.
10/8/04
9-15/11/04
30/11/04
First application of raw DW to the PSP as a lysimeter
Fifth intensive monitoring period (last one to include
sampling and assay of Total Thermotolerant Coliforms)
Start of 1st 2-week cycle for the OFP system
7-13/12/04
14/12/04
28/12/04
11/1/05
25/1/05
8/2/05
15-21/2/05
22/2/05
8/3/05
22/3/05
5/4/05
Sixth intensive monitoring period
Start of 2nd 2-week cycle for the OFP system
Start of 3rd 2-week cycle for the OFP system
Start of 4th 2-week cycle for the OFP system
Start of 5th 2-week cycle for the OFP system
Start of 6th 2-week cycle for the OFP system
Seventh intensive monitoring period
Start of 7th 2-week cycle for the OFP system
Start of 8th 2-week cycle for the OFP system
Start of 9th 2-week cycle for the OFP system
Start of 10th 2-week cycle for the OFP system
35 of 217
Comment
Percolation plot system reassigned as a large-scale
lysimeter
Completed on schedule
Change needed to provide
data to test mathematical
model
Completed on schedule
Completed on schedule
Date
5-11/4/05
19/4/05
3/5/05
17/5/05
31/5/05
Event
Eighth intensive monitoring period
Start of 11th 2-week cycle for the OFP system
Start of 12th 2-week cycle for the OFP system
Start of 13th (final) 2-week cycle for the OFP system
End of Trial 2 - all three systems and lysimeters shut down, as
planned.
Comment
Completed on schedule
Completed on schedule
As in Trial 1, sampling and data collection were undertaken weekly, with additional daily sampling
(on weekdays) during four intensive monitoring periods. A similar weekly routine was adopted except
that after 30/11/04, the OFP system was operated on a fortnightly cycle, which continued until the end
of the Trial, as detailed in Table 2.1.5. The sampling, sample photography, process monitoring and
field application monitoring strategies remained largely as summarised in Tables 2.1.2 and 2.1.3.
2.2
INTENSIVE AERATION PLANT (CoB, TC, EN)
2.2.1 Description of process and equipment
2.2.1.1
Plant overview: location, configuration and operation
The purpose of the IAP used for DW-STOP was to reduce the concentration of pollutants in dairy
farm DW using a combination of biological and physical processes, as listed in Table 2.2.1
Table 2.2.1 Key processes used in the IAP
Class of
process
Biological
Component removed
soluble organic matter
ammoniacal nitrogen
Physical:
insoluble matter (including
biomass)
Measured property
BOD5 (included in
COD)
TAN, (included in
TN)
TSS (included in TS)
Means of removal
Biological oxidation
Nitrification and
denitrification
screening and
sedimentation
In practice, the IAP consisted of a series of tanks, pumps, stirrers and ancillary equipment mounted
onto two steel frames (modules). Separate DW storage tanks were used, to facilitate transport of the
IAP. The main parts of the IAP are summarised in Fig. 2.2.1, which include five unit treatment
operations, as follows:
•
primary sedimentation,
•
aerobic treatment by CSTR (continuous stirred tank reactor),
•
secondary sedimentation,
•
aerobic treatment by HRTF (high rate trickling filter), and
•
tertiary sedimentation.
36 of 217
Rx1
S2
P1
t3
t2
L1
T9
T7
T5
T1
T2
P4
P2
V1
T3
B1
P7
t4
Rx2
L2
t1
T4
T8
L3
P3
L5
L4
P5
MODULE 1
P6
T10
P8
MODULE 2
Fig. 2.2.1 Schematic diagram of the intensive aeration plant (IAP) showing the
configuration used in Trial 1
Note: all references in Chapter 2 to IAP tank numbers (i.e. T1- T10) and pump numbers (i.e. P 1 –
P8) refer to Fig. 2.2.1
The IAP was installed at Pallinghurst Farm in March 2003. It was located next to the main slurry
lagoon, and obtained its supply of DW from a separate 10 m3 stirred tank sited alongside (Fig. 2.1.12).
This was replenished from the three sources of effluent as described in Sections 2.1.3.1 and 2.1.3.3.
For part of Trial 1, some of the treated effluent from the IAP was sent for a final treatment by the DRB
system located nearby. During the second Trial, the arrangements were changed, as detailed in
Section 2.2.1.8, so that partly treated DW could be supplied to the DRB system.
Throughout both Trials, all remaining treated effluents from the IAP were combined with those from
the DRB system and were pumped to Site 2. All sludges produced by the IAP were pumped back to
the slurry lagoon nearby. The initial design and operation of the IAP are described below, and
illustrate the arrangements that were in place for Trial 1. Section 2.2.1.8 details the changes that were
made before Trial 2.
2.2.1.2
Primary sedimentation
DW from the 10 m3 stirred tank was periodically pumped by a scroll and stator pump (P1) to the
primary sedimentation tank (T1), at a rate of 550 L/d. The plant operated according to a repeated 2
hour control cycle in which 48.5 L of raw DW was added at the beginning of each cycle. The duration
of each operation of P1 determined the volume of DW transferred. T1 had nominal capacity of 1000
litres and a baffle in this tank minimised disruption of the settling phase by the incoming flow of DW
(Fig. 2.2.1).
T1 was fitted with a run-down screen to remove coarse debris such as straw, twine, undigested forage,
etc. However, this was not used because the loss of liquid from the screen substantially impaired the
flow metering accuracy of P1, so that in consequence, the amount of DW would have been unknown.
T1 removed settleable material including inert matter such as soil particles and small stones. Hence, it
was unlikely to have much effect on BOD5 concentration of the DW although, depending on the nature
of the DW had the potential to reduce TSS concentrations substantially. T1 also protected the
downstream parts of the IAP from damage due to hard solid matter.
37 of 217
Settled sludge collected in the conical base of T1 from where it was removed periodically by pump P2.
The amount of sludge removed needed to be set at a minimal value to avoid the discharge of too much
sludge, but not so low that clarification was impeded. The mathematical model of the process, based
on a mass balance around the plant, and in the form of an Excel computer spreadsheet, was used to
assist the selection of initial settings. Hence, a sludge removal rate (i.e. duration of each P2 operations)
was adopted that amounted to 7.5 % of the DW feed rate (i.e. 3.6 L/cycle). Once the IAP was running,
it was possible to sample and compare the TS concentrations of samples of the incoming feed DW,
plus the sludge and supernatant from T1. Hence, it was possible to adjust the amount of sludge
removed by P2 to achieve a sludge TS concentration of about 30g/l. Lower values indicated that too
much sludge was being removed. A higher value combined with poor clarification implied
insufficient sludge removal.
T1 was fitted with a temperature probe (labelled “feed temperature”) and a low level switch for plant
protection purposes. Clarified DW from T1 overflowed via a weir to the next stage (see Section
2.2.1.3).
2.2.1.3
Aerobic treatment by continuous stirred tank reactor (CSTR)
Clarified effluent from T1 overflowed directly into the first biological treatment stage, tank T2. This
was an aeration unit with a working volume of 2.2 m3 and it functioned as a continuous stirred tank
reactor (CSTR). Hence, all of its contents were similar to the treated effluent that overflowed to the
next stage. T2 was expected to remove up to 90% of the incoming BOD5; in theory, 100% removal
was possible but the continuous nature of a CSTR makes this impracticable. Normally, removal of
BOD5 would also be lead to a similar reduction in the chemical oxygen demand (COD), so that the
process performance could be measured using either property. Accordingly, COD removal was
expected to follow the correlation:
CODout/CODfeed =
[0.33 / (1+0.4R)] + 0.535
Equation 2.2.1
Where R is hydraulic residence time in days (Burton, 1992). Hence, a four day treatment was
expected to reduce COD by 34%.
Aerobic treatment in the CSTR was also expected to reduce the ammoniacal nitrogen concentration,
provided that nitrification was encouraged by high aeration levels plus R > 3 d). However, this was
rarely the case due to the required high throughput of DW, and so little nitrification occurred in T2. In
the event, this was not important because the process was completed in the second biological stage,
(T7, see Section 2.2.1.5).
Air entered T2 through three diffuser disks located near the base of the tank. The air was supplied by
a blower (B1), which typically supplied 3-5 l/s at 30 kPa (i.e., a little more pressure than the
hydrostatic head of liquid in T2). The blower (Fig. 2.2.2) started and stopped according to control set
points that responded to signals from a single redox probe immersed at 50% of the liquid depth in T2. .
The redox control value was -50mV Ecal and the plant’s control sequence was programmed to
interrogate this value at 20 minute intervals. If the redox value was found, upon interrogation, to be >
-50mV Ecal, the blower was turned off, and vice-versa if the redox was < -50mV Ecal. The redox
probe also included a temperature sensor.
38 of 217
Fig. 2.2.2 Blower (B1) for supplying air to the diffuser disks, plus the mechanical air flow meter
and pressure gauge. A water-filled manometer was also fitted both to check pressure and to
provide pressure relief in the event of blockage
Besides aerating the liquid, the air bubbles from the diffuser disks also agitated the vessel contents. A
mechanical foam breaker was fitted to the top of T2, approximately 0.5m above the liquid level. This
consisted of a 0.4 m diameter thin steel disk that rotated on a vertical axis at approximately 1000 rpm,
and thus projected liquid droplets, at high speed, radially from its centre. Hence, any rising foam was
effectively contained beneath the disk and the “umbrella” of droplets.
The discharged flow from T2 was simply the effluent that was displaced by the incoming feed from
T1. The discharge flow from T2 first passed to a tipping bucket flow meter (T3), and then to a small
tank (T4, Fig. 2.2.3), from where it was pumped by P3 to the secondary sedimentation vessel (T5).
The operation of P3 was controlled by float switches (L2 and L3).
Fig. 2.2.3 Detail of the feed vessel T4 supplying the transfer pump P3. The pump was activated by
a high level switch and turned off by a low level switch. The operation software prevented running
for more than a few minutes to protect equipment in the event of a fault.
39 of 217
2.2.1.4
Secondary sedimentation
The secondary sedimentation vessel (T5) was similar in design to the other sedimentation vessels (T1
and T9). However, unlike T1, settled material in T5 was more likely to be active bacterial floc (i.e.
“biomass”), rather than largely inactive, insoluble organic and inorganic matter. The formation of
bacterial flocs along with the breakdown of some of the organic matter in the CSTR (T2) was
expected to enable better clarification in T5 than in T1.
Sludge was removed from T5 by pump P4 and its operation followed the same strategy as P2.
However, some sludge could be returned to the CSTR (T2), controlled by the setting of a motorized
valve (V1), thus following the strategy of the activated sludge process used in the sewage industry.
This sludge recycle increased the biomass concentration and thus activity, in T2. In this way, the
volume of sludge removed from T5 was equivalent to 20% of the raw DW input flow to T1. Half of
this (i.e. 4.9 L/cycle) was returned to T2. Without this recycle, the TS of the effluent in the CSTR
would have been similar to that in the feed (e.g. 5 to 10 g/l); by recycling, this concentration was
expected to rise to 20 to 30 g/l. Higher recycling rates were not used because they have a diminishing
effect as the TS concentration of the CSTR effluent gradually approaches that of the sludge.
Since recycling was used, the flows recorded by the tipping bucket (T3) required correction, as
follows: If the flow rate from the tipping bucket is T tips/hour, where each tip discharges 14 l, the flow
rate from the CSTR, F is given by the equation:
F (l/h) =
14 x T - 6.5 x (P4) t
Equation 2.2.2
Where: (P4) t is the hourly run duration of P4 in minutes, and P4 is assumed to achieve a mean flow
rate of 6.5 l/min. Clarified supernatant from T5 overflowed by displacement and gravity to the next
stage.
2.2.1.5
Aerobic treatment by high rate trickling filter (HRTF)
The second biological treatment stage (T7) differed from the CSTR mode of operation used in the first
stage (T2). In particular, T7 operated as a high rate trickling filter (HRTF), in which microbes were
immobilised on inert packing (Fig. 2.2.4), whilst the effluent to be treated trickled over this packing.
This arrangement was able to sustain more biomass in an effluent depleted of BOD5, than would have
been possible in a second CSTR. Hence, this matched the lower oxygen demand of discharged
effluent from T2, which was greatly diminished in reactive organic matter, leading to a lower
biological loading of T7 compared with T2. Hence, the biological oxidation of organic matter (i.e.
removal of BOD5) in T7 was expected to be small, although this could be enhanced by extending the
residence time in T7.
40 of 217
Fig. 2.2.4 Freshly installed plastic packing rings in the HRTF (T7).
Subsequently, it was necessary to irrigate these rings with recycled DW
for several days to establish an active biofilm layer
If the BOD5 concentration in the effluent from T2 remained low, the consequently small biological
loading of T7 meant that despite the relatively short contact time as DW trickled over the packing in
HRTF, enough oxygen transfer occurred from the naturally ventilating air to sustain nitrification. In
principle, nitrification will result in ammonia being oxidized to nitrates (and possibly, nitrites also);
subsequent anaerobic storage will result in the breakdown of these with the nitrogen being released as
(mostly) di-nitrogen gas (N2) and some nitrous oxide (N2O).
Operation of the HRTF (T7) required a steady flow of DW from pump P5 in the compartmented sump
(T8) to nozzles located at the top of the packing. The P5 flow rate was typically 100-150 l/min, so the
entire contents of T8 were recirculated every few minutes. The arrival of discharge flow from T5
caused the liquid in the main part of T8 to overflow into a separate compartment from where it was
periodically pumped by P6, controlled by float switches L4 and L5. Temperature and redox values
were monitored in T8 but neither was used to control the process.
2.2.1.6
Tertiary sedimentation
The design of the final sedimentation vessel (T9) was similar to T1 and T5 except that it operated as a
clarification process rather than primary settlement or sludge collection. Hence, the main purpose of
T9 was to collect any biomass washed off the packing and other surfaces in T7. Consequently, it was
unlikely to produce much sludge but the clarification achieved in T9 was important because it
controlled the quality of the treated effluent leaving the process. Hence, a generous volume of sludge
was removed from T9 to minimise the chance of sludge in the final effluent. This sludge volume was
equivalent to 10% of the raw DW feed (i.e. 4.9 L/cycle);. This sludge was conveyed by pump P7 to
the CSTR (T2). Therefore, this avoided the need to discharge of large volumes of dilute sludge,
although it reduced the effective residence time in T2.
P7 was controlled in conjunction with P6 as follows: on each occasion that the liquid level in T8 rose
to the high level switch (L5), P7 first removed some settled matter from T9, ahead of the subsequent
operation of P6. Since the volume of liquid delivered from P7 was less than that from P6, operation
of P6 caused clarified supernatant to overflow from T9 to tank T10. This flow was the final treated
effluent and was delivered by pump P8 to a local collection point, as required.
41 of 217
2.2.1.7
Control cabin
The entire plant was controlled and monitored by equipment located in the control cabin. A main
control panel contained relays to operate the process pumps, stirrers and other electrical equipment.
All of the power consumed was recorded using an electro-mechanical Watt-hour meter. All power
supplies were protected by contact breakers and thermal overloads. The outputs from the main panel
comprised a series of sockets, so that the two IAP modules and the cabin could be readily
disconnected for transport (Fig. 2.2.5).
Input signals from
probes
Main panel
Rdx1
Rdx2
run
auto
fail
Instruments
Power
man
auto
off
signal
processing
STOP
Protection
devices
Run signals
Inputs
Outputs
Power supply to
electrical devices
Modem
link
Control
Operation
Data-logging
Fig. 2.2.5 Schematic diagram of the IAP control equipment
2.2.1.8
Changes implemented for Trial 2
Before the start of Trial 2 on 6/7/04, the IAP was changed as shown in Fig. 2.2.6 and as summarised
below:
(a) During the Trial 1, there were periods of sustained high concentration of BOD5 in the DW at Site 1
(see Section 3.1.1.2), and during these periods, the aeration capacity of the system was sometimes
insufficient to maintain the minimum redox set point of -50mV Ecal. Therefore, before stating the
second trial, a second blower was added; this also allowed volume of DW treated to be increased from
500 l/d to 800 l/d. Following this modification, B1 operated continuously whilst the second blower
(B2, Fig. 2.2.7) operated only when the redox level fell below the set point. Both blowers delivered air
via the original diffusers.
42 of 217
(b) The IAP was modified before Trial 2 to enable an increased throughput of 800 L/d. A new tank
was added (T11) to collect the partly treated effluent from the CSTR (T5). Approximately 250 l / d of
this was transferred to the HRTF by an additional pump (P9), whilst a further 250 l / d was pumped to
the DRBs by a centrifugal pump in T11, which was controlled by the DRB control system (as
described in Section 2.3). Any remaining DW in T 11 was combined with the yard run-off liquid that
contributed to Pallinghurst Farm’s total DW supply.
Rx1
S2
P1
t3
t2
L1
T9
T7
T5
T1
T2
P4
P7
P2
t1
V1
T3
B1
t4
Rx2
L2
B2
T4
L3
T8
P3
MODULE 1
L5
L4
P5
P6
T10
P8
MODULE 2
T11
P9
Fig. 2.2.6 Schematic diagram of the intensive aeration plant (IAP) showing the
revised configuration used in Trial 2 (picture)
Fig. 2.2.7 Blower B2 used in parallel with B1 in Trial 2. Maximum
output exceeded 10 l/s and the maximum delivery pressure was 500 kPa.
43 of 217
2.2.2 Plant operation
2.2.2.1
General plant operation
Each of the dozen or so electrical items (pumps, stirrers, blowers, etc) had a local isolator to enable
safe maintenance and the operational status each one was indicated by lights on the panel, which
showed a state of “off", "running", or "tripped out". The latter status meant that an overload current
had occurred and that the protective circuits had operated to isolate the affected motor, pump, blower,
etc. Depending on the control requirements of each electrical unit, relevant switches enabled manual
selection of three settings:
(a) OFF (the item only being used occasionally)
(b) ON (the item being run continuously)
(c) AUTO (the item being run by the computer)
Under "AUTO", relays were operated by external signals. Examples included:
(a) A control signal from the personal computer (PC) which controlled the main time-based sequence
of operations (e.g. P1, P2 and P4). The same computer also provided logging facilities for all
equipment, including the redox and temperature probes,
(b) Level control sequence, and
(c) The on/off signal from the redox meter in T2 (to control the blowers, B1 and B2).
The time-based operating sequence operated via a computer program based on Visual Basic, written
by SRI staff. This programme enabled the computer, via an output card, to switch on and off any
device set to “AUTO” on the control panel. The program structure included a series of operation
sequences that could be triggered either by time or by liquid level switches (e.g. L5). One routine
operated the blowers and included an input from the redox probe in T2 (i.e. Rx1).
In parallel with the plant operation, the program also recorded all inputs (probes and equipment
operation) to a data file. Operational data were also logged separately to a “history file” to assist with
diagnosis in the event of plant disruption. The PC was connected to a modem link to allow remote
monitoring, data collection and operational changes.
2.2.2.2
Two-hourly control sequence
Plant operation followed a repeated two hour control sequence that started on each even hour. Thus at
any time, the operational state of the plant could be anticipated. Compared with true continuous
operation, this method of control was simpler and more robust. Most operations took place in
response to the time schedules in the program, but others were triggered by float switches and so,
where necessary, operated simultaneously with other functions.
As an example, the control system used in Trial 2 is set out in Table 2.2.2. Thus, (in this example), the
first operation at the beginning of a cycle is programme sequence 1 which is started (in this example)
at 0 minutes, i.e. at the beginning of the cycle. For its first activity, the programme starts the stirrer in
the feed tank (S1) - this runs for 120 s duration before the feed pump P1 is switched on as well. Both
devices run for a further 620 s, which, with a flow rate of 6.3 l /s will deliver 65 l of feed DW to T1.
With 12 x 2 hour cycles per day, this equates to a daily feed of 780 l as specified for Trial 2.
Separately, but still under sequence 1, pump P6 is run for a maximum of 15 minutes (or until the high
level switch goes off). The logic in coupling these two operations is to ensure that there is a “space”
in the HRTF sump (vessel T8) to receive the effluent displaced as a result of the feed cycle.
44 of 217
Table 2.2.2 Control system for IAP in Trial 2. The sequence start time is in bold.
Sequence
1
Title
P1 (feed)
2
Blower
(compressor)
cycle A
3
P4-W (waste)
4
P7 (sludge)
Start signal
Units
Start time
Duration
involved
(minutes)
(seconds)
Timer
S1
0
120
Timer
S1, P1
2
620
Timer + level L5
P6
12
up to 900
Timer
B2
5, 23, 41
up to 2280 total
Timer
(optional)
B2,
(optional)
Timer
V1
50
5
Timer
P4
51
50
Timer
P2
55
7
Timer
P7
56
7
S2
B2 + 2mins
(optional)
5
P8 (transfer)
Timer + float switch
P8
60
up to 300
6
Blower
(compressor)
cycle B
Timer
B2
65, 83, 101
up to 2280 total
Timer
(optional)
B2, S2
(optional)
B2 + 2mins
(optional)
P6 (transfer)
Timer + level L5
P6
80
up to 900
Timer
P9 (AUX2)
95
91
10
7
P4-R (recycle)
Timer
P4
110
50
8
P2 (sludge)
Timer
P2
115
14
9
P3 (transfer)
Level L2
P3
anytime
up to 30 s
n/a
P5 (HRTF pump)
Continuous
P5
continuous
continuous
n/a
B1 (blower)
Continuous
B1
continuous
continuous
n/a
S2 (foam breaker)
Continuous
S2
continuous
continuous
Before the first sequence is completed, the second one will start allowing the operation of the blower
(B2) (if the redox value is below the chosen set point). Once activated, the routine would keep B2 on
for 18 minutes before checking again the redox value; if still below the set point, B2 would remain on
for a second period of 18 minutes and again for a third period if necessary; in this way, damaging
on/off switching was avoided. Sequence 2 includes the option of a delayed start to the foam breaker
(S2) to save electricity. The sequence completes at 59 minutes but a second blower sequence (no. 6)
comes in at 65 minutes hence there is a 54 minute blower cycle each hour.
Other sequences start at various points throughout the 2-hour cycle including:
•
•
•
Sequence 3 (secondary sludge to waste),
Sequence 4 (tertiary sludge to recycle) and
Sequence 5 (transfer pump P8 for final treated waste): P8 would run only if (a) the sequence
was activated and (b) the float switch was showing a high level.
45 of 217
Sequence 9 was activated by float switch rather than by timer but then operation would follow a set
pattern as defined in the routine.
2.2.2.3
Explanation of individual control sequences - an example
An example programme routine is given below, featuring the routine “P4-Waste” that is called by
sequence 7 at 110 minutes into the cycle: As the title implies, the primary purpose of this routine is
the removal of sludge to waste which involves the pump, P4 and the motorised valve, V1. The code
runs as shown in Table 2.2.3 with numbers added to aid with the explanation only.
Table 2.2.3: example of control system code
Instruction
Action
Explanation
1
Delay 5
Confirm signal
2
Ctrlon 12
3
Delay 30
motorized valve switched to pump to waste
4
If Dig(15) off
5
Ctrlon 4
6
Delay 17
)
pump P4 running for 17 s (to waste
7
Ctrloff 4
8
Endif
9
Ctrloff 12
10
Delay 60
pump P4 locked out (60 s)
11
Ctrloff 4
Instruction 1 delays any activity by 5 seconds to prevent the unintentional clash of instructions from
other routines that may be running in series. The second instruction turns on the motorised valve, V1,
(given the code 12). This, when off, is set at recycle to minimise the risk of liquid loss in the event of
process disruption. In consequence, the valve must be activated to switch the direction of sludge flow
to waste. This requires a few seconds, but the programme allows 30 seconds (line 3) to ensure against
a stiff valve. The plant has protection should the pump P4 be started prematurely (via pressure relief
system) but it is preferable not to rely on this in normal operation. Instruction 4 checks the level probe
in the first sedimentation tank T1 (probe L1). Given the code 15, a positive signal would indicate a
low level and stop the operation of the pump. Pump P4 is given the code 4 and switched on (line 5)
and run for 17 seconds (line 6). Beyond the loop (lines 4 to 8), the motorised valve is switched off
(line 9) and allowed 60 seconds to do this (line 10). The routine needs a definite instruction for
completion (not a delay instruction) hence the seemingly redundant stopping of the pump a second
time (line 11). The routines used with the other sequences followed a similar logic but in some cases,
several activities were combined in one procedure for simplicity.
2.2.2.4
Data logging procedure
All plant signals were conveyed to the PC but only some were recorded. The key data elements were
temperature and redox values (T2 and T8). All switching data of pumps, motors etc was recorded in a
“history file” which was used primarily for diagnosis when problems occurred. Otherwise, it was
assumed that equipment ran as prescribed in the operation program (see Section 2.2.2.3)
Temperatures were recorded every 5 minutes and comprised a set of four measurements: ambient air;
liquid in T1 (i.e. raw DW); liquid in T2 (i.e. CSTR contents); and liquid in T8 (i.e. HRTF contents).
The values from the two redox probes, in T2 and T8 were sampled more frequently, at one minute
intervals, so that the cyclic effects of feed additions could be analysed from the recorded data files.
46 of 217
Figures 4 and 5 show the redox values of both the CSTR and the sump of the HRTF, which were
measured and logged every minute during both Trials. In addition, the temperatures of ambient air,
raw DW, CSTR contents and HRTF contents were logged every 5 minutes.
Manual check sheets were completed weekly and included readings of the electricity meters. A
mechanical, integrating gas flow meter was connected in line with the output from blower B1 - this
was also noted in the check sheets. However, the same meter could not be used for blower B2 because
it could not operate under the potentially higher pressures. Thus for Trial 2, no cumulative air
consumption data were recorded.
2.2.2.5
Sampling procedure
Weekly samples of raw and treated effluent were taken for biochemical analyses, as detailed in
Sections 2.1.4 and 2.1.5. Throughout all of Trials 1 and 2, these samples included:
•
•
•
the DW in the 10 m3 stirred tank (i.e. untreated DW at Site 1),
the treated effluent from T9, and
sludge from T5.
In addition, during the latter part of Trial 1, and through Trial 2, further weekly samples were taken
from the supernatant flow from T5. This allowed the performance of the CSTR (T2) and the HRTF
(T7) to be distinguished. From the perspective of plant operation, all of these samples identified
particular process problems and helped to ensure satisfactory resolution.
The wider set of properties measured in the daily samples taken during the intensive monitoring
periods provided a more comprehensive indication of plant performance, although the limited
frequency of these data sets meant that they were less useful for diagnosis of plant operational
difficulties.
Lastly, samples of the raw feed effluent were taken on two occasions and analysed for the metal ions,
sodium, magnesium and calcium
2.3
REED BEDS (AnB, GF, JW)
2.3.1
2.3.1.1
Description of process and equipment
Plant overview: location, configuration and operation.
As described above, the DRB treatment system was located at Site 1, adjacent to the IAP. The DRB
process consisted of 5 portable experimental DRB units, and 5 storage tanks. Figure 2.3.1 shows the
flow diagram of the system. Wastewater entered the system via tank 1 from either the farmyard
storage tank containing raw DW or from the IAP, as described in Section 2.1. In order to avoid the
possibility of shock loads of pollutant entering the beds and damaging the reeds, it was decided to
operate the process with a recycle of treated DW that was returned from tank 4 and then mixed with
fresh feed in tank 2. DRBs 1-3 comprised the primary treatment stage, and were planted in individual
metal tanks. Within each stage, the DRBs were operated in rotation such that one was in use whilst
the others were “resting”. Tank 3 was a holding tank between the primary and secondary treatments.
DRBs 4 and 5 comprised the secondary stage of treatment, and were planted in a single PVC tank,
such that recirculation between the two halves of the DRB was possible. Treated water discharged to
tank 5 and was subsequently pumped to the 3 m3 pyramid-bottom tank as described in Section 2.1.
47 of 217
Fig. 2.3.1 Layout of the DRB treatment system at Pallinghurst Farm (plan view).
48 of 217
2.3.1.2
Reed bed design
Figure 2.3.2. shows the DRB installation. The DRBs each have a plan area of 1 m2 and a depth of 0.6
m. All of the 5 DRBs used at Pallinghurst Farm were of the tidal operation type. The DRB matrix
consisted of gravel, in several layers of graded particle sizes ranging from 2 mm fine gravel at the top
to stones of 30-60 mm diameter at the bottom.
Fig. 2.3.2 The DRB treatment system installed at Pallinghurst Farm.
DRBs 1, 3, 4 and 5 were filled with graded layers of gravel, but DRB 2 was filled with pea gravel of
fixed diameter (5 mm). The height of the beds was of 60 cm. The typical thickness and particle size
range of the gravel in the graded DRBs is shown in Table 2.3.1.
Table 2.3.1. Thickness and particle size of the gravel layers of DRBs 1, 3, 4 and 5
Layer Thickness Gravel particle size (diameter)
13 cm
Ø = 0.6 - 0.8 cm
22 cm
Ø = 1.5 -1.7 cm
15 cm
Ø = 3 – 3.5 cm
All the DRBs constituting the pilot scale system contained plastic aeration pipes of about 10 cm
diameter. This enhanced the oxygen rate transfer into the rhizome area of the reeds.
DRBs 1, 3, 4 and 5 were planted with common reeds, (Phragmites Australis). DRB 2 was planted
with mace reed (Typha Latifolla).
2.3.1.3
Tanks and auxiliary items.
The intermediate tanks on the DRB plant were used to provide storage for water awaiting treatment
and temporary storage for treated effluent. Although theoretically the oxidation of pollutants mainly
took place in the DRBs, some oxidation of pollutants may have also occurred in the storage tanks.
Tank 2 was a cylindrical glass fibre vessel, and tank 5 a cylindrical PVC vessel, whilst the other tanks
were rectangular metal troughs. The dimensions of the tanks are given in Table 2.3.2.
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Table 2.3.2. Dimensions of the intermediate tanks on the DRB system
Feature
Height (cm)
Length (cm)
Width (cm)
Tank 1 Tank 2 Tank 3 Tank 4
62
180
81
66
240
124
154
61
Ø = 100
92
89
The pumps were submersible centrifugal pumps, located within the tanks. These were switched on
and off by the timer sequence in the control cabin, with the exception of limits on the levels in tank 2,
which were controlled by float switches. The tanks and DRBs were connected together by reinforced
plastic hose of 50 mm (2 inches) diameter.
2.3.2 Plant Operation
2.3.2.1
General plant operation.
During the course of the project, the DRB system was operated in four different modes, corresponding
to different flow rates of raw and recycled DW. In mode 1, the influent water supplied to the system
was pre-treated by both stages of the IAP, i.e. the CSTR and HRTF. Table 2.3.3 shows the relevant
flow rates for each mode of operation, and the periods of operation in each mode. In Modes 1 – 3 the
timer sequence operated on a 24 hour cycle.
Table 2.3.3. Operation modes of the DRB system with dates and flow rates
MODE
Period
1
06/05/2003 09/09/2003
09/09/2003 17/02/2004
17/02/2004 - 09/03/05
09/03/2005 31/05/2005
2
3
4
Throughput
(L/d)
1600
1150
800
800( 2×400)
Input flow (L/d)
500 (from IAP: CSTR +
HRTF)
50 (raw)
380 (from IAP: CSTR)
380 (from IAP:
CSTR)(2×190)
Recycled
(L)
1100
1100
420
420 (2×210)
The process operated with a recycle of treated water that was pumped from tank 4 to dilute the fresh
feed in tank 2. The two main reasons for operating with a recycle were:
•
•
To avoid possible situations of shock loads of pollutants entering the beds and damaging the
reeds and
To enhance nitrification by preventing the washout of nitrifying bacteria and reducing the BOD5
towards 100 mg/l, at which concentration nitrification starts to become significant.
In Mode 1, the overall flow rate through the beds after mixing influent and recycled water was 1600
l/day.
During Mode 2, it was decided to operate the DRB as a stand-alone treatment system, being fed with
raw DW. The objective was to test the test the treatment performance of the DRB independently of
the other systems, as required to fulfil the contracted protocol for Trial 1. The flow rate of recycle
water was maintained at 1100 l/day, but due to the higher concentration of pollutants in the raw water,
a flow of only 50 l/day of raw water was applied, such that a high dilution with recycle occurred.
Over the course of operation in Mode 2, the resistance to flow through the DRBs increased, which was
thought to be due to the high solids content of the raw DW applied during that period.
During Mode 3, the total throughput was reduced to 800 l/day in order to reduce the problems of
ponding observed in mode 2. Also, the 380 l/day of influent water were pre-treated in the CSTR stage
of the IAP, such that the DRB operated as a secondary treatment.
50 of 217
In Mode 4, the same daily throughput of influent and recycled water as mode 3 was used, except the
control timer operated on a 12 hour cycle, so that the application of pulses of water to the primary and
secondary DRBs were staggered. This also reduced the volume of water pumped in a single event, to
avoid overflow from tank 4. The two-fold objectives of changing the operating sequence during Mode
4 were to achieve, overall, a more consistent pattern of hydraulic flow through the system and also to
stagger the phasing of transfer of effluent to the first and second sets of DRBs. Therefore, the 24 hour
cycle was divided into two, providing a 12 hour cycle regime that, due to the smaller volumes being
applied into the beds, enhanced the hydraulic stability of the system, avoiding any overflow
occurrence. Thus, two cycles per day, with 400 litres throughput per cycle were applied to the beds.
The 400 litres were made up from 190 litres of pre-aerated (IAP-CSTR) dairy water, diluted with 210
litres of recycled effluent. Consequently, the overall daily throughput in Modes 3 and 4 were the
same.
2.3.2.2
Control sequence.
During the tidal cycle of operation of each DRB, 200 litres of water were applied to the bed at a flow
rate of 1 – 3 L/min, depending on which pump was being used to dose the pulse on to the bed. Table
2.3.4 gives the flow rates of pumps A – D. Following the application of a pulse of water, a waiting
period of the timer sequence of up to 165 minutes was allowed. The exact duration depended on which
mode of operation was in effect. During this waiting period, each bed drained under gravity, via hoses
connected to the holding tank.
Table 2.3.4. Pump flow rates.
PUMP A
PUMP B
PUMP C
PUMP D
FLOW RANGE (L/s) 1.00 – 1.06 2.65 – 2.70 1.8 – 2.00
2.00
The DRB system was controlled by an RS 328-134 programmable electronic sequence controller
which permitted eight 5A relays to be switched on and off, in any combination, during a 20-step
sequence. Table 2.3.5 shows the timing sequence as commissioned, and Table 2.3.6 shows the
modified sequence during Mode 3 of operation. This sequence controlled the pulses of water applied
to the DRBs, and also the recycle of treated DW to tank 2, thus keeping the system in mass balance
and preventing overflow of the storage tanks.
Table 2.3.5. Timer sequence control for Mode 1.
Step
0
1
2
3
4
5
6
7
8
9
10 + 0
10 + 1
10 + 2
10 + 3
Control
PA
PB PC
WAIT
PB PC
WAIT
PB PC
WAIT
PD
PB PC
WAIT
PB PC
WAIT
PB PC
WAIT
Status
Duration
Volume
ON
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
(Minutes)
4 m 10
1 m 40
165 m 00
1 m 40
165 m 00
1 m 40
165 m 00
2 m 15
1 m 40
165 m 00
1 m 40
165 m 00
1 m 40
165 m 00
(Litres)
500
200
51 of 217
200
200
270
200
200
200
Step
10 + 4
10 + 5
10 + 6
10 + 7
10 + 8
10 + 9
RESET
TOTAL
Control
PB PC
WAIT
PB PC
WAIT
PD
WAIT
RESET
Status
Duration
Volume
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
RESET
(Minutes)
1 m 40
165 m 00
1 m 40
165 m 00
6 m 55
93 m 20
RESET
1440 m 00
(Litres)
200
200
830
RESET
Table 2.3.6 Operating sequence for Mode 3.
Step
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
2.3.2.3
Control
PA
PB
WAIT
PC
WAIT
PB
WAIT
PC
WAIT
PD
PB
WAIT
PC
WAIT
PB
WAIT
PC
WAIT
PD
WAIT
RESET
Status
Duration
Volume
ON
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
ON
ALL OUTPUTS OFF
RESET
(Minutes)
6 min 20 sec
74 sec
157 min
110 sec
157 min
74 sec
157 min
110 sec
157 min
1 min 45 sec
74 sec
157 min
110 sec
157 min
74 sec
157 min
110 sec
157 min
1 min 45 sec
157 min
RESET
~1440 min
(Litres)
380
200
200
200
200
210
200
200
200
200
210
RESET
800
Sampling procedure.
Samples of DW were collected from tanks 1 and 4 on a weekly basis and sent for analysis at Direct
Laboratories, Wolverhampton. During the intensive monitoring periods, samples were taken from the
same tanks of the system on a daily basis. From the weekly samples and during the intensive
monitoring periods, the BOD5, TS and TSS concentrations were measured. In addition, nitrogen,
ammonia, phosphorus, COD and thermotolerant coliforms were measured by Direct Laboratories from
the samples collected during the intensive monitoring periods.
The above analyses provided information on the overall treatment efficiency of the DRB system.
However, it was decided to investigate further the individual performance of the primary and
secondary DRBs by taking extra samples for separate analysis at the University of Birmingham.
These extra samples were collected from tanks 1, 2, 3 and 4, during monthly visits to the site.
52 of 217
2.3.2.4
BOD5 analysis at the University of Birmingham
In these analyses, BOD5 was measured using a dissolved oxygen probe. A prepared stock of dilution
water was brought to incubation temperature (20 ± 0.5°C) and kept at that temperature for
approximately 13 hours. Then the water was saturated with oxygen by gently bubbling organic-free
filtered air for 1-hour ± 10 minutes. This ensured that the sample became saturated with dissolved
oxygen (DO). This was checked by the DO measurements performed using the WTW Oxi 330
Oximeter (Oxical ® SL), that showed saturation values of about 9.2 mg/L.
An appropriate dilution ratio was selected, typically being 1 or 2 % sample to 98-99 % dilution water.
Around 5 ml of bacterial seed were added to each litre of dilution water as component of the feed for
any aerobic process. A good quality settled sewage effluent was used for this purpose. Measurements
of the initial and final DO (before and after incubation period) were used to calculate the oxygen
consumed during the five-day test. The measurements and analysis carried out at the University of
Birmingham are fully documented in the MPhil thesis of Gari Fernandez (Appendix 3).
2.4
PERCOLATION SOIL PLOT SYSTEM (DCh)
2.4.1 Design and construction
The PSP was set up at Site 2 at Pallinghurst Farm between the 9th and 11th September 2002 when
conditions were dry and favourable for moving/handling soil. The soil at Site 2 could not be used for
constructing the system, since it had very high P and organic matter contents as a result of a long-term
history of DW applications (see Table 1). Therefore, a similar soil type was imported from a nearby
source. The soil was supplied by a local top-soil distributor and it had been screened to remove large
stones and roots. Samples of this soil were analysed for total P and organic matter content (see Section
3.4.3). The soil was a clay loam (sand 40%, silt 32%, clay 28%). An excavator was used to grade the
base (on a slope of c. 2 %) of the system to facilitate flow to the bottom corner.
Earth bunds were pushed into place and a geo-textile sheet was used to protect a plastic liner from
puncture by roots and stones. A collection drain was placed along the base of the two lower sides (in
gravel) to facilitate movement of effluent to the outlet pipe. The soil depth was c. 1 m and the surface
area was 7 m x 7 m. The PSP is shown in Fig. 2.4.1, shortly after it was installed. Care was taken to
minimise the degree of compaction by the excavator. After the plot had been established, it was sown
with Lolium perenne (variety ‘Moy’).
Fig. 2.4.1. The IGER percolating soil plot (PSP), February 2005
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2.4.2 DW
application system
Raw DW was applied to the soil surface of the PSP via a 500 L header tank mounted on pallets to
increase the pressure head for DW distribution (Fig. 2.4.1). The raw DW was supplied as described in
Section 2.1.3. A pump in the raw DW tank at Site 2 was used to fill the PSP header tank each day. A
float valve within the header tank ensured accurate filling. Float switches were eventually fitted in the
top and bottom of the header tank to log the filling and emptying. A solenoid valve was used to allow
the DW to flow into a gutter distribution system by gravity. The opening and closing of this valve was
controlled by a data logger. Initially, the DW was distributed onto the surface of the plot using a gutter
distribution system. However, this was replaced with the sprinkler system after 3 months to improve
distribution across the plot. When the PSP distribution system was changed to an irrigator system, the
solenoid valve-gravity feed system was replaced by an additional pump within the header tank. The
DW delivery rate from the sprinkler was set at 10 mm / hour.
The sprinkler could distribute the DW to only one quarter of the surface area of the PSP, so it was
moved between each of the four quarters of the plot on a weekly basis. DW applications of 10 mm
were applied on 5 consecutive days per week allowing a 2 day recovery period. DW was applied in 3
‘doses’. Initially, these were applied during the early hours of the morning (01:00, 02:00 and 03:00),
but this delivery timing was changed to 00:00, 03:00 and 06:00 hours to allow more time for the DW
to infiltrate the soil and avoid surface ponding. The finalised irrigation system is shown in Fig. 2.4.2.
Fig. 2.4.2. The DW irrigation system on the PSP, February 2004
2.4.3 Effluent collection system
A collection pit was dug down to approx. 120 cm ( ~ 1m x 1 m) and exterior grade plywood was used
to shutter the sides, which were held in place with wooden struts. A solid plastic drainage pipe from
the PSP was placed so that drainage was directed into a plastic water storage tank (200 L). A tipping
bucket was placed directly under the pipe, above the storage tank, to allow the volume of leachate to
be quantified (Fig. 2.4.3). Samples were taken automatically from the collection tank each day. The
collection pit was drained with a three-inch perforated drainage pipe to a point down-slope in nearby
woodland. An excavator was used to dig the trench and backfill with soil.
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Fig. 2.4.3. The collection pit, tank and tipping bucket for the PSP
January 2004.
The tipping bucket was calibrated periodically by pouring 10 L of DW through it and counting the
number of tips. The tipping bucket was connected to a data logger sited in a nearby shed and the time
at which each tip occurred was logged. Hence, the flows leaving the lysimeter, and their periodicity,
were recorded. Effluent samples were taken once per week during the routine measurement period and
sent for analysis as described in Section 2.1.4. Soil temperature (5 cm depth) was measured with a
thermistor and logged on the data logger housed in the site shed. Rainfall data were also collected and
logged by Cranfield University (see Section 3.5).
2.5
OVERLAND FLOW SYSTEM (PL-H, MD)
2.5.1 Design and operation of the Overland Flow Plot
Based on previous experience of overland treatment, the system installed at Site 2 at Pallinghurst Farm
was a grassed treatment plane as shown in Fig. 2.5.1. This was constructed in the form of a gently
inclined plot of vegetated clay soil with a slope of approximately 1% and measuring 10 x 7.0 x 0.3 m,
which was hydrologically isolated with an 800 gauge polythene membrane of the sort usually used for
horticultural poly tunnels. The soil for the plot was sourced from Pallinghurst Farm. A 1% slope was
used because experience had shown that steeper slopes lead to channelling of flow and uneven
distribution of water across the slope.
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From dirty water store
Re-circulation tanks
Pump 1
Distribution
channel
1% slope
10 m
sump Gravel filled trench
Pump 2
7m
Fig. 2.5.1 Plan layout of the overland flow plot (OFP)
Six recirculation/distribution tanks each of 1 m3 capacity were installed at the upper end of the slope to
contain the DW. These tanks were interconnected to provide a large capacity reservoir, and were
filled with DW as described in Section 2.1.3. In front of these tanks was a distribution trough across
the width of the plot. At the bottom of the slope was a gravel drain, which flowed into a sump in
which a float operated pump (pump 2) re-circulated the water to the tanks.
Once a day, at 6 pm, the DW in the tanks was pumped by pump 1 into the distribution trough and
flowed across the plot to the sump, via the drain, for a period of two hours. The plot then drained for
22 hours allowing air to enter the soil. Pump 2 returned water that collected in the sump to the
distribution tanks, so that the cycle could be repeated 24 hours later. In the initial phase of operation,
which included all of Trial 1 plus approximately the first four months of Trial 2, the treated DW was
sampled and removed after seven days and then replaced with more untreated DW (see Sections 2.1.3,
- 2.1.5). The original design of the overland flow plot at Pallinghurst is shown in Fig. 2.5.2
Fig. 2.5.2 Original design of the overland flow plot (OFP) at Site 2,
Pallinghurst Farm
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2.5.2 Design Modifications
2.5.2.1
Sodium and Sodicity
The sodium content of the DW that was applied to the OFP led to some operational problems. It was
not anticipated that the DW would contain sodium from parlour washings in concentrations that would
be harmful to the soil, as there was very little in the literature regarding this issue. However, during the
early stages of Trial 1, between June and September 2003, the soil became heavily waterlogged and
gleyed. It was suggested that this was due to high levels of sodium entering the plot, leading to clay
dispersion. This phenomenon is well understood and is common in surface irrigated clays in semiarid areas of the world. High sodium concentration displaces calcium on the clay complex allowing a
thickening of the diffuse double layer that forces charged particles apart at which point, close range
van der Waals forces cease to operate and the soil structure collapses.
The sodium adsorption ratio (SAR), and the electrical conductivity (EC) of the soil and DW were
determined by sampling and analysis. It was found that the soil had a high SAR of 46 and an EC of
2.5 dSm-1. For a soil to be considered sodic, it must have a SAR in excess of 13 and an EC of less
than 4 dSm-1 (Brady and Weil, 1999); thus, the soil had become extremely sodic. It was confirmed that
the source of the sodium was the influent DW, which was found to have a SAR in excess of 16. A
sample of the original clay soil that was used in the construction of the plot was also tested and found
to be 1.4. As there were no inputs to the system other than DW and rainfall, it was deduced that the
increase in the SAR of the soil was caused by sodium from the DW.
2.5.2.2
Remediation
There were two possibilities for remediating this situation. One was to replace the soil in the plot with
one of a coarser texture, and the other was to attempt remediation. Replacement was considered
impractical and too costly. Moreover, the project was intended for farms with heavy soils. Therefore,
it was decided to correct the sodicity problem. A tried and tested method of remediating sodic soils is
through the application of gypsum, CaSO4.2H2O (Bresler et al., 1982; Brady and Weil, 1999), which
is readily available. When the calcium ions enter the soil matrix, sodium ions bond with the SO4,
which is then leached, and replaced by the calcium ions.
The gypsum requirement was calculated using a method described by Brady and Weil (1999), which
estimated that approximately 200 kg of agricultural gypsum were required to remediate the entire plot.
This equated to approximately 3 t ha-1. The gypsum was applied in October 2003 by sprinkling
approximately half of the required amount evenly over the plot, then rotavating, then reapplying, and
then rotavating again. This was done in an attempt to ensure that the gypsum was mixed thoroughly
through the soil profile. Failure to do this could have resulted in a crust forming at the surface of the
soil. The grass that was on the plot was rotavated into the soil, rather than mown and removed. It was
felt that the added organic matter would help to improve soil structure and increase aggregate stability.
Immediately after the gypsum had been applied, the plot resembled a ploughed field. Clods appeared
to form, although they were still very fragile, possibly because the soil was very wet. The surface of
the soil was rather rougher than would have been ideal, but it was hoped that as the soil drained, the
effects of weathering would smooth it out.
When the OFP was visited in December 2003, there was a covering of grass (if slightly sparse), the
soil was load bearing under foot and no longer appeared to be gleyed. A sample of the soil was
retrieved for laboratory analysis by Cranfield University staff and it was determined that the SAR had
been reduced to approximately 1, indicating that the remediation with the gypsum had been successful.
As described in Sections 2.1.4 and 2.1.5, Concentrations of BOD5. TS and TSS were measured every
week that the system was in operation. There were intensive monitoring weeks when total nitrogen,
total phosphorus, magnesium, sodium, nitrate nitrogen, ammoniacal nitrogen, nitrite nitrogen and
57 of 217
chemical oxygen demand (COD) were also measured. In addition, records of the quantity of DW in
the holding tanks at the start and finish of the treatment period were also kept.
2.5.2.3
Operational changes following remediation
The initial arrangement of the OFP irrigated the whole plot each day and for each weekly batch of
DW. Following remediation, the plot was split into two equal halves so that one half of the plot could
be irrigated with a batch of DW whilst the other half remained unirrigated until the batch was replaced
with fresh DW (Fig. 2.5.3). This allowed some recovery of the soil by permitting the grass to dry out
the unirrigated area. The split was achieved by placing commercially available lawn edging along the
centre of the plot from the higher inlet end to the gravel drain. In addition, the single inlet gutter at the
head of the plot was replaced by two shorter ones and the system was operated so that the flow from
pump 1 was directed to either the right hand or left hand side of the plot for each batch of DW.
Distribution
tanks
Straw
insulation
Distribution
trough
Lagged
pipe
Lawn edging
Bund
Turf
Gypsum
trench
Sump pump
Gravel bags
Lagged pipe
Fig. 2.5.3 Plan layout of the overland flow plot (OFP) highlighting the revisions to
the plot design.
2.6
LYSIMETERS (field and lab-scale) (DCh)
2.6.1 Plot lysimeters at Pallinghurst Farm
Diamond lysimeters were constructed in order to determine the ability of the soil to remove BOD5 and
TS from both raw and treated DW under a range of environmental and edaphic conditions. Three
diamond lysimeters were constructed, 5 m x 5 m and 30 cm deep:
•
•
•
The first received raw DW from the raw DW tank at Site 2,
The second received bulk treated DW (i.e. collected and amalgamated from each of the
treatment systems) from the treated DW tank at Site 2, and
The third did not receive any DW and was there as a control.
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An excavator was used to remove the top 30 cm of soil (as this was contaminated with years of DW
applications and P in particular, see Table 2.6.1). The excavator was also used to grade the slope (c. 2
%
) towards the bottom corner of the each diamond to aid collection of leachate/drainage.
Each lysimeter was lined with a single layer of geo-textile fabric and then covered with a layer of
plastic. Three-inch perforated plastic drainage pipe was placed along the two ‘collection’ edges. One
piece of this pipe was then pushed through the plastic sheeting into a solid pipe that drained the
leachate from the bottom corner of each lysimeter into a collection pit. The perforated pipe was
pushed through the plastic sheeting into a solid pipe to make a good seal. The bottom corner of each
diamond lysimeter was then filled with washed gravel (10 mm) before filling the whole lysimeter with
the same imported, screened top-soil used for the PSP. Care was taken not to compact the soil.
DW (raw or treated) was pumped from the relevant tanks at Site 2 to the header tanks once every two
weeks. Each header tank was fitted with a float valve that was adjusted and calibrated to fill to a
volume of 250 L, resulting in a daily application (once per fortnight) of 10 mm. Once full, a tap was
manually opened and the DW allowed to flow by gravity into the gutter distribution system.
Each lysimeter drained to a collection pit via a tipping bucket in the same way as described for the
PSP (see Section 2.4). The floor of each pit was covered with gravel to keep it clean. Each tipping
bucket was connected to the data logger to record when each tip occurred. The lysimeters were also
sown with Lolium perenne. Effluent samples were taken approximately 2 hours after application.
Concurrent samples from the control lysimeter were taken if the drain was flowing.
During the operation of the lysimeters and the PSP system, the grass was periodically cut and
removed. However, no herbage samples were taken for yield or nutrient analysis. Figure 2.6.1 shows
the raw DW lysimeter after grass establishment.
Fig. 2.6.1 The raw DW lysimeter, January 2004.
2.6.2 Laboratory-scale column lysimeters
2.6.2.1
Overall approaches
The diamond lysimeters at Pallinghurst Farm were constructed to determine the ability of that soil to
remove residual BOD5 and other potential pollutants after land spreading. However, the data generated
from these lysimeters was inevitably limited to one soil type. It would have been inaccurate to
59 of 217
extrapolate the results from this soil to other types of soil. Therefore, a smaller scale approach was
adopted to determine the ability of different soils to remove potential pollutants from treated and raw
DW.
2.6.2.2
Excavating intact soil cores
One metre deep cores of soil were obtained, assuming that nutrients and organic matter below 0.9 m
would be leached. Plastic piping, 200 mm diameter, was cut into 1.5m lengths and one end was filed
to give a sharpened edge.
The pipe was then placed, sharpened edge downwards, inside a specially designed steel cylindrical
case, which had a sharpened bottom edge and steel handles to assist pulling it back out of the soil
(Figure 2.6.2). It was essential that the plastic pipe fitted snugly inside the metal case so that the metal
case absorbed the shock of being forced into the soil and not the plastic pipe, which would otherwise
have distorted. A suitable area (i.e. an area that had had no grazing livestock or had grazing livestock
or dung for the past six months) was selected for taking the soil core and the metal case was positioned
upright with sharpened edge on the soil surface. A block of wood supported by steel strapping and
with shaped steel over one end formed a lid that was placed over the end of the metal case. A line was
drawn on the outside of the steel case to mark a depth of 1 m and a tractor mounted impact post driver
was used to force the metal case into the soil to a depth of 1 m.
Fig. 2.6.2 Method of extracting intact soil cores.
Once the metal case, containing the plastic pipe, had reached the required depth an excavator was used
to extract the metal case from the soil by digging out around it and then pushing the metal case over,
before pulling it to the surface, using a rope. If the metal case was dragged vertically out of the soil,
there was more chance of the soil core dropping out as it was lifted. The plastic case containing the
initial soil was then removed from the metal casing.
Complete soil cores were transported to the lab, where soil was removed from some of the cores so
that each core was 1m in length. The cores and plastic pipes were pushed vertically into plastic endcaps filled with drainage beads and with a hole drilled in the bottom of each cap to provide leachate
collection (Figure 2.6.3). The caps were held onto the plastic pipes using couplers sealed with
silicone sealant. Pipes were attached to the drainage holes to transfer leachate into 5L plastic
containers. For the duration of the experiments, the soil cores were placed on purpose built steel
supporting stands inside a cool greenhouse. Temperature measurements were recorded on a daily
basis. The average temperature over the duration of the experiment was xx oC.
60 of 217
Fig. 2.6.3 End cap containing drainage beads and fitted with
a drainage hole.
The five soils were chosen to represent a range in soil textures and pH. They were:
•
•
•
•
•
Coarse, freely draining sandy loam (Crediton series) (CRED1)
Loamy sand over soft sandstone, porous freely draining, neutral to slightly acidic, (Bridgnorth
series) (BRID1)
Porous and freely draining, highly calcareous silty clay loam over chalk (Andover series)
(ANDO1)
Clay loam over culm shale (Halstow series) (HAL1)
Well drained silty clay (Denbigh series) DENB1.
Further details are listed in Table 2.6.1
Table 2.6.1 Soil Texture Analysis
Texture
% sand
% silt
% clay
Crediton
Coarse sandy loam
66
20
14
Bridgnorth
Loamy sand
74
17
9
Andover
Silty clay loam
10
24
66
Halstow
Clay loam
21
46
33
Denbigh
Silty clay
25
52
23
2.6.2.3
Water and DW
Raw and bulk treated DW was obtained from the appropriate tanks at the Pallinghurst Farm site and
transported to IGER in 60 litre containers. The deionised water or DW was applied to the soil columns
by pouring the liquid carefully from a beaker onto the soil surface. Each soil column was covered
with a loosely fitting plastic bag to reduce evaporation.
There were two phases of application of DW. The Phase 1 consisted of 5 mm applications of DW once
per week for 10 weeks. Each week there were two additional applications of simulated rainfall (4 mm
on each occasion). In Phase 2, which continued for 7 weeks, the DW application was increased to 10
mm per week. The simulated rainfall continued in the same manner as in Phase 1.
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2.6.2.4
Water Sample collection and analysis
Leachate samples were collected once per week. Leachate volumes were measured and samples of
leachate and DW applied were analysed for BOD5, as described by Committee of Analysts (1988),
MRP (Murphy and Riley, 1956), NO3--N (Kamphake et. al., 1967) and TAN (Searle, 1984) and
chloride (Cl-). The Cl- was already present in the DW and was not added as a ‘spike’. This was used to
determine the time at which effluent breakthrough occurred in the different soils.
2.6.2.5
Statistical analysis
Data, consisting of BOD5 concentrations, nutrient concentrations and DW volumes, was summarised
using Excel spread sheets. Mass nutrient data was then analysed by Anova using Genstat (Lawes
Agricultural Trust, 1993). The experiments were set up in a replicated design, with 4 replicates for
each soil type. Analysis of variance on % reduction data was analysed using angular transformation as
the use of this analysis was appropriate for % data where values were clustered.
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CHAPTER 3 RESULTS, MODELLING AND DISCUSSION
Chapter authors: Trevor Cumby, Andrew Barker, Colin Burton, David Chadwick, Marc Dresser, Gari
Fernandez, John Gregory, Peter Leeds-Harrison, Ian Muir, Elia Nigro, Ken Smith and Joe Wood
3.1
OVERVIEW COMPARISON (TC, KS, IM, JG)
3.1.1 Characteristics of the DW supplied to the four treatment processes.
3.1.1.1
Comparisons with DW on other farms.
The DW at Pallinghurst Farm provided a rigorous test, with TS and BOD5 values ranging from 300 to
18000 mg/l and from 600 to 12000 mg/l respectively, with substantial variations. Comparison with
previous survey data showed that these characteristics were close to the median values recorded on a
sample of UK dairy farms, and thus confirmed the validity of investigation (Figs 3.1.1 and 3.1.2).
Concentration, mg/l
25000
20000
15000
10000
5000
0
TS
BOD5
MMB survey mean
MMB survey median
Farm DW: Trial 1
Farm DW: Trial 2
Field DW:Trial 1
Field DW: Trial 2
Fig.3.1.1 Comparison of TS and BOD5 data from DW-STOP with previous MMBsponsored survey data (from Cumby, et al 1999)
63 of 217
Concentration, mg/l
2000
1500
1000
500
0
COD / 10
Total N
Amm N
MMB survey mean
Farm DW Trials 1 and 2
Total P
MMB survey median
Field DW Trials 1 and 2
Fig.3.1.2 Comparison of Total N, Amm-N, COD and Total P data from DW-STOP
with previous MMB-sponsored survey data (from Cumby, et al 1999)
3.1.1.2
Seasonal variations in the DW at Pallinghurst Farm and between Sites 1 and 2
Figures 3.1.3 and 3.1.4 compare the TS and BOD5 concentrations of the untreated DW samples at
Sites 1 (farm) and 2 (field), throughout Trials 1 and 2. These data show that considerable variations
occurred from season to season, to the extent that the differences between the highest and lowest
concentrations in both parameters readily exceeded on order of magnitude. This has clear implications
for the specification and design of all DW management systems, and for systems that involve
treatment, this variation imposes an particular need to ensure that the periods of highest demand can
be accommodated, whilst allowing the extent (and hence cost) of the process to be scaled-down during
less demanding periods.
25000
TS mg/l
20000
15000
10000
5000
0
1-Apr-03
18-Oct-03
5-May-04
21-Nov-04
9-Jun-05
date
Farm DW - TS Trial 1 2003
Field DW - TS Trial 1 2003
Farm DW - TS Trial 1 2004
Field DW - TS Trial 1 2004
Farm DW - TS Trial 2 2004
Field DW - TS Trial 2 2004
Fig.3.1.3 Comparison of the variations in the Total Solids (TS) concentration in the
untreated DW in the two supply tanks during Trial 1 and Trial 2
64 of 217
15000
BOD5 mg/l
10000
5000
0
1-Apr-03
18-Oct-03
5-May-04
date
21-Nov-04
9-Jun-05
Farm DW - BOD5 Trial 1 2003
Field DW - BOD5 Trial 1 2003
Farm DW - BOD5 Trial 1 2004
Field DW - BOD5 Trial 1 2004
Farm DW - BOD5 Trial 2 2004
Field DW - BOD5 Trial 2 2004
Fig.3.1.4 Comparison of the variations in the 5-day Biochemical Oxygen Demand
(BOD5) concentration in the untreated DW in the two supply tanks during Trial 1 and
Trial 2
Generally, the results in Figs 3.1.3 and 3.1.4 show that the measures introduced in 2003 to minimise
defences in the composition of the DW between the two experimental were largely effective, although
occasional differences occurred, especially during the winter periods, when some high peak
concentrations were observed. These differences were greatest on connection with the measurement
of TS, and comparison with Fig. 3.1.5, which presents corresponding TSS data from Trial 2, shows
similar trends, (TSS was not measured during Trial 1). In particular, some high TSS and TS values
were observed occasionally in the samples from Site 2 (field). The TSS data suggested that these
might have reflected sampling errors that derived from the limited mixing capacity available at this
Site (see Section 2.1.3.2). Accordingly, a revised sampling procedure introduced early in 2005,
which increased the duration of mixing before sampling, helped to reduce the incidence of sampling
errors during the latter part of Trial 2.
20000
TSS mg/l
15000
10000
5000
0
22-Jun-04
31-Aug-04
9-Nov-04
18-Jan-05
29-Mar-05
7-Jun-05
date
Farm DW - TSS Trial 2 2004
Field DW - TSS Trial 2 2004
Fig.3.1.5 Comparison of the variations in the total suspended solids (TSS)
concentration in the untreated DW in the two supply tanks during Trial 1 and Trial 2
Besides the variations in DW composition shown in Figs 3.1.3 – 3.1.5, all of the treatment systems
were subject to natural variations in ambient temperatures, as shown in Figs 3.1.6 and 3.1.7 for Trials
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1 and 2 respectively. In each case, the temperatures ranged between winter minima of about -5 oC
and summer maxima of between 30 and 35 oC.
40
35
Daily Minimum
Daily Maximum
Daily Average
30
Temperature, oC
25
20
15
10
5
0
-5
09-May-03 20-Jun-03
01-Aug-03 12-Sep-03 24-Oct-03
05-Dec-03 16-Jan-04
27-Feb-04
09-Apr-04
Fig.3.1.6 Variations in daily minimum, maximum and average ambient temperatures
during Trial 1, recorded at Site 1
40
35
Daily Minimum
Daily Maximum
Daily Average
25
o
Temperature, C
30
20
15
10
5
0
-5
12-Jul-04
23-Aug-04
04-Oct-04
15-Nov-04
27-Dec-04
07-Feb-05
21-Mar-05
02-May-05
Fig.3.1.7 Variations in daily minimum, maximum and average ambient temperatures
during Trial 2, recorded at Site 1
3.1.2 Comparison of system performance – Total Solids (TS) concentration
Figure 3.1.8 compares system performance based on the ratio of the output TS concentrations to the
input TS concentrations from Trials 1 and 2. In view of the ways in which the systems periodically
worked in combinations (as detailed in Tables 2.1.1 and 2.1.2 and 2.1.5), the performance of the IAP
is expressed using data both for the whole system, and for each of its separate biological treatment
stages, the CSTR and the HRTF. The ratio values for the former are based on the TS concentrations in
the untreated DW and the effluent leaving the CSTR. For the latter, the values refer to the ratio of the
concentrations in the HRTF effluent to those of the CSTR effluent. This reflects their sequence of
66 of 217
operation, and in this way, the product of the two ratios is approximately equal to the overall IAP
performance ratio. This relationship is approximate due to the statistical variability reflected by the
error bars in Fig. 3.1.8. The ratios for the DRB during Trial 2 are calculated in a similar way to those
of the HRTF, since this also received effluent from the CSTR as its source of DW. Thus, the Trial 2
ratio values for these systems may be compared directly.
Data from the OFP system include an indication of its performance whilst operating with a two-week
treatment cycle. All data were calculated using the untreated DW values obtained one week before the
treated DW values, to reflect the weekly recharge mode of operation. Since the PSP system was
converted to a lysimeter at the start of Trial 2, its performance data relate to only Trial 1.
The results showed that the OFP system achieved the largest reduction in TS concentration, especially
during the two-week treatment cycles, followed by the IAP. The CSTR achieved a ratio of 69%.
However, since the DRB recorded lower output concentrations than the HRTF (i.e. respective ratio
values of 85% and 104%), it follows that the overall ratio for the combination of the CSTR and the
DRB was (69% * 85% = 59%), and was therefore better than the CSTR and HRTF combination
(measured combined ratio = 67%).
160%
140%
Output TS (%)
120%
100%
80%
60%
40%
20%
0%
CSTR
HRTF
IAP
Reed bed
Trial 1
PERC
OVERLAND OVERLAND
(one week)
(two week)
Trial 2
Fig. 3.1.8 Comparison of system performance expressed as mean values of the ratios
of output to input TS concentrations. Error bars represent +/- one std deviation
Figure 3.1.9 compares system performance based on the cumulative amounts of solid matter (TS)
removed by each system during Trials 1 and 2. Since the DRB was loaded more lightly than the other
systems, its recorded performance lagged behind the others. The decrease in the slope of the line
representing the OFP system shows the reduced rate of TS removal associated with the change to a
two-week treatment cycle. Occasionally, the latter system leached solid matter, as shown by the small
falls in cumulative removal of TS.
67 of 217
1400
Cumulative TS removed, kg
1200
1000
800
600
400
200
0
1-Apr-03
18-Oct-03
5-May-04
21-Nov-04
9-Jun-05
Date
IAP data trial 1
OVERLAND data trial 1
Reed bed data trial 2
IAP data trial 2
OVERLAND data trial 2
PERC data
Reed bed data trial 1
Fig. 3.1.9 Cumulative mass of TS removed
3.1.3 Comparison of system performance- 5-day Biochemical Oxygen Demand (BOD5)
The comparisons made in Figs 3.1.8 and 3.1.9 are echoed in Figs 3.1.10 and 3.1.11, which use the
same style of presentation to show the effects of the four systems on BOD5.
80%
Output BOD (%)
60%
40%
20%
0%
CSTR
HRTF
IAP
Reed bed
Trial 1
PERC
OVERLAND OVERLAND
(one week) (two week)
Trial 2
Fig. 3.1.10 Comparison of system performance expressed as mean values of the ratios
of output to input BOD5 concentrations. Error bars represent +/- one std deviation
These results show that the IAP (i.e. CSTR + HRTF) was the most effective system for reducing the
BOD5 concentration, and that the great majority of the BOD5 removal occurred in the CSTR.
However, the DRB performance almost matched that of the HRTF, and so the CSTR + DRB
combination was almost as good, with an overall ratio of 3%, compared with 2.1% for the IAP
combination. Although the sample size was small, the overall performance of the OFP system
operating with a two-week cycle was also effective. However, it must be noted that the treated effluent
68 of 217
from this system was subject to considerable dilution with rainfall during the reported period of twoweek operation. This effect is evident from Fig. 3.1.11, which shows that the overall BOD5 mass
removal rate of the OFP system was reduced during this period.
Figure 3.1.11 shows that the IAP curve had a steeper gradient during Trial 2 than in Trial 1, reflecting
the increased aeration capacity described in Table 2.1.4.
This slope was also sensitive to the
concentration of the DW supplied to the CSTR. Higher concentrations increased the slope because of
the IAP control system, which matched aeration effort to demand (as detected by Redox concentration
in the CSTR).
Cumulative BOD 5 removed, kg
1000
800
600
400
200
0
1-Apr-03
18-Oct-03
5-May-04
21-Nov-04
9-Jun-05
Date
IAP data trial 1
IAP data trial 2
Reed bed data trial 1
PERC data
OVERLAND data trial 1
OVERLAND data trial 2
Reed bed data trial 2
Fig. 3.1.11 Cumulative mass of BOD5 removed
3.1.4 Comparison of system performance - Total Suspended Solids (TSS) concentrations
System performance levels during Trial 2 are compared in Fig. 3.1.12, showing that the OFP system,
recorded the biggest percentage reductions in TSS. The IAP (i.e. CSTR + HRTF) was almost as good,
although the specific values for both the CSTR and the HRTF were subject to large error bars. These
arose from two specific observation of TSS:
•
•
8/3/05: the TSS in the outflow from the CSTR was observed to be over 34 times that in the
input flow (compared with an average of 43% obtained from the other observations), and
6/11/04: the TSS in the outflow from the HRTF was over 4.6 times that in the corresponding
output from the CSTR (compared with an average of 65% obtained from the other
observations).
The results also show that the DRB performed better than the HRTF, so that the combined ratio of the
CSTR + DRB was marginally better than that of the IAP.
69 of 217
Output TSS (%)
150%
100%
50%
0%
CSTR
HRTF
IAP
Reed bed
Trial 1
PERC
OVERLAND OVERLAND
(one week) (two week)
Trial 2
Fig. 3.1.12 Comparison of system performance expressed as mean values of the ratios
of output to input TSS concentrations. Error bars represent +/- one std deviation
3.1.5 Comparison of system performance - Nitrogen, Phosphorus and Chemical Oxygen
Demand
The eight intensive monitoring periods (IMPs) detailed in Tables 2.1.1 and 2.1.5 enabled several
properties to be measured that were not possible during the regular weekly monitoring operations.
These included Total (Kjeldahl) Nitrogen (TN); Total Ammoniacal Nitrogen (TAN); Total
Phosphorus (TP) and chemical oxygen demand (COD). Combined results for each of these, for all of
the treatments are illustrated in Figs 3.1.13 – 3.1.16. However, since the duration of each IMP was
one week, it was not possible to determine the specific two-week performance of the OFP system.
Figures 3.1.13 and 3.1.14 show that the IAP (i.e. CSTR +HRTF) achieved the biggest reductions in
both TN and TAN. (NB the TN values included the corresponding TAN values). Much of this
nitrogen removal was achieved in the HRTF, which embodied technology previously known to be
effective for nitrification of ammoniacal compounds. However, it is important to note that the values
for the IAP presented in Figs 3.1.13 and 3.1.14 exclude data from the first IMP because this took place
before the nitrification process had become established in the IAP. It is interesting to compare the
HRTF performance with that of the DRB system, which shows that the former produced the greatest
degree of nitrification.
70 of 217
100%
Output Total N (%)
75%
50%
25%
0%
CSTR
HRTF
IAP
Reed bed
PERC
OVERLAND
-25%
Fig. 3.1.13 Comparison of system performance observed during the eight intensive
monitoring periods (IMP), including Trials 1 and 2, expressed as mean values of the
ratios of output to input total nitrogen concentrations. IAP data from the first
intensive monitoring period are omitted since the nitrification process had not begun
at that time. Error bars represent +/- one std deviation
100%
Output Ammoniacal N (%)
75%
50%
25%
0%
CSTR
HRTF
IAP
Reed bed
PERC
OVERLAND
-25%
-50%
Fig. 3.1.14 Comparison of system performance observed during the eight intensive
monitoring periods (IMP), including Trials 1 and 2, expressed as mean values of the
ratios of output to input ammoniacal nitrogen concentrations. IAP data from the
first intensive monitoring period are omitted since the nitrification process had not
begun at that time. Error bars represent +/- one std deviation
Figure 3.1.15 shows a close similarity in the performance of the OFP system and the IAP, in removing
phosphorus but is also interesting to note that the DRB performed slightly better than the HRTF.
71 of 217
100%
Output Total P (%)
75%
50%
25%
0%
CSTR
HRTF
IAP
Reed bed
PERC
OVERLAND
-25%
Fig. 3.1.15 Comparison of system performance observed during the eight intensive
monitoring periods (IMP), including Trials 1 and 2, expressed as mean values of the
ratios of output to input total phosphorus concentrations. Error bars represent +/- one
std deviation
In keeping with the results for removal of BOD5 shown in Fig. 3.1.10, the IAP was also the most
effective system for removal of COD, as shown in Fig. 3.1.16. However, in this case, the DRB
appeared to achieve a slightly greater reduction than the HRTF.
100%
Output COD (%)
75%
50%
25%
0%
CSTR
HRTF
IAP
Reed bed
PERC
OVERLAND
Fig. 3.1.16 Comparison of system performance observed during the eight intensive
monitoring periods (IMP), including Trials 1 and 2, expressed as mean values of the
ratios of output to input ratios of output to input chemical oxygen demand (COD)
concentrations. Error bars represent +/- one std deviation
72 of 217
3.1.6 Comparison of system performance - during the intensive monitoring periods: Total
thermotolerant coliforms and other properties
During the four intensive monitoring periods of Trial 1 plus the first one of Trial 2, the concentrations
of total thermotolerant coliforms (TTCs) were determined daily in the two supply tanks and in the
flows of treated effluent from the four treatment processes. The resulting data are summarised in Fig.
3.1.17., showing that the IAP achieve the largest mean reduction in numbers of TTCs, although since
the reduction factor was less than 102, this was judged to be insufficient to claim any degree of
effective pathogen inactivation.
Mean log(10) reduction in TTC
concentrations
3
2
1
0
IAP
Reed bed
PERC
Overland
Overland
(end point
only)
Combined
treated
-1
Fig. 3.1.17 Comparison of changes in the concentrations of total
thermotolerant coliforms observed during the eight intensive monitoring periods
(IMP), including Trials 1 and 2. Error bars represent +/- one std deviation.
3.1.7 Comparison of visual appearance of untreated DW
four treatment processes
and treated effluent from each of the
Examples of the photographic records of sample appearance are presented in Figs 3.1.18 - 3.1.23, in
pairs. In each case, two views are presented, showing firstly, the freshly collected samples (Figs
3.1.18, 3.1.20 and 3.1.22), and secondly, the same sample after being left undisturbed for at least 7
days (Figs 3.1.19, 3.1.21 and 3.1.23). These views represent the appearance of both the untreated
(“raw”) and treated DW, at both Site 1 (“Farm”) and Site 2 (“Field”). The sampling dates represented
include one during Trial 1 (13/1/04, Figs 3.1.18 - 3.1.19), and two during Trial 2 (28/9/04 and 19/4/05,
Figs 3.1.20 - 3.1.21 and Figs 3.1.22 - 3.1.23 respectively). These photographs are provided as
examples of the more comprehensive record obtained. Space precludes the inclusion of the full
photographic record in this report, but full the full set of data are included Elsewhere (Cumby et al,
2005).
As described above (Section 2.1.4.2), the samples were photographed in front of a light source,
including an opaque band to indicate sample turbidity (through loss of contrast). Therefore, turbidity
could be assessed in each case by comparison with the sample container at the right hand end of the
set of containers. Throughout all of the photographs, this contained clean tap water.
The turbidity comparison shows that on all three of the sampling dates, the treatment processes
produced at least some improvements in the appearance of the DW, although there was much
variability in the extent of this. There was also variability between the untreated DW samples from
the two sites, as reflected in the corresponding BOD5, TS and TSS (Trial 2 only) data.
73 of 217
Farm Raw Field Raw
3690
1120
9630
4490
IAP
477
5410
Reed
653
3890
Overland
1020
3070
PERC
2
934
Treated
193
BOD mg/l
2010
TS mg/l
Fig. 3.1.18 Appearance and biochemical properties of fresh samples collected and
photographed on 13 January 2004
Farm Raw Field Raw
3690
1120
9630
4490
IAP
477
5410
Reed
653
3890
Overland
1020
3070
PERC
2
934
Treated
193
BOD mg/l
2010
TS mg/l
Fig. 3.1.19 Appearance and biochemical properties of fresh samples collected on 13
January 2004 and photographed on 20 January 2004
In most cases, the seven day settlement period between the “fresh” and “settled” observations
produced some improvements in clarity. However, there were some exceptions to this, e.g. the
untreated DW at Site 1 (“Farm Raw”), which was also the most concentrated sample.
Figures 3.1.20 - 3.1.23 show that the treated DW samples obtained during Trial 2 were of generally
better appearance than those from Trial 1, thus indicating that most of the processes were operating
more effectively than during the earlier period. However, this observation must be tempered by the
higher BOD5 and TS concentrations at the earlier date.
74 of 217
Farm
Field Raw
Raw
(21/9/04)
(21/9/04)
1370
518
1880
3080
5210
7870
IAP
Reed
Overland
3
72
3160
2
66
2230
44
208
3470
PERC
Treated
19
134
2900
BOD
TSS
TS
mg/l
mg/l
mg/l
Fig. 3.1.20 Appearance and biochemical properties of fresh samples collected and
photographed on 28 September 2004
Farm
Field Raw
Raw
(21/9/04)
(21/9/04)
1370
518
1880
3080
5210
7870
IAP
Reed
Overland
3
72
3160
2
66
2230
44
208
3470
PERC
Treated
19
134
2900
BOD
TSS
TS
mg/l
mg/l
mg/l
Fig. 3.1.21 Appearance and biochemical properties of fresh samples collected on 28
September 2004 and photographed on 5 October 2004
75 of 217
Farm Raw Field Raw
(12/04/05) (12/04/05)
2240
446
1330
760
3980
3710
IAP
Reed
Overland
PERC
Treated
26
204
2790
54
196
2280
10
156
1760
<2
37
1400
23
516
2270
BOD mg/l
TSS mg/l
TS mg/l
Fig. 3.1.22 Appearance and biochemical properties of settled samples collected and
photographed on 19/4/05
Farm Raw Field Raw
(12/04/05) (12/04/05)
2240
446
1330
760
3980
3710
IAP
Reed
Overland
PERC
Treated
26
204
2790
54
196
2280
10
156
1760
<2
37
1400
23
516
2270
BOD mg/l
TSS mg/l
TS mg/l
Fig. 3.1.23 Appearance and biochemical properties of settled samples collected on
19/4/05 and photographed on 26/4/05
76 of 217
3.2
INTENSIVE AERATION PLANT (CoB, TC, EN)
3.2.1 Experimental programme and operation of the IAP
3.2.1.1
Flows and volumes of DW treated
During Trial 1 (May 2003 to April 2004), the IAP treated a nominal throughput of 550 L/d. The plant
operated according to a repeated 2 hour control cycle in which 48.5 L of raw DW was added at the
beginning of each cycle. Effluent passed through the plant as a series of stages, which were operated
according to a control programme as described in Section 2.2.2. Hence, the following amounts of
sludge were removed within each cycle:
•
•
•
First settling tank (T1): 7.5% of the raw DW feed (i.e. 3.6 L/cycle);
Second settling tank (T5) (after the CSTR): 20% of the raw DW feed (i.e. 9.7 L/cycle), but half
of this was returned to the CSTR, leaving a net discharge of 4.9 L/cycle; and
Third settling tank (T9) (after the HRTF): 10% of the raw DW feed (i.e. 4.9 L/cycle), but all of
this was returned to the CSTR.
Thus, the total net volume of sludge removed from the plant was equivalent to 17.5% of the feed
volume (i.e. 3.6 + 4.9 = 8.5 L/cycle). All of the remaining DW flow through the IAP proceeded to the
final effluent discharge point, i.e. no other effluent flows were withdrawn from intermediate stages of
the process.
The IAP was modified before Trial 2 to enable an increased throughput of 800 L/d. This required an
increased aeration capacity compared with Trial 1. Hence, a compressor was fitted in parallel with the
original blower. The latter ran continuously whilst the compressor was controlled by the redox value
of the liquid in the CSTR. As for Trial 1, the redox control value was -50mV Ecal with an observed
cycling around this point when the process was under control, as described in Section 2.2.2. Flows
were also modified compared with Trial 1; the flow of partly treated effluent from the CSTR was
divided into three: 250 L/d to the HRTF, 250 L/d to the DRB system and the remainder was
discharged to the Pallinghurst farm DW system.
3.2.1.2
Redox and temperature
Figures 3.2.1 and 3.2.2 show the redox values of both the CSTR and the sump of the HRTF, which
were measured and logged every minute during both Trials. In addition, the temperatures of ambient
air, raw DW, CSTR contents and HRTF contents were logged every 5 minutes. These data were used
primarily for plant monitoring and no further analysis is included here. At no time did the CSTR
temperature fall below 5 oC and more often it was above 10 oC. Generally, it was 5-10 oC warmer than
the feed, confirming biological activity.
77 of 217
250
Redox value, mV Ecal
150
50
-50
-150
-250
-350
09-May-03
18-Jun-03
28-Jul-03
06-Sep-03
16-Oct-03
25-Nov-03
04-Jan-04
13-Feb-04
24-Mar-04
Fig. 3.2.1 Recorded redox values in the CSTR during Trial 1
150
Redox value, mV Ecal .
50
-50
-150
-250
-350
12-Jul-04
21-Aug-04
30-Sep-04
09-Nov-04
19-Dec-04
28-Jan-05
09-Mar-05
18-Apr-05
28-May-05
Fig. 3.2.2 Recorded redox values in the CSTR during Trial 2
Operation of the plant was also monitored by recording all events such as pumps switching on and off
in a history file. These data were primarily used for diagnosis purposes in the event of malfunction.
Following commissioning, most malfunctions arose from the frequent breaks in mains power supply.
In view of this problem, early in Trial 1 the plant was modified to restart automatically in the event of
a power cut. However, on a few occasions, a combination of events frustrated the protection devices
leaving the plant idle until its routine inspection once a week.
3.2.2 Removal of specific components from DW
3.2.2.1
BOD5 and COD
Reductions in BOD5 and COD concentrations provide the most reliable indicators of the extent of
biological activity in an aerobic system such as the IAP. Since BOD5 concentrations indicate the
amounts of the most reactive organic matter present in DW, it was expected that the IAP would
remove most of the BOD5; Fig. 3.2.3 indeed confirms that this was the case throughout Trials1 and 2.
78 of 217
100000
Raw DW
Treated DW from CSTR
Treated DW from HRTF
BOD 5 mg/litre
10000
1000
100
10
1
09-Apr-03
07-Aug-03
05-Dec-03
03-Apr-04
01-Aug-04
29-Nov-04
29-Mar-05
27-Jul-05
Fig. 3.2.3 BOD5 concentrations in raw DW at site 1 compared with those in treated
DW from the CSTR and HRTF stages of the IAP
The BOD5 concentration of the raw DW at Site 1, as supplied to the IAP, was mainly between 1000
and 5000 mg/L. The unusually high values in the Autumn of 2003, which exceeded 10,000 mg/L,
overstretched the plant leading to a low redox value for a period (see Fig. 3.2.1). Consequently, the
values for the effluent leaving the CSTR varied widely from 50 to 500 mg/l or more (typically range:
100 to 200 mg/l). Despite this, the BOD5 concentration of the treated effluent from the HRTF rarely
rose above 100mg/l and was more typically in the range 10 to 50 mg/l. This prompts an important
observation concerning the relative contribution of the HRTF: when the CSTR was loaded within
normal capacity, there was little requirement for the HRTF to remove BOD5. However, with the CSTR
heavily loaded, the HRTF was able to remove much of the remaining BOD5, so leaving a consistent
effluent and helping to ensure against accidental pollution during periods of peak loading. The higher
throughput of Trial 2 did not greatly change overall performance.
Table 3.2.1 lists the COD data collected during the periods of intensive monitoring, and Table 3.2.2
expresses these as percentages of the corresponding concentrations in the raw DW. Unlike BOD5
concentrations, COD values include the oxygen demands arising from non-biodegradable matter,
which often includes insoluble components. Hence, COD concentrations can be reduced significantly
by physical settlement. Thus, the final treated effluent from the HRTF and its related settlement stage,
improved significantly on the quality of effluent from the CSTR. In particular, the COD
concentrations of the effluent from the CSTR were between 20 and 41 % of that in the raw DW,
compared with a corresponding range from 11 to 27 % in the final effluent from the HRTF stage.
Table 3.2.1 Average values for key analytes during the eight intensive periods of sampling
Trial 1
June 2003
Raw
Treated HRTF
July 2003
Raw
TN
P
NO3-
NO2-
COD
TAN
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
336
179
65
31
4890
745
239
135
277
92
5352
186
79 of 217
pH
Treated HRTF
Sept 2003
Raw
Treated HRTF
Mar 2004
Raw
Treated CSTR
Treated HRTF
Trial 2
Nov 2004
Raw
Treated CSTR
Treated HRTF
Dec 2004
Raw
Treated CSTR
Treated HRTF
Feb 2005
Raw
Treated CSTR
Treated HRTF
April 2005
Raw
Treated CSTR
Treated HRTF
TN
P
NO3-
pH
NO2-
COD
TAN
mg/l
18
mg/l
8
mg/l
20
8.2
mg/l
0
mg/l
611
mg/l
2
743
102
107
47
0
180
7.4
8.4
0
3
8548
1608
600
12
370
192
53
71
42
37
0
0
150
7.2
7.7
7.3
0
0
0
5294
1296
1061
222
122
6
473
110
26
83
42
13
1
165
294
7.3
7.7
8.0
0
12
2
6254
1240
760
281
19
1
1188
588
153
173
37
29
1
27
317
7.1
8.6
8.0
0
0
38
13260
3732
1856
665
376
33
554
215
101
68
43
29
1
124
246
7.6
7.7
7.7
0
2
3
5608
2274
1518
371
77
4
386
146
67
72
51
43
1
172
246
7.2
7.4
7.1
1
0
1
4874
1334
843
228
41
4
Table 3.2.2 Average values for key analytes during the eight intensive periods of sampling
treated expressed as percentages of corresponding concentrations in raw DW (pH expressed as
differences, positive difference implies that the treated DW was more alkaline than the raw DW)
Trial 1
June 2003
Treated HRTF
July 2003
Treated HRTF
Sept 2003
Treated HRTF
Mar 2004
Treated CSTR
Treated HRTF
Trial 2
Nov 2004
Treated CSTR
Treated HRTF
Dec 2004
Treated CSTR
Treated HRTF
Feb 2005
Treated CSTR
TN
%
P
%
53
COD
%
TAN
%
48
15
57
6
9
11
1
14
44
-1.0
19
2
52
14
59
52
-0.5
-0.1
24
20
55
3
23
5
51
16
-0.4
-0.7
20
12
7
1
50
13
21
17
-1.5
-0.9
28
14
57
5
39
64
-0.1
41
21
80 of 217
pH
difference
Treated HRTF
April 2005
Treated CSTR
Treated HRTF
3.2.2.2
TN
%
18
P
%
42
pH
difference
-0.1
COD
%
27
TAN
%
1
38
17
71
59
-0.2
0.1
27
17
18
2
Total suspended solids (TSS)
TSS removal is very much a clarification process and thus largely independent of the biological
processes. There is a link though as microbe activity tends to break down proteins and certain
hydrocarbons that act as surfactants that sustain the suspension. Good activity can thus be expected to
enhance the settlement process. Figure 3.2.4 illustrates the effect of treatment on TSS for Trial 2. An
incremental reduction in the two major stages of the IAP is clearly evident: TSS concentrations in the
raw DW ranging from 2000 to 6000 mg/l were reduced to 500 to 2000mg/L after settling following
the CSTR stage and then to 70 to 700 mg/L after the HRTF stage and it associated settlement tank.
10000
TSS mg/litre
1000
100
Raw DW
10
23-Feb-04
13-May-04
Treated DW from CSTR
01-Aug-04
20-Oct-04
Treated DW from HRTF
08-Jan-05
29-Mar-05
17-Jun-05
Fig. 3.2.4 TSS concentrations in raw DW at site 1 compared with those in treated DW
from the CSTR and HRTF stages of the IAP
3.2.2.3
Total solids (TS)
The characteristics of TS removal tend to combine aspects of both BOD5 and TSS removal. Figure
3.2.5 shows that the results reflect this, with incremental reductions across the CSTR and HRTF, such
that a typical concentration of 5,000 mg/L in the raw DW fell to 2000 mg/L in the final effluent.
However, the TS reductions were less variable that those observed with TSS, and a closer relationship
was apparent between the concentrations in the raw DW and the in final treated effluent. In part, this
was because TS includes soluble components such as salts, which, largely, are not removed by either
settling or biological processes. Table 3.2.3 provides a limited indication of the presence of certain
metal ions (sodium, magnesium and calcium) in the raw DW supplied to the IAP.
81 of 217
100000
TS mg/litre
10000
1000
Raw DW
100
09-Apr-03
07-Aug-03
Treated DW from CSTR
05-Dec-03
03-Apr-04
Treated DW from HRTF
01-Aug-04
29-Nov-04
Sludge
29-Mar-05
27-Jul-05
Fig. 3.2.5 TS concentrations in raw DW at site 1 compared with those in treated DW from the
CSTR and HRTF stages of the IAP, plus those in the sludge discharged from the IAP
Table 3.2.3 Metal ion concentrations measured in
raw (feed) effluent to intensive aeration plant.
Date of
sampling
11 Nov 2004
17 Feb 2005
Na+
mg/l
Mg++
mg/l
Ca++
mg/l
94
94
67
59
186
160
The TS concentration of the sludge (also shown in Fig. 3.2.5) varied between 10,000 and 20,000
mg/L, which is relatively dilute compared with other processes. This suggests that the sludge removal
rate was high, and thus incorporated additional DW in the sludge, with a consequent diluting effect.
Arguably therefore, the sludge removal settings detailed in Section 2.2 could have been reduced,
although from a practical perspective, this would have increased the risks of pump and pipe blockages,
because the removal of extra DW with the sludge ensured that these components were flushed
periodically.
3.2.2.4
Phosphorus (P)
Since much P is contained in insoluble matter, settling and clarification are the key actions in this
process. Thus, P removal can be expected to resemble the characteristics of TSS reduction. However,
Tables 3.2.1 and 3.2.2 show that the removal of P varied widely from a maximum reduction of 91 %
(i.e. 9% remaining in the final effluent) to a minimum of 29% (i.e. 71% remaining). In all cases, the
second HRTF stage contributed to the process. It is known that reductions in pH will increase the
solubility of P compounds in water, and Table 3.2.1 shows that the pH of the final effluent was
generally about 8.0 whilst that of the raw DW was typically 7.5. However, these observed changes in
pH did not correspond nor explain the much greater variability in phosphorus removal.
3.2.2.5
Nitrogen (TAN and TN)
82 of 217
The effects of the IAP processes on TAN were markedly different from those on TN. The latter
effectively represents all nitrogen, including the organic component, which is largely unaffected by
short-term biological treatment. However, this organic component tends to be partly insoluble and
thus it can be removed by settlement. Table 3.2.2 shows that up to 50% of the TN was removed by the
CSTR stage, and that this increased to more than 80% following the HRTF stage.
TAN can be removed from liquid effluents either by stripping (as ammonia gas) or by the nitrification
process. The latter is to be expected in any well controlled aerobic process and the production of
nitrites and nitrates in the intermediate and final streams at concentrations comparable with the TAN
in the raw DW provides evidence of this (Table 3.2.1). The data show that contribution of the CSTR
to nitrification was variable, reflecting changes in the CSTR throughout the two trials; the crucial
microbes for nitrification (Nitrosomonas and Nitrobacter) do not flourish in the presence of a very
active heterotrophic bacterial population. However, as with BOD5 removal, it is evident again that
when nitrification rates in the CSTR were reduced, the HRTF was able to complete the task, producing
a consistently high removal of ammonia with a final concentration often below 10 mg/l.
The nitrates produced in the final effluent can be expected to break down during subsequent storage,
mainly to di-nitrogen (N2) gas and some nitrous oxide. Although project resources did not permit
measurement of this effect, there is a great deal of published work that supports this assumption. This
also supports the subsequent assertion that where nitrification takes place (as in the IAP), most of the
TAN will be removed from the DW prior to land application, (Burton et al, 1993; Greatorex and
Burton, 1995; Gronestein and Faassen, 1996; Willers et al, 1996; Pahl et al, 1997).
3.2.3 Operational factors
3.2.3.1
Redox values
Figures 3.2.1 and 3.2.2 show the progress of the redox values monitored for the CSTR over the course
of the two Trials. For much of the time, the values remained between -100 and +100 mV. The cyclic
operation was an expected feature of the control system used; this had no detrimental effect on the
treatment process and simplified plant operation. However, there were some periods when the redox
value in the CSTR fell below -300mV. This was due to the supply of concentrated raw DW early in
Trial 1 followed by a series of interruptions caused by power cuts. These prevent the plant from reestablishing normal operation
In the middle of Trial 2, the CSTR redox probe became fouled and this was not diagnosed for several
weeks and therefore incorrect low values were logged during this period. Thereafter, routine cleaning
was undertaken. However, despite the variations in the observed redox values, the effect on BOD5 and
other indicators was surprisingly small and no obvious correlation could be found.
3.2.3.2
Redox cyclic trends
Figure 3.2.6 provides more detail of the redox values over a limited period at the end Trial 1 when the
plant was well controlled. The higher values correspond to the redox measured in the HRTF: this
was uncontrolled but was rarely challenged by a heavy BOD5 load. Thus, with an adequate aeration
regime for a small BOD5 load, the redox remained much higher than that for the CSTR, as expected.
Two cyclic patterns are evident, one corresponding to the two-hour feed, the other (presumably) due to
temperature or some other diurnal effect as this is clearly on a 24-hour cycle. These daily variations
may be compared with the concurrent temperature data shown in Fig 3.2.7, which indicate that the
temperature in the CSTR varied less than that in the HRTF, and that the changes that were observed in
the CSTR followed trends observed in the raw DW fed to the CSTR. Temperatures in the HRTF were
generally higher that those in the CSTR, but were affected more by variations in ambient
temperatures.
83 of 217
200
20
HRTF redox potential
Ambient temperature
15
0
10
Redox value, mV Ecal
100
-100
5
-200
0
12
24
36
48
60
72
84
96
Ambient temeprature, oC
CSTR redox potential
0
120
108
Elapsed time (hours)
Fig. 3.2.6 Example of trends in the measured redox values taken over a 5-day period in March
2004. Both 2-hour and 24-hour cycles are evident. The 2-hour cycle reflects the period supply of
raw DW, whilst the 24 hour cycle reflects the system’s response to ambient temperature.
20
CSTR temperature
Feed temperature
HRTF temperature
Ambient temperature
o
Temperature, C
15
10
5
0
0
12
24
36
48
60
72
84
96
108
120
Elapsed time, hours
Fig. 3.2.7 Example of daily temperature variations in the IAP recorded over a 5-day period in
March 2004 .
3.2.3.3
Power consumption
Figure 3.2.8 shows the cumulative electrical power consumed by the IAP over Trials 1 and 2. The
figures include electricity to operate the DRB system and, in winter, to heat the IAP control cabin.
However, most power went to the IAP and the graph gives some indication. The steeper slope in Trial
2 reflects both the higher throughput and the use of a rather inefficient compressor. The latter was
used out of necessity for the experiment, but a better efficiency would have been achieved by using
more diffuser disks in the CSTR and a second side-channel blower instead of the compressor. Overall,
the plant used 39 and 74 kWh/d for Trials 1 and 2 respectively. By deducting and estimated 20% for
non-plant power requirements, this equates to a power consumption of 60 kWh per tonne of effluent
processed.
84 of 217
Electricity used, kWh
65000
55000
45000
35000
25000
09-Apr-03
07-Aug-03
05-Dec-03
03-Apr-04
01-Aug-04
29-Nov-04
29-Mar-05
27-Jul-05
Fig. 3.2.8 Cumulative electricity consumption during Trials 1 and 2. The mean value for Trial 1
was 39 kWh/d (around £2 at 5p per kWh), with a nominal IAP throughput of 550 L/d. The
increase to 800 L/d in Trial 2 is reflected in the higher energy consumption of 74 kWh/d.
3.3
REED BEDS (AnB, GF, JW)
3.3.1 Removal of total solids
Figure 3.3.1 shows the cumulative mass removal of TS in the DRB system. In Modes 1, 3 and 4 the
rate of cumulative removal of TS was significantly higher than in Mode 2, as the slope of the graph
was steeper during those periods. It appears from observations at the site that some TS are removed in
the upper layers of the DRB, but these can lead to blinding of the gravel surface and subsequent
ponding of water and blockage problems. During Mode 2, the TS loading of the raw influent water
was up to 15,200 mg/l, and after mixing with the recycle of treated water, the resulting mixture applied
to the primary DRBs contained up to 5,200 mg/l TS. Despite the reduced flow of raw DWduring
Mode 2, the TS content of the waste applied to the beds was quite high, and possibly led to an
accumulation of sludge in both the DRBs and holding tanks. In turn, the settled solids deposited may
have led to the blinding and blockage of the DRBs.
35
Cumulative removal of Total Solids, kg
30
25
20
15
10
5
0
01-May-03
30-Jul-03
28-Oct-03
26-Jan-04
25-Apr-04
24-Jul-04
22-Oct-04
20-Jan-05
20-Apr-05
Date
Fig. 3.3.1 Cumulative mass of TS removed by the DRB during Trials 1 and 2
85 of 217
Figure 3.3.2 displays the percentage removal of TS in the DRB system overall. Table 3.3.1 shows the
mean and standard deviations of the TS concentrations in the streams connected with the IAP and
DRB. It is observed from Fig. 3.3.2 that the TS removal by the system was very variable and typically
ranged from +70 % to -70 %. The occurrence of apparently negative TS removal corresponded with
instances in which the TS content of the outlet flow was higher than the inlet. This situation could
occur if settled solids previously accumulated in the DRB gravel matrix are sloughed off and thus
released into the flow of treated effluent. Sloughing could be increased by heavy rainfall, periodic
mechanical unblocking the DRBs or by back-flushing during maintenance periods. Overall, the
removal of TS in the DRBs was poorer than expected.
100
Total Solids Removal, %
50
0
-50
-100
-150
-200
01-May-03
09-Aug-03
17-Nov-03
25-Feb-04
04-Jun-04
12-Sep-04
21-Dec-04
31-Mar-05
Date
Fig. 3.3.2 Percentage removal of Total Solids in the DRB during Trials 1 and 2
Table 3.3.1. Mean and standard deviation of BOD5 and TS concentrations in the raw DW at Site 1,
IAP-CSTR effluent, IAP-HRTF effluent, DRB influent after mixing with recycle, and DRB effluent.
µ is the mean value, σ is the standard deviation.
MODE 1
BOD5, mg/l
µ 3347.1
σ 2577.9
-
TS, mg/l
µ 5501.9
σ 2612.4
-
MODE 2
BOD5, mg/l
µ 2650.1
σ 1552.9
-
MODES 3 & 4
TS, mg/l
µ 6775.9
σ 3140.8
-
BOD5, mg/l
TS, mg/l
Raw DW
µ 2762.2
µ 6251.8
σ 3735.2
σ 3001.8
CSTR outlet
µ 135.0
µ 3859.4
σ 147.5
σ 1852.1
HRTF outlet
µ 47.0
µ 2668.7
µ 122.3
µ 4421.4
µ 50.1
µ 3924.0
σ 47.4
σ 1813.6
σ 133.6
σ 1469.2
σ 44.6
σ 1459.6
DRB inlet
µ 21.9
µ 2089.0
µ 393.8
µ 4245.5
µ 96.7
µ 3380.0
after recycle
σ 24.2
σ 1302.9
σ 258.9
σ 709.2
σ 95.6
σ 1226.7
DRB outlet
µ 11.7
µ 1851.0
µ 291.2
µ 4130.5
µ 64.3
µ 3093.6
σ 18.1
σ 1131.8
σ 226.4
σ 731.0
σ 91.2
σ 786.1
Note: The values for DRB influent after mixing with recycle were determined by calculation using the
appropriate influent concentration data together with the relevant recycle flow data and DRB effluent
concentrations.
In order to reduce blocking problems, other researchers have experimented with different grades and
stratification of gravel in the DRB and their effect on the blockage characteristics of the DRBs. For
86 of 217
example, Zhao et al (2004) reported that a possible reduction in blockage of the gravel matrix may be
achieved by using an anti-sized DRB, in which the gravel at the top of the bed is largest, and that at
the bottom of the bed is smallest, in contrast with the grading used in this work. However, the severity
of the blockage problems encountered during this project suggests that some pre-treatment of the
waste is required. This might include installation of a settling tank prior to the DRBs, or the use of a
secondary lagoon in which the raw water can collect and settle-out some of the solids before
application on to the DRB.
3.3.2 Removal of BOD5
3.3.2.1
Mode 1 operation
Table 3.3.1 shows the mean and standard deviations of BOD5 concentrations in the raw DW at Site 1,
IAP-CSTR effluent, IAP- HRTF effluent, DRB inlet after recycle and DRB effluent in all modes of
operation. As Modes 3 and 4 operated with the same recycle ratio, the data have been averaged for
these periods of operation.
In Mode 1, the average removal of BOD5 in the combined IAP and DRB system was 99.6 %. The
removal across the whole DRB plant was 75.1 %, though after allowing for the dilution effect of the
recycle, a removal of 46.6 % in the primary and secondary DRBs occurred. These data show that the
DRB could remove small quantities of BOD5 from the DW that were not previously removed by
aeration. However, the large standard deviations in the measurements indicate the large degree of
variability in both the quality of the raw and treated effluents. The latter is further illustrated in Fig.
3.3.3, which shows the BOD5 concentrations at the outlets of the DRB system and IAP during Mode 1
of operation. The trends of the data showing BOD5 variations were similar for both the IAP and DRB
effluents. It was noted that for the DRB, during May, July and August 2003. the effluent BOD5 was
usually lower than the typical discharge consent limit of 20 mg/l, whereas for the IAP the discharge
limit was only met during about 3 weeks of July. This result suggests that the DRB played a valuable
role in polishing the effluent from the IAP to achieve the discharge limit required. The low BOD5
values measured between July and August 2003 indicated a good quality effluent, which may be
related to the high rate of activity or population of the micro-organisms due to the high summer
temperatures. However, during this season the treated effluent from the IAP-HRTF had a low BOD5
concentration, therefore placing a lower load on the DRB system. Peak BOD5 values were measured
at the beginning of June and September 2003, showing concentrations of up to 170 mg/l for the IAP
and 75 mg/l for the DRB, which did not meet the discharge consent limit.
87 of 217
200
180
160
DRB
IAP
140
BOD, mg/l
120
100
80
60
40
20
0
01-May-03
21-May-03
10-Jun-03
30-Jun-03
20-Jul-03
09-Aug-03
29-Aug-03
18-Sep-03
Date
Fig. 3.3.3 Comparison of BOD5 concentrations at the outlet of the IAP and DRB
operating in Mode 1
Under steady ideal conditions, the IAP reduced the BOD5 concentrations to around 30-50 mg/l and the
DRB reduced it further to 5-15 mg/l, within the consent discharge limit of 20 mg/l. This suggests that
for BOD5 removal, the IAP followed by DRB treatment as a polishing step offers a potentially
satisfactory solution to the problem of DW treatment.
3.3.2.2
Mode 2 operation
Figure 3.3.4 shows the BOD5 at the outlet of the DRB and IAP in Mode 2, between October 2003 and
February 2004. This was the only period when the DRB feed was not pre-aerated. A small amount of
raw DW (50 litres) was diluted with 1100 litres of recycled effluent. The average removal of BOD5 in
the DRB system during Mode 2 was 89 %, though again the quality of the raw DW and treated
effluent varied widely, as indicated by the standard deviations reported in Table 3.3.1.
1000
900
DRB
IAP
800
BOD5, mg/l
700
600
500
400
300
200
100
0
27-Aug-03
26-Sep-03
26-Oct-03
25-Nov-03
25-Dec-03
24-Jan-04
23-Feb-04
24-Mar-04
Date
Fig. 3.3.4 Comparison of BOD5 concentrations at the outlet of the IAP and DRB
operating in Mode 2
88 of 217
Although it was expected that the DRB should be capable of dealing with such diluted input, the
gathered results showed quite high BOD5 concentrations in the DRB outlet, especially towards
January 2004, when these reached a peak of 850 mg/l. Clearly, this level of organic matter in the
discharge was unacceptable.
Possible reasons for the high values of BOD5 at the DRB outlet during Mode 2 included the following:
•
•
•
•
Very high solid loading imposed by receiving raw DW, which caused both a significant
reduction in the flow of effluent through the first set of DRBs and blockages in the outlet ports
from all DRBs.
Lower rate of BOD5 removal at low temperatures.
Seasonal die back of the reeds leading to lower oxygenation rates in the beds in winter
compared with during lush growth of summer.
Occasional freezing of pipes and beds, reducing flow through the system and treatment
efficiency
The above factors were compounded by the fact that the quantity and composition of dairy farm DW
varied significantly throughout the year, winter being the critical period, when cattle were mainly kept
in the dairy yards, leading to larger quantities of manure than in summer. It was during this period
when the daily production of 27 m3/day showed the highest TS concentration of the year, (15,200
mg/l, on 16/12/03) illustrating that in some instances the DW at Pallinghurst Farm contained an
exceptionally high amount of TS and thus resembled slurry on these occasions. The high TS content of
the raw DW was thought to be the most likely reason for the malfunctioning and blockage of the beds
during Mode 2.
As shown also in Figure 3.3.4, the IAP showed a slightly better performance in terms of effluent
quality compared with the DRB, but still a maximum outlet BOD5 of 500 mg/l from the IAP was
recorded in January 2004.
3.3.2.3
Modes 3 and 4 operation
During Mode 3, the overall DRB throughput of influent water mixed with recycled effluent was
reduced to 800 l/d. This corresponded to a flux of 1 m3m-2d-1, and a corresponding drainage rate of 0.7
l/min from a DRB with a surface area of 1 m2. These were the recommended design figures from
ARM Ltd. Once the DRBs were cleared of accumulated settled solids, the measured drainage rate was
close to the recommended values given above.
Figure 3.3.5 shows the outlet BOD5 concentrations from the DRB, the IAP-CSTR and the IAP-HRTF
during Modes 3 and 4. Two values are given for the IAP since, in effect, the DRB and IAP-HRTF
operated in parallel, with the IAP-CSTR supplying both. From July – September 2004 the discharge
from the DRB system and the IAP-HRTF were mainly within the discharge consent limit of 20 mg/l,
as also observed during Mode 1. Towards the winter of 2004, the outlet BOD5 concentrations from
the IAP-CSTR, IAP-HRTF and the DRB increased substantially. For example, in early January 2004,
the DRB outlet reached a BOD5 concentration of 220 mg/l. It was also observed that the IAP-CSTR
and DRB showed some unusual fluctuations in BOD5; some particularly high isolated values of BOD5
were recorded around the period 8- 15 March 2005. However, these could be related to
unrepresentative samples, obtained when the tanks were low in volume or the sediment at the bottom
had been disturbed.
89 of 217
1000
800
BOD5 , mg/l
DRB
600
IAP-CSTR
IAP-HRTF
400
200
0
12-Jun-04
01-Aug-04
20-Sep-04
09-Nov-04
29-Dec-04
17-Feb-05
08-Apr-05
28-May-05
17-Jul-05
Date
Fig. 3.3.5 Comparison of BOD5 concentrations at the outlet of the IAP and DRB
operating in Mode 3
Operation of the DRB system with effluent that had received only single stage pre-treatment in the
IAP-CSTR generally showed removal performance between that of Modes 1 and 2, when dual stage
aeration and no aeration were respectively used. When operating with single stage aeration, the DRB
was effective as a polishing step during the summer months in reducing the BOD5 concentration of the
effluent to within the discharge consent limit. However, when highly loaded with TS and
concentrated effluent during the winter, it showed a poorer performance, which again failed to meet
the discharge consent limit. This suggests that single stage aeration is acceptable for less concentrated
wastes of low volumes, but heavy sludges require dual stage aeration before application on to the DRB
system.
Previous studies (Sun et al., 1998) of DRB hydraulics and pollutant removal suggested that greater
removals were observed if smaller amounts of strong wastewater were applied into the beds.
Consequently, significant reductions in both BOD5 and COD concentrations at the DRB outlet were
expected for operation Mode 4. The data in Figure 3.3.5 after 9/3/05 show the BOD5 outlet
concentrations during operation Mode 4. It was during this period that a large isolated peak of 650
mg/l in the BOD5 at the DRB outlet occurred on 15/3/05. This was during the period when the DRB
was settling back into steady operation following the changes made to the operating sequence, and as
already mentioned, may have been an unrepresentative sample. In the following period up to the end
of the data set, the BOD5 of the DRB outlet BOD5 was always lower than 150 mg/l and fell as low as
the consent discharge limit of 20 mg/l on 10/5/05. Overall, the results from Mode 4 of operation did
not show consistently better quality effluent than Mode 3, because of the change in timer sequence.
3.3.3 Removal of Suspended Solids
Figure 3.3.6 shows the TSS concentration in the raw DW and the outlets of the IAP and DRB system.
The raw DW contained up to 6000 mg/l TSS, which is consistent with other observed high levels of
TSS; Sun et al (1999) reported that DW can contain up to 8,000 mg/l TSS. It was observed that the
effluent from the IAP-CSTR showed some isolated incidences of unusually high TSS concentrations.
In four instances, these values were substantially higher than the corresponding TSS concentrations in
the raw DW. (These comparisons were between each sample of raw DW and the treated effluent from
the IAP-CSTR sampled during the following week, thus allowing for hydraulic residence time effects
in the CSTR). These high TSS values may have been due to the re-entrainment of previously settled
solids. During the early period of recording TSS concentrations, between June and November 2004,
90 of 217
the values at the DRB outlet were consistently lower than 200 mg/l. However, from November 2004
to March 2005, the values were typically around 350 mg/l, and occasionally rose as high as 2800 mg/l.
10000
Total Supended Solids (TSS), mg/l
8000
DRB
IAP-CSTR
RAW
IAP-HRTF
6000
4000
2000
0
01-Jul-04
30-Aug-04
29-Oct-04
28-Dec-04
26-Feb-05
27-Apr-05
Date
Fig. 3.3.6 Variations in the concentrations of total suspended solids in the raw DW at Site 1, and in
the outlet streams from the IAP-CSTR, IAP-HRTF and the DRB
The average removal of TSS in the IAP-CSTR was 43.9 %; the average removal in the DRB system
was a further 44.7 %, compared with 26.5 % removal in the IAP-HRTF. This analysis of results
indicates that the DRB performed favourably compared with the IAP-HRTF. Further improvements in
solids removal could be achieved by the installation of a second lagoon or a series of settling tanks, in
which to settle out solids before applying the water to the biological treatment systems.
3.3.4 Removal of nitrogen, phosphorus, ammonia and COD
3.3.4.1
Ammoniacal nitrogen and total nitrogen
Figures 3.3.7 and 3.3.8 show the measured concentrations of TAN and TN respectively. Comparison
of the data in Figure 3.3.7 shows that the removal of TAN in the DRB system is generally very
effective, indeed vertical flow DRBs are known to be effective for nitrification of ammonia (Cooper et
al, 1997; Morris and Herbert, 1997). The intensive monitoring periods represented in Figs 3..3.7 and
3..3.8 correspond to the following modes of DRB operation: MODE 1, June – Sep 03; MODE 3, Mar
04 – Feb 05; and MODE 4, Apr 05.
91 of 217
900
800
Raw
Total Ammoniacal Nitrogen, mg/l
700
DRB
600
IAP-HRTF
500
400
300
200
100
0
June 03
July 03
Sep 03
Mar 04
Nov-04
Dec-04
Feb-05
Apr-05
Date
Fig. 3.3.7 Removal of TAN in the DRB, and the IAP-HRTF.
1.4
Raw
1.2
DRB
Total Nitrogen, g/l
1
IAP-HRTF
0.8
0.6
0.4
0.2
0
June 03
July 03
Sep 03
Mar 04
Nov-04
Dec-04
Feb-05
Apr-05
Date
Fig. 3.3.8 Removal of TN in the DRB, and the IAP-HRTF
The nitrification process is more efficient once most of the BOD5 has been removed from the water,
thus explaining the high removals of ammonia observed in Mode 1 of operation. Recirculation has
been shown to improve the NH4-N nitrification (Sun et al, 2003). Connolly et al (2004) found that
NH4-N was removed by a two-stage process of adsorption on to the DRB media followed by
nitrification into NO3-N and NO2-N. It was observed from Figure 3.3.8 that TN removal in the DRB
was initially very effective, but nitrogen levels in the treated effluent generally increased through the
project, and were particularly high in December 2004. The increase may be a result of build up of
nitrogen-containing material in the bed matrix, perhaps through nitrification of TAN to produce NO3N and NO2-N. In addition, in winter months, the die back of reeds could lead to lower TAN and TN
removals due to the slowing down of the nitrification process.
92 of 217
3.3.4.2
Phosphorus
Figure 3.3.9 shows the measured phosphorus concentrations corresponding with the TAN and TN
values shown above. The intensive monitoring periods represented in Figs 3.3.9 correspond to the
following modes of DRB operation: MODE 1, June – Sep 03; MODE 3, Mar 04 – Feb 05; and MODE
4, Apr 05.
Total Phosphorous, mg/l
200
180
Raw
160
DRB
140
IAP-HRTF
120
100
80
60
40
20
0
June 03
July 03
Sep 03
Mar 04
Nov-04
Dec-04
Feb-05
Apr-05
Date
Fig. 3.3.9 Removal of Total Phosphorus in the DRB, and the IAP-HRTF
Initially, the phosphorus concentration detected at the DRB outlet was very low, but increased
generally through the period of operation, with a peak in November 2004. Phosphorus is taken up by
the reeds and adsorbed on to the gravel matrix (Headley et al, 2003). It may be expected that
phosphorus removal is a breakthrough process whereby the DRB eventually becomes saturated to
capacity with phosphorus, explaining the rise in outlet phosphorus concentration with age of the plant.
It should also be noted that the DRBs used in this study were previously used on a different site, such
that their capacity to adsorb phosphorus may have been lower than that of a completely new system.
Previous studies suggest that the lifetime of a DRB may be limited by its capacity to adsorb
phosphorus, and may need to be regenerated after 5 – 25 years by washing the gravel matrix and
replanting (Cooper, 2005).
3.3.4.3
Chemical oxygen demand
Figure 3.3.10 shows the COD concentrations for the raw DW at Site 1, plus the corresponding IAP
and DRB effluents. It was observed that, compared with the COD of the raw DW, the COD
concentrations of the DRB and IAP effluents were much lower, the IAP showing an average removal
of 84 % and the DRB effluent an average removal of 86 % (compared with the raw effluent).
Therefore, on average, the DRB removed only 2 % of the COD in the raw DW COD, which is a little
disappointing.
93 of 217
14000
12000
Raw
COD, mg/)
10000
DRB
IAP-HRTF
8000
6000
4000
2000
0
June 03
July 03
Sep 03
Mar 04
Nov-04
Dec-04
Feb-05
Apr-05
Date
Fig. 3.3.10 Removal of COD in the DRB, and the IAP-HRTF
From Fig. 3.3.10, it was observed that in the earlier months of operation from June 2003 to September
2003, the COD from the DRB was always lower than the COD in the aeration plant effluent, which
was as expected for Mode 1, when the DRB was polishing the effluent from the HRTF stage of the
IAP. In subsequent months, the graph provides a comparison between the performance of the DRB
and the HRTF as polishing steps. In December 2004 and April 2005 the effluent of the DRB was
higher in COD than the effluent of the HRTF, showing a poorer removal. But during the other
remaining months, the DRB showed an equal or slightly higher removal than the HRTF for COD.
3.3.5 Treatment efficiency based on additional sampling by University of Birmingham
The objective of this additional sampling was to determine the treatment efficiency of the primary and
secondary DRBs specifically, rather than the overall treatment efficiency of the two combined stages
as measured by project general sampling and analysis strategy.
Regular visits to Pallinghurst Farm provided the opportunity to collect further samples from
intermediate tanks 2 and 3. The samples analysed for BOD5 concentration provided the necessary data
to calculate the individual efficiency of the primary and secondary DRBs, rather than the treatment
system as a whole. Since the raw DW was obtained from the normal dairy operations at Pallinghurst
Farm, the BOD5 measurements were subject to more variation and uncertainty than if an “artificial
source” of wastewater had been used.
A total of 10 visits were made, during which samples were collected according to the procedure
described in Section 2.3.2.4, and subsequently analysed at the University of Birmingham for BOD5.
Five sets of results were gathered in 2004 and the other five were obtained during 2005. The BOD5
results obtained are presented in Tables 3.3.2 and 3.3.3, in chronological order; therefore allowing
observation of the system response to both different operation modes and seasonal conditions.
Table 3.3.2 Results from University of Birmingham BOD5 analysis 2004
SAMPLING
POINT
14/07/04
26/08/04
21/09/04
26/10/04
23/11/04
BOD5
(mg/L)
BOD5
(mg/L)
BOD5
(mg/L)
BOD5
(mg/L)
BOD5
(mg/L)
94 of 217
SAMPLING
POINT
T1
T2
T3
T4
14/07/04
26/08/04
21/09/04
26/10/04
23/11/04
BOD5
(mg/L)
163
110
76
88
BOD5
(mg/L)
246
169
110
122
BOD5
(mg/L)
289
109
184
174
BOD5
(mg/L)
429
174
239
99
BOD5
(mg/L)
169
167
161
78
Table 3.3.3 Results from University of Birmingham BOD5 analysis 2005
SAMPLING
POINT
T1
T2
T3
T4
09/03/05
12/04/05
26/04/05
03/05/05
10/05/05
BOD5
(mg/L)
435
279
138
135
BOD5
(mg/L)
355
271
231
214
BOD5
(mg/L)
269
176
404
131
BOD5
(mg/L)
176
96
125
80
BOD5
(mg/L)
275
136
181
132
In general, the trend in values following the route of the water flow from tank 1 to tank 4 displayed a
progressive reduction in BOD5 concentrations. However, as expected, some anomalies were observed
due to the generally high level of variability in the recorded data. For example, it was noted that on
the 14/07/04 the BOD5 in tank 3 was 12 mg/l lower than the one recorded in tank 4. The same trend
was found in the samples taken on the 26/08/04, where the dairy water in tank 3 was 60 mg/l lower in
concentration than the fully treated one. Sampling from tanks filled to different levels could influence
the results, as suspended solids were more likely to be entrained in the sample if it was taken whilst
the tank was nearly empty than if it was full. It was also noted that changes in the composition of the
influent DW could take several days to show up later in the system, due to the long residence times
involved. Therefore, it is quite possible that if the BOD5 of the influent DW decreased significantly,
tanks 2, 3 or 4 may contain stronger waste than tank 1, even though they have been diluted with
recycle and, in the case of tanks 3 and 4, undergone some treatment.
The data collected on 26/4/05 show such an anomaly, in that the concentration in tank 3 was higher
than even the raw effluent in tank 1. If the result from 26/4/05 is excluded from the data set, over the
whole set of results from 2004 and 2005 the average BOD5 removal in the primary DRBs was 4.3 %,
whilst the average removal in the secondary beds was 22.3 %. However, due to the large amount of
variability in the data, these figures should not be over interpreted to suggest that the secondary DRBs
are much more efficient than the first set. Nevertheless, it may be concluded that on some occasions a
significant removal of 20 – 50 % per stage may occur in the primary or secondary beds. Blockages
caused by the high TSS loadings, variability of the raw wastewater quality and other environmental
factors, meant that this performance was not maintained consistently.
The variability of system performance is related to the issue of DRBs in exposed locations, where they
are affected by weather variations and other factors. Low temperatures, for instance, may provoke two
problems: slow down any bacterial activity and freezing episodes that may block pipework and outlet
ports. Additionally, rainfall occurrences may dilute the organic matter, which may become evident
through a lower BOD5 measurement than expected.
It is commonly assumed that organic pollutants, expressed mainly as BOD5, are retained in the DRBs,
being aerobically decomposed by micro organisms during the intervals between intermittent feeding
when oxygen can diffuse into the beds more easily. Previous research studies focused on the
agricultural wastewater treatment in DRBs found removals of around 74.3 % for BOD5 and of 53 % in
case of the COD (Sun, 1998). In the current case, if BOD5 concentrations of both tank 1 and tank 4
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are considered, an average removal of 53.34 % was observed, suggesting that the performance of the
DRB in this project was somewhat poorer than would normally be expected from a typical system.
3.4
PERCOLATION SOIL PLOT SYSTEM (DCh)
3.4.1 Removal of BOD5 and TS
Samples of raw DW and effluent leaving the system were anlaysed for BOD5 and TS on a weekly
basis. Figure 3.4.1 shows the data for BOD5. The break in the data signifies the point at which there
were problems with the system draining efficiently and the soil had to be replaced. However, prior to
this point, i.e. during Trial 1, the soil was able to reduce the BOD5 concentration significantly
throughout the year even when input BOD5 concentrations were high in the summer. Nevertheless,
imperfect drainage did become a problem and DW applications in addition to rainfall often exceeded
infiltration rates, resulting in surface ponding.
12000
BOD applied
10000
BOD (mg/l)
soil replaced
BOD effluent
8000
6000
4000
2000
0
20-Mar
09-May
28-Jun 17-Aug
06-Oct
25-Nov
14-Jan
04-Mar
23-Apr
Fig. 3.4.1 BOD5 concentrations in raw dirty water applied to the percolation
system and in effluent draining from the system
There was a gradual decrease in the treatment efficiency of the system, particularly for TS (Fig. 3.4.2)
in the first phase of this monitoring period. This was accompanied by a notable increase in surface
ponding of the soil. This was due to soil structural collapse, possibly accelerated by the high sodium
loading in the DW. At this time, it was decided to replace the soil with fresh soil. Care was taken to
source a more freely draining soil with a low P content from a local contractor. However, the ‘chosen’
soil was not delivered and the second filling of the PSP was completed essentially with a similar soil
type to the first. The second part of the data-set reflects the ability of the ‘fresh’ soil to remove BOD5 ,
TS and TSS.
96 of 217
Total solids (mg/l)
20000
16000
TS applied
TS effleunt
12000
8000
4000
0
20-Mar
09-May
28-Jun
17-Aug
06-Oct
25-Nov
14-Jan
04-Mar
23-Apr
Fig. 3.4.2 TS concentrations in raw DW applied to the percolation
soil plot and in effluent draining from the system
3.4.2 Removal of COD, TN, TAN and P
During the intensive application and sampling weeks (in June ‘03, July ‘03, September ‘03 and March
’04) COD, TN, TAN and Total P concentrations of the DW applied to the PSP were determined,
together with the corresponding concentrations in the treated effluent. The results in Table 3.4.1 are
mean treatment percentages (i.e. mean treatment efficiency for the 5 days in which samples were
taken) for the different weeks. It is clear that treatment efficiency was greatest during the first week (in
June ‘03) and declined thereafter. For example, removal of P by the system decreased from 85% to
57% over the following 9 month period.
Table 3.4.1. Treatment efficiency achieved by the PSP during intensive sampling weeks. Values
are expressed as means of the 5 sample day collections during each week.
June ‘03
BOD5
COD
Total solids
Total N
Ammonium-N
Total P
%
83.7
74.7
39.7
72.7
85.4
85.1
se
3.56
5.13
8.43
1.90
3.91
4.09
July ‘03
%
55.1
34.7
5.9
49.7
52.7
61.5
se
8.81
12.01
12.17
9.84
9.22
9.84
September ‘03
%
58.3
40.6
14.6
50.8
60.8
68.6
se
3.41
3.53
3.59
4.79
6.13
3.54
March ‘04
%
53.1
53.6
28.9
47.5
68.7
57.2
se
12.82
10.69
13.20
11.65
9.83
5.88
3.4.3 Changes in PSP performance
3.4.3.1
Changes in hydraulic conductivity
Attempts at reducing the ponding during Trial 1 by ‘spiking’ to a depth of 20cm were not successful.
Therefore, DW applications were stopped on 30 Sept 2003, to allow the PSP to dry out and have a
period of rest before the winter rain began (in addition to DW applications). Regular observations by
ADAS field officers highlighted that surface ponding was still evident after 14 days despite no
additions of DW, and that the effluent was still flowing from the base of the system. This suggested
that the soil system had been storing DW
and that the poor infiltration and low hydraulic
conductivity of the soil had resulted in water logging and some preferential flow through cracks.
There was also evidence from the OFP (Cranfield University), that the high sodium content of the DW
(derived from the use of cleaning chemicals in the milking parlour) had caused structural deterioration
97 of 217
of the soil, thus reducing hydraulic conductivity even further. The soil structure in the OFP was
restored firstly by incorporating gypsum into the soil and secondly by installing a gypsum-filled
trench. This sodium problem probably affected soil structure in the PSP system in a similar manner.
The increased TS concentration leaving the PSP (Fig. 3.4.2) could have been partially of soil origin
and not from the DW, suggesting that structural breakdown also occurred in this clay loam soil.
Adding gypsum to the PSP was not considered to be an effective option, since the depth of soil was
>1m. Gypsum could have been rotavated into the top 20-30 cm, but the soil below this layer would not
have received the benefit of the gypsum application.
3.4.3.2
Nutrient accumulation
During the course of Trial 1, soil samples (0-5 cm) were taken at random locations from the PSP and
combined to generate one sample on each occasion. These samples were analysed to determine the
accumulation of nutrients, particularly P. The samples were taken only from the 0-5 cm layer in order
to maintain the integrity of the soil and prevent rapid flow of effluent down artificial ‘macro-pores’
left by the sampling. Table 3.4.2 shows how the P accumulated throughout the first phase of operation.
Based on this rate of accumulation and the Heckrath et al. (1995) critical threshold of 65 mg/l Olsen P,
the system had reached the point of requiring some additional remediation to address the potential for
P breakthrough after only 3 months of operation.
Table 3.4.2 Accumulation of nutrients in the upper soil layer (0-5 cm) of the PSP treatment
system * = sample taken immediately after installation of the soil.
01-Nov-02*
18-Jun-03
15-Jul-03
02-Sep-03
Organic matter (%w/w)
2.5
3.5
3.4
3.5
pH
7.4
7.4
7.3
7.4
Total N (%w/w)
0.12
0.16
0.16
0.18
Total C (%w/w)
2.00
2.56
2.37
2.42
Total P (mg/kg)
686
1117
1281
1085
Ammonium-N (mg/kg)
0.93
0.43
1.02
0.88
Nitrate-N (mg/kg)
36.6
114.3
163.0
248.1
Olsen P (mg/l)
19.6
33.0
64.2
94.4
The increase in nitrate concentration suggests that the soil was nitrifying the applied ammoniacal -N.
However, on the occasions that nitrate-N concentration of the drainage from the percolation system
was determined, values were often low. It has been shown previously (Chadwick et al, 2000) that the
PSP treatment system can result in high nitrate concentrations in drainage.
3.4.3.3
Remedial action
The most promising option to restore the PSP in November 2003 was to excavate the existing soil
from the system and to replace it with a coarser grained soil type, preferably a sandy loam with a
greater inherent hydraulic conductivity. A previous study using a disturbed coarse sandy loam
demonstrated no problems associated with infiltration and hydraulic retention at the same application
rates being used in the Pallinghurst project (Chadwick et al, 2000).
When the soil was replaced, considerable care was taken to source a suitably textured soil with low P
status. Once the laboratory results confirmed the low P status of the chosen soil, arrangements were
made for a local contractor to deliver the soil to the site and ‘re-fill’ the PSP system. After a period of
rest, this was re-started in late November 2003. Unfortunately, it became clear that the soil that was
delivered was not that that was carefully selected and that the ‘new’ soil was in fact very similar in
texture to the original one (Table 3.4.3).
98 of 217
Table 3.4.3. Accumulation of nutrients in the upper soil layer (0-5 cm) of the replacement soil in
the PSP treatment system * = sample taken immediately after installation of the soil.
17-Nov-03*
02-Mar-04
pH
7.6
7.7
Total N (%w/w)
0.12
0.12
Total C (%w/w)
2.16
2.19
Total P (mg/kg)
510
590
Olsen P (mg/l)
20.4
26.8
3.4.4 Consequences of soil replacement
Figures 3.4.1 and 3.4.2 illustrate the effect of the new soil on BOD5 and TS concentrations. Despite
early signs of good treatment, ponding began to occur again as infiltration rates declined and treatment
efficiencies decreased (Fig. 3.4.3). The hydraulic conductivity of the 0-10cm soil layer was 0.055
mm/s, suggesting that infiltration rate per se in the top 10 cm was not restricting downward flow of the
effluent and that infiltration was impeded at a lower depth.
Using this soil type would restrict the frequency of application and volumetric loading rate to prevent
hydraulic overload and therefore reduce the volume of DW that could be treated per day.
Fig. 3.4.3 The percolation soil system showing ponding, April 2004.
Given that soil replacement had not resolved the earlier problems, this presented three options:
(a) Further replacement of the soil, or
(b) Roofing the system to prevent rainfall ingress, or
(c) Cease DW applications in ‘treatment’ mode, but instead apply less raw DW to create a deeper
lysimeter than the 30 cm deep diamond lysimeters.
After much consultation with collaborators and funders options (a) and (b) were rejected because (a)
involved a risk of further problems with soil characteristics and (b) was likely to be too expensive for a
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practical farm scale system. Thus, option (c) was adopted and subsequently the PSP received raw DW
every 2 weeks.
3.4.5 Previous research on development of percolation soil systems and practicalities of using
soils of different textures
A previous MAFF funded project (Chadwick et al, 2000) demonstrated that a coarser textured soil, a
coarse sandy loam of the Crediton Series could be used to treat DW without problems of ponding.
With this soil in either an intact state or using disturbed soil (as at Pallinghurst Farm) DW applications
of up to 75 mm/wk could be treated whilst maintaining substantial removal of BOD5 (Fig. 3.4.4), TAN
and P. During this previous project, no ponding or other problems associated with high Na inputs
were observed. However, it must be noted that the BOD5 of the DW was ‘standardised’ (by addition
of milk) at ca. 1500 mg/l. Hence, the system was not challenged to the same degree as that at
Pallinghurst Farm.
BOD
%red-dist
120
100
80
60
40
20
0
%red-int
Expon. (BOD redn- as
applied- disturbed (%))
Linear (%red-int)
0
50
100
150
200
250
300
Dirty water application rate (mm/wk)
Fig. 3.4.4 Percentage reduction in BOD5 concentrations of leachates from disturbed
and intact soils for different applications of DW (from Chadwick et al, 2000)
Of course, the irony is that soils with higher clay content have a greater ability to remove BOD5 than
those with a greater sand content, assuming the same rate of BOD5 input, and that effluent passes
through the soil matrix and not down macro-pores. This is due to the greater exposed surface area and
generally lower hydraulic conductivity of the clay soils. These factors increase the retention time
within the soil and hence allow micro organisms greater time in which to act upon the incoming
organic matter. However, as the current study has shown, some clay soils can suffer with poor
infiltration rates, even if that is not obvious at the surface.
Also, in project WA0518, the soils in England and Wales were ranked according to whether they
would be suitable for percolation treatment systems. In this ranking it was assumed that some clay top
soils would be suitable, as long as the hydraulic conductivity was high enough. It was speculated that
some heavier soils could be used if some method was developed to break these soils up into peds that
could increase infiltration rate. In light of the DW-STOP project, it is necessary to revise these
conclusions and to state that heavy soils are best suited to the OFP system.
The soil type that is available on a farm will influence the choice of soil-based treatment systems, their
efficacy of operation and environmental impact. Pertinent soil properties for an engineered soil based
treatment system are texture (particle-size class and organic matter content), porosity and structure,
chemical properties and profile depth. Relevance of several soil features, especially those affecting
infiltration behaviour, will vary depending on whether the system operates by percolation or overland
flow. If final effluents are applied to adjacent land, its soil properties also require assessment.
100 of 217
3.5
OVERLAND FLOW SYSTEM (PL-H, MD)
3.5.1 Hydraulic loading
The plot received water from the header tanks once a day. When operating at full capacity, the weekly
amount handled by the OFP was 2400 l. This was equivalent to an application to the half plot of 35
m2, i.e. 67 mm of water. On average, this was much higher than the average daily rainfall or
evapotranspiration from the plot. However, in weeks with heavy rainfall, the amount of water at the
end of the treatment was greater than at the start. Conversely, in dry summer months there was a net
loss of water. The ratio of the input volume to the output volume averaged 0.89 but ranged from 0.46
in wet winter conditions to 2.0 in a dry summer week.
Figure 3.5.1 shows the rainfall over the period of two-week batch treatment during the latter part of
Trial 2. The very high rainfall in October 2004 gave rise to a net accumulation of water in the tanks
whereas the dry weather in March and May led to a net loss of water. Note that a 10 mm rainfall event
resulted in a 0.7 m3 increase in water volume even when only half the plot was being used for
treatment. Analysis of the treatment of BOD5 showed no relationship between the mass BOD5
removed and rainfall, suggesting that the system treated pollutants by soil based bio-reactions and not
by dilution.
40
35
Rainfall (mm)
30
25
20
15
10
5
21
28/10
/0
04 /10/ 4
0
11/11 4
/0
18/11 4
/0
25/11 4
/0
/
1
02 1 4
/0
09 /12 4
/
0
/
16 12 4
/0
/
1
23 2 4
/0
30/12 4
/1 /04
06 2/
0
13/01 4
/0
/
0
20 1 5
/0
27 /01 5
/
0
/
03 01 5
/0
/
0
10 2 5
/0
17/02 5
/0
/
24 0 2 5
/
0
/
03 02 5
/0
/
0
10 3 5
/0
17/03 5
/
0
/
24 0 3 5
/0
/
0
31 3 5
/0
07/03 5
/0
/
14 04 5
/
0
/
21 04 5
/0
/
0
28 4 5
/0
05 /04 5
/0
/
12 05 5
/0
/
0
19 5 5
/0
26/05 5
/0 /05
5/
05
0
Date
Fig. 3.5.1 Rainfall over the period from October 2004 to May 2005
3.5.2 Removal of BOD5
3.5.2.1
One-week treatment periods
Throughout the early operation of the treatment plane during Trial 1, the BOD5 concentration was
lowered, on average, by 91% with a median of 95% and standard error of 2.5 %. This was a
remarkably consistent degree of change, which appeared to be independent of the inlet concentration,
even though this varied over a wide range - the high influent concentration was 11400 mg/l and the
lowest was 585 mg/l. Figure 3.5.2 shows the variations in input and output BOD5 concentrations
during the period from May 2003 to April 2004. The zone with no data was during the remediation
and recovery phase of the plot.
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12000
BOD5 (mg l-1)
10000
8000
BOD5 In
BOD5 Out
6000
4000
2000
06/05/03
20/05/03
03/06/03
17/06/03
01/07/03
15/07/03
29/07/03
12/08/03
26/08/03
09/09/03
23/09/03
07/10/03
21/10/03
04/11/03
18/11/03
02/12/03
16/12/03
30/12/03
13/01/04
27/01/04
10/02/04
24/02/04
09/03/04
06/04/04
0
Week Commencing
Fig. 3.5.2 The variation in BOD5 concentration during the period from May 2003 to
April 2004
Because the plot was subject to evapotranspiration and rainfall, the concentration data were converted
to load data, thus expressing BOD5 in mass units. Figure 3.5.3 shows the mass of BOD5 removed as a
function of the starting mass during each batch of DW treated for one-week periods. The data show
that that there were occasions when the mass removed was negative i.e. the mass of BOD5 in the
treated water was greater than at the start of the treatment cycle. This occurred either when the
influent water had a very low BOD5 load and the DW already in the dead space at the base of the tanks
and in the sump was at a higher level than the influent water or when the soil structure had collapsed
so that good soil/DW contact was not maintained. The relationship is highly significant (p<0.001) and
the slope implied that 95% of the BOD5 load was removed by this system.
BOD Mass Removed (kg)
25.00
y = 0.9564x - 0.7109
20.00
R 2= 0.9033
15.00
10.00
5.00
0.00
5.00
-5.00
10.00
15.00
20.00
25.00
BOD Mass In (kg)
-10.00
Fig. 3.5.3 Mass removal of BOD5 during the one-week treatment periods
Figure 3.5.4 contrasts the data for BOD5 load removed during summer and winter conditions.
Analysis of the two data sets suggests that that there was no difference in performance between these
two periods. There is a suggestion of slightly improved summer time treatment when temperatures
102 of 217
were higher than winter but the data suggest that for design of a full scale system the mass removal
rates seen here may be treated as one population.
18.00
BOD removed mass (kg)
16.00
y = 0.83x - 0.0654
14.00
R2 = 0.9705
12.00
10.00
8.00
6.00
y = 0.9004x + 0.0627
summer 2004
R2 = 0.9763
Linear (summer 2004)
winter 2004
4.00
Linear (winter 2004)
2.00
0.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
BOD in mass (kg)
Fig. 3.5.4 Comparison between summer and winter BOD5 removal
3.5.2.2
Two-week treatment periods
The system was operated on a two-week cycle from November 2004 to May 2005. During this period,
the system performed well with no mechanical failures of pumps or sump and without any obvious
soil structure problems. The data collected over this period is perhaps the most valuable in terms of
consistency and in its demonstration of the system’s capability.
Figure 3.5.5 shows that the changes in BOD5 concentration effected by the OFP were consistently
different between the first week of treatment and the second. Figure 3.5.6 shows that while the first
week treatment removed over 90% of the BOD5 the second week only removed 55% of the BOD5
remaining after the first week. Overall, over the two week period the system removed 95.7% of the
BOD5 load. This difference between the first treatment week and the second was significant, p = 0.05.
103 of 217
4500
4000
BOD5 (mg/l)
3500
BOD in
3000
BOD out
2500
2000
1500
1000
500
0
03/05/2005
19/04/2005
05/04/2005
22/03/2005
08/03/2005
22/02/2005
08/02/2005
25/01/2005
11/01/2005
28/12/2004
14/12/2004
30/11/2004
Fig. 3.5.5 Changes in BOD5 concentration over 13 two-week cycles
% removal BOD mass
100.0
average removal over 2 weeks = 95.7%
90.0
80.0
LSD = 23.64% (p = 0.05)
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
BOD week 1
BOD week 2
Treatment period
Fig. 3.5.6 Difference in BOD5 load removal between week one and week two of the
two-week treatment. Bars show standard errors.
The BOD5 removal was expected to follow a first order decay. Such decay in concentration occurs
where the rate of change of concentration due to treatment is proportional to the concentration. If this
were the case, the second week would remove less BOD5 than the first. To investigate this hypothesis,
the changes in BOD5 concentration recorded daily over two intensively monitored weeks of two-week
operation were considered. Figure 3.5.7 shows the change of BOD5 concentration over the first week
of treatment that started on 5 April 2005. It is noted that while an exponential decay provides a good
explanation (p<0.05) of the fall in BOD5 concentration, the very rapid fall during the first day of
treatment is not well represented. It was proposed that this fall resulted from dilution in the plot and
sump rather than from treatment by aerobic bacteria. Figure 3.5.8 shows a much stronger relationship
when the first day is removed from the data set.
104 of 217
1000
900
800
BOD5 (mg/l)
700
y = 446.74e-0.3165x
R2 = 0.8288
600
500
400
300
200
100
0
0
1
2
3
4
5
6
7
8
Treatment day
Fig. 3.5.7 Daily change in BOD5 concentration over the first seven days of treatment
in a two-week treatment period
250
BOD5 (mg/l)
200
y = 272.56e -0.2217x
R2 = 0.9809
150
100
50
0
0
1
2
3
4
5
6
7
8
Treatment Day
Fig. 3.5.8 Daily change in BOD5 concentration over days one to seven of the first
week of a two-week treatment period.
To confirm that dilution was responsible for the early poor fit to an exponential decay, data from the
second week of a two-week treatment period was fitted to a first order decay (Fig. 3.5.9). An
exponential decay curve fitted the data well and gave a very significant relationship (p<0.005). The
decay exponent was different in the two cases but of the same order. This may be explained by the
different weather conditions in the two cases and by the different starting concentrations, which
influenced the amounts of dilution and aeration in the system.
105 of 217
120
100
BOD5 (mg/l)
-0.1466x
y = 111.56e
2
R = 0.9394
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9
Treatment day
Fig. 3.5.9 Change in BOD5 over a seven day period in the second week of a two-week
treatment period in February 2005
The two week treatment regime continued for 26 weeks giving the opportunity to investigate the rate
of change of BOD5 concentration over two weeks, albeit based on only three values (i.e. BOD5
concentrations at the start of week 1, start of week 2 and end of week 2). The 13 sets of data from the
26 weeks were normalised with respect to the BOD5 concentration at the start of week 1, and the
resultant relationship is shown in Fig. 3.5.10.
1
0.9
Normalised BOD5
0.8
-0.214x
y=e
0.7
2
R = 0.78
0.6
P<0.001
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
12
14
16
Treatment day
Fig. 3.5.10 Normalised BOD5 data for the 13 two-week treatment
periods (Bars show the standard error of the mean, n=13)
Interestingly, the resulting rate exponent (Fig. 3.5.10) is similar to that for the weekly data shown
above (Figs 3.5.8 and 3.5.9). This further suggests that the process can be adequately described as a
first order process.
106 of 217
3.5.3 Removal of suspended and total solids
3.5.3.1
One-week treatment periods
Suspended Solids Removed (kg)
Suspended solids form a key parameter in water quality measurement and in addition, the solid
material introduced into a vegetated treatment system such as the PSP used at Pallinghurst, the DRB
or the OFP may be compromised by clogging effects from such solids. Figure 3.5.11 shows the
removal of suspended solids (expressed as mass of TSS) during the one-week treatment periods.
High suspended solid loads were observed at times and these coincided with discharges of slurry into
the farm’s DW system. A strong relationship between TSS removed and that entering the OFP is
noted, with apparently 99% of the TSS being removed. This removal is biased by the occasionally
large inflow loads, but it indicates that such vegetated systems are efficient at removing TSS.
45.00
40.00
y = 0.9937x - 1.1843
R2 = 0.9923
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
0
5
10
15
20
25
30
35
40
45
-5.00
-10.00
Suspended Solids In (kg)
Fig. 3.5.11 Suspended solids removal during the one-week treatment periods in the
overland flow plot (OFP)
In addition, it should be noted that there were occasions when the removal of TSS load was negative,
implying that more suspended solids were in the DW after treatment than before. This occurred
mainly when the influent TSS load was low. This probably occurred because the OFP system allowed
solid material to accumulate in the bottom of the tanks and this could mix with the fresh DW, so
increasing the initial condition. Also, solids in the form of biological slimes may have been sloughed
off the soil and vegetative surfaces during periods of heavy rainfall. Lastly, when the soil was
dispersed due to the high sodium levels in the influent water more solids may have collected in the
treated water as soil may have been washed off the plot into the sump.
Total solids (TS) load removal is shown in Fig. 3.5.12. The scatter in the data is greater than for the
TSS values and the data suggest that 81% of the TS load was removed by the OFP system on average.
Again, some negative removal values imply that as with TSS, material may have been mobilised in the
header tanks and the sump, thus appearing as an increased TS load in the treated effluent.
107 of 217
Solids Removed (kg)
50
y = 0.8083x - 4.1485
40
R2 = 0.7557
30
20
10
0
0
10
-10
20
30
40
50
60
Solids In (kg)
-20
Fig. 3.5.12 Total solids removal during the one-week treatment periods in the overland
flow plot (OFP)
3.5.3.2
Two-week treatment periods
The data show that the overall change in TSS load was around 78% but that there was a significant
reduction in solids removal in the second week compared to the first week. Indeed, the second week
often showed a gain in TSS load. This was presumably due to the effects already discussed, whereby
solids were initially removed from the DW, but that remobilisation of soils in the second week
increased the TSS load in the treated effluent.
It was not clear why the TSS load removal was poorer during the two-week treatment periods,
compared with the one-week periods. Average removal from the DW during the first week of
treatment was 83%. Possibly, solids that were difficult to remove from the tanks and sump had, by
this time, accumulated to such an extent that they compromised the system.
3.5.4 Removal of Nitrogen
3.5.4.1
Total Nitrogen (TN)
Nitrogen occurred in the OFP system in mineral and organic forms. Nitrogen concentrations were
only measured during the intensive monitoring weeks so the information is limited but overall, TN
was lost from DW in the system. The average reduction in TN concentrations measured during the
intensive weeks was 72% with final concentrations varying depending on the influent concentration,
(Figs 3.5.13 and 3.5.14). The maximum TN concentration discharged was 0.254 g/l, and this occurred
when the influent DW was 0.851 g/l at the start of the treatment week. Rapid removal of nitrogen was
always observed on the first day of treatment suggesting again that dilution was an important factor on
the first day of a new batch being treated.
108 of 217
% total N concentration remaining
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.13 Average normalised reductions in TN concentration with time, following
applications of raw DW to the OFP during the intensive monitoring weeks
100%
90%
% total N remaining
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.14 Average normalised reductions in total mass of TN with time, following
applications of raw DW to the OFP during the intensive monitoring weeks
A large variation in percentage reduction of TN concentration was observed on the first day of
treatment; the average removal was 53% with a standard error of 12.5%. The standard error of the
mean removal on the last day of the treatment week was 5.4%. The data suggest that reduction of TN
concentration obeyed a first order decay, but unlike the BOD5 data, no common decay rate term could
be found, suggesting perhaps that the TN decay depended on complex interacting factors, with various
nitrogen pools changing depending on loading and water status of the plot.
3.5.4.2
Total Ammoniacal Nitrogen (TAN)
TAN was also measured during intensive monitoring weeks and the data show that, on average, TAN
concentrations were reduced by 78% with a standard error of 6.2% (Figs 3.5.15 and 3.5.16). However,
these concentrations were always higher than would be allowed for discharge to a watercourse, even
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after the two-week treatment cycle, except in the case of the first intensive monitoring week, when the
whole plot was used for treatment and the influent DW had a low initial concentration.
% TAN concentration remaining
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.15 Average normalised reductions in TN concentration with time, following
applications of raw DW to the OFP during the intensive monitoring weeks
100%
90%
80%
% TAN remaining
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.16 Average normalised reductions in total mass of TAN with time, following
applications of raw DW to the OFP during the intensive monitoring weeks
In most cases, but not all, there was a rise in nitrate concentration, as shown in Fig 3.5.17, indicating
that loss of TAN might be explained by oxidation to nitrate. NO3-N concentrations were low in most
of the weeks during which this parameter was measured (the intensive weeks) with the highest
recorded value at the end of a treatment week being 20 mg/l. The mean concentration at the start of
the treatment weeks was 0.48 mg/l and the mean end of week concentration was 8.92 mg/l. This
contrasted with a change in TAN from a mean value at the start of the treatment weeks of 346 mg/l to
a mean of 64 mg/l at the end of the treatment weeks. Because the volumes of water associated with
these concentrations were the same for the nitrate and TAN concentrations, it follows that insufficient
110 of 217
NO3 –N was produced to account for the fall in TAN. This may be explained by either denitrification
processes in the system, thus releasing nitrogen gases to the atmosphere or some ammonification in
storage of the DW in the tanks and sump.
NO 3 as a percentage of start concentration
2500%
2000%
1500%
1000%
500%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.17 Average normalised increase in NO3-N with time, following applications of raw DW to
the OFP during the intensive monitoring weeks
The oxidation of ammonia/ammonium may have been reduced because of the high BOD5 loads, which
may have reduced redox potentials in the soil. No redox levels were recorded for this data set so this
hypothesis could not be tested.
3.5.5 Removal of Phosphorus
3.5.5.1
Total Phosphorus
Phosphorus was also only measured in the water samples on the intensive weeks. The average
reduction in phosphorus concentrations was 80% with a standard error of 3.8% over the full working
phase of the system % (Figs 3.5.18 and 3.5.19). The highest influent phosphorus concentration was
125 mg/l and this was reduced to 22 mg/l in the week starting 2 September 2003. The lowest influent
phosphorus concentration of 45.4 mg/l was in the first intensive monitoring week, which started on 3
June 2003 and this was reduced to 3.8 mg/l.
111 of 217
100%
% P concentration remaining
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.18 Average normalised reductions in total P concentration with time,
following applications of raw DW to the OFP during the intensive monitoring weeks
100%
90%
% total P remaining
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
day
Fig. 3.5.19 Average normalised reductions in mass of total P with time, following
applications of raw DW to the OFP during the intensive monitoring weeks
3.5.5.2
Sustainability of the plot
In the OFP, phosphorus was mainly lost from the DW by accumulation and immobilization in the soil
of the plot. It has been shown in work at Rothamsted Experimental Station (Heckrath et al., 1995) that
there is likely to be an upper limit of phosphorus absorption in soil beyond which phosphorus will
leach with drainage water. This loading will therefore mark the point at which the system will no
longer be able to treat phosphorus successfully. This point was not reached in the two years of the
trials at Pallinghurst Farm. O’Keefe (2004) describes the methodology developed in this work.
112 of 217
The outcome of the phosphorus sorption studies was expressed as a function showing the change in
percentage sorption of phosphorus over a period of ten years. Figure 3.5.20 predicts that sorption at or
around 90-95% could proceed on the Pallinghurst Farm soil for about 5 to 6 years before falling
rapidly, at which stage leaching would occur. These predicted values were compared with
measurements of the phosphorus sorption achieved at Pallinghurst Farm, based on the amounts of P in
the raw DW applied to the OFP, and in the corresponding treated DW collected. These measurements
indicted a somewhat lower figure of 80% phosphorus sorption. The difference may have arisen
because the calculation assumed that the whole soil mass was available for sorption, but in practice,
bypass flow reduced the effective volume of soil for phosphorus retention.
Fig. 3.5.20 Predicted changes in the percent of the applied phosphorus load that will
be sorbed by the overland flow plot at Pallinghurst Farm
3.5.6 Modelling and scale-up
3.5.6.1
DW supply reservoir
The OFP system used at Pallinghurst Farm was a small trial system only dealing with 2.4 m3 of DW
per week. In practice, the average production of DW at the farm was approximately 25 m3/d or 175
m3 / week. The recorded data show that concentrations of the treated effluent from an OFP are
unlikely to be at levels suitable for discharge into a water course (see Section 3.5).
Throughout this work, the key pollutant parameters were BOD5, TS and TSS. Based on the data and
model shown in Fig. 3.5.10 it was estimated that for an influent BOD5 concentration of 5000 mg/l, a
median value based on earlier work (Cumby et al. 1999), a two-week treatment period would bring the
BOD5 to 300 mg/l. A one-week treatment would only reduce the input to 1100 mg/l. This suggests
that if a one-week retention period is to be used, as has been assumed above, then a buffer reservoir of
at least 175 m3 is required for each of the two plots. If a two-week period is used, then two 350 m3
reservoirs are required. (i.e. 175 * 2). These reservoirs could be in the form of ditches. For example,
a 175 m3 reservoir would measure 72 m long x 2.2m wide x 1m deep to contain a one-week batch at
Pallinghurst.
3.5.6.2
Area required for treatment
The data for removal of the BOD5 load shown in Fig. 3.5.3 indicates that the system did not become
overloaded, even when it received a load of 20 kg of BOD5 in a single batch. Similar characteristics
113 of 217
were observed for TSS and TS removal (Figs 3.5.11 and 3.5.12). These observations suggest that the
half plot size of 35 m2 was sufficient to treat the volumes of DW that were applied, even though these
had varying pollutant loads. However, it was clear from the experience and observations of the OFP
system at Pallinghurst Farm that the hydraulic loading was at a level where any increase would result
in a danger of erosion, excessive flooding and slow drainage. Alternating the half plots during
treatment so that each half plot was allowed to drain helped to maintain the integrity of the system.
Based on this experience, the system may be scaled using a factor that expresses the required total plot
area per unit flow of DW. The data implied a value of 400 m2 m-3 d-1 for a twin-plot system using a
two-week treatment period (i.e. 2*(35 / (2.4/14)) = 408).
3.6
LYSIMETERS (FIELD AND LAB-SCALE) (DCh)
3.6.1 Plot lysimeters at Pallinghurst Farm
3.6.1.1
Raw DW
lysimeter: removal of BOD5, TS and TSS
Raw DW was applied to this diamond lysimeter every 2 weeks during the periods when the treatment
systems were functioning. The results illustrated in Figs 3.6.1 – 3.6.4 show that the soil in the
lysimeter reduced BOD5, TS and TSS concentrations to varying degrees throughout the study. The
peak concentrations of BOD5 in the first summer were significantly reduced as were the peaks in TS
and TSS concentrations in the final winter period. Reductions in BOD5 concentrations would have
been the result of microbial decay of organic material as well as dilution by rainfall. The reductions in
TS concentrations would have been the result of physical filtration and dilution by rainfall.
The average reduction for BOD5 concentrations for all the raw DW applications was 57%. The
reductions in TS and TSS concentrations were 40% and 48%, respectively. As discussed, the degree of
BOD5 removal by the soil was controlled by the prevailing soil and weather conditions and this
relationship was explored in developing the soil model (see Section 3.6.3).
y
BOD (mg/litre)
12000
10000
BOD applied
8000
BOD effluent
6000
4000
2000
0
28-Feb
28-Jun
26-Oct
23-Feb
22-Jun
20-Oct
17-Feb
17-Jun
Fig. 3.6.1 BOD5 concentrations in raw DW applied to and in effluent draining from
the “raw DW” diamond lysimeter
114 of 217
25000
TS applied
TS (mg/litre)
20000
TS effluent
15000
10000
5000
0
28-Feb
28-Jun
26-Oct
23-Feb
22-Jun
20-Oct
17-Feb
17-Jun
Fig. 3.6.2 TS concentrations in raw DW applied to and in effluent draining from the
“raw DW” diamond lysimeter
% reduction in BOD and TS
120
BOD % redctn.
100
TS % redctn.
80
60
40
20
0
28-Feb
-20
28-Jun
26-Oct
23-Feb
22-Jun
20-Oct
17-Feb
17-Jun
-40
Fig. 3.6.3 BOD5 and TS removal efficiency from raw DW applied to the “raw DW”
diamond lysimeter
25000
SS (mg / l)
20000
SS applied
SS effluent
15000
10000
5000
0
28-Feb 28-Jun
26-Oct 23-Feb
22-Jun
20-Oct
17-Feb 17-Jun
Fig. 3.6.4 TSS concentrations in raw DW applied to and in effluent draining from the
“raw DW” diamond lysimeter
3.6.1.2
Raw DW
lysimeter: removal of COD, TN, TAN and P
115 of 217
Periodically, DW (raw or treated, as appropriate) was applied to the diamond lysimeters each day for 5
days, in order to determine the ability of the soil to accept repeated applications within a short time
frame period. The data illustrated in Figs 3.6.5 and 3.6.6 are the mean analyses for each 5 day period,
and the data demonstrate the ability of the soil to remove a range of other nutrients, e.g. TN, TAN, and
total P, as well as the consequences on nitrate leaching.
(g / l)
Total N concentrations in DW applied and leachate
leaving the RAW DW lysimeter
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Total N applied
Total N leachate
03/06/2003
02/09/2003
09/03/2004
07/12/2004
15/02/2005
Fig. 3.6.5 A comparison of TN concentrations in raw DW applied to and effluent
draining from “raw DW” diamond lysimeter.
800
Ammonim-N, nitrate-N and total P concentrations in DW
applied to and leachate leaving the RAW DW lysimeter
NH4-N applied
NH4-N leachate
NO3-N applied
NO3-N leachate
Total P applied
Total P leachate
700
( mg /l)
600
500
400
300
200
100
0
03/06/2003
02/09/2003
09/03/2004
07/12/2004
15/02/2005
Fig. 3.6.6 A comparison of TAN, NO3-N and total P concentrations in raw DW
applied to and effluent draining from the “raw DW” diamond lysimeter.
The TN concentration of the raw DW was reduced by >50% on all occasions, presumably as a result
of grass uptake (not measured) and gaseous losses from the soil (e.g. via ammonia volatilisation and
losses of nitrous oxide and di-nitrogen (N2) , products of denitrification.
3.6.1.3
Raw DW
lysimeter: effect of soil depth
Following the decision to stop using the PSP in treatment mode, it received subsequently applications
of raw DW on the concurrently with applications to the “raw DW” diamond lysimeter. The soil in both
facilities was not exactly the same because the PSP soil had been replaced in November 2003.
116 of 217
However, the soil texture was the same and the organic matter, total P and TN contents were similar
(Table 3.6.1). Therefore, a comparison of the ability of the soil in the PSP system and in the diamond
lysimeter was expected to provide evidence of the effect of soil depth on reduction of BOD5 and TS
concentrations. The depth of soil in the PSP was 100 cm, compared with 30 cm in the diamond
lysimeter.
Table 3.6.1. Characteristics of the replacement soil in the percolation
soil block and the soil in the diamond lysimeters.
Lysimeter soil
Percolation soil plot soil
(after soil replacement)
Soil depth (cm)
30
100
Total N (%w/w)
0.12
0.12
Total C (%w/w)
2.00
2.12
Total P (mg/kg)
686
510
Olsen P (mg/l)
19.6
20.4
pH
7.4
7.6
Organic matter (%)
2.5
3.0
Figure 3.6.7 shows that the BOD5 concentration of the raw DW was reduced after passing through the
30 cm depth of soil (diamond lysimeter). However, greater reductions were observed in the effluent
leaving the 100 cm depth of soil (PSP). Hence it was concluded that the greater the volume of soil that
the effluent travels through, then the greater opportunity for BOD5 to be removed. Similarly, the
greater depth of soil reduced the TS and TSS concentrations, although not to the same extent (Figs
3.6.8 and 3.6.9).
25000
TS applied
1 m TS effluent
30 cm TS effluent
(mg / l)
20000
15000
10000
5000
0
01- Aug-04
20- Sep- 04
09- Nov-04
29-Dec-04
17-Feb-05
08-Apr- 05
28-May-05
Fig. 3.6.7 The effect of soil depth (“raw DW” diamond lysiemeter 30 cm deep; PSP
100 cm deep) on BOD5 removal from raw DW.
117 of 217
p
25000
TS applied
1 m TS effluent
30 cm TS effluent
20000
(mg / l)
15000
10000
5000
0
01-Aug-04
20-Sep-04
09-Nov-04
29-Dec-04
17-Feb-05
08-Apr-05
28-May-05
Fig. 3.6.8 The effect of soil depth (“raw DW” diamond lysimeter 30 cm deep; PSP
100 cm deep) on TS removal from raw DW.
25000
Effect of soil depth on SS concentrations
SS applied
(mg / l)
20000
1 m SS effluent
30 cm SS effluent
15000
10000
5000
0
01-Aug-04 20-Sep-04 09-Nov-04 29-Dec-04 17-Feb-05 08-Apr-05 28-May-05
Fig. 3.6.9 The effect of soil depth (“raw DW” diamond lysimeter 30 cm deep; PSP
100 cm deep) on TSS removal from raw DW.
3.6.1.4
Treated DW
lysimeter: removal of BOD5, TS and TSS
Figures 3.6.10 – 3.6.13 demonstrate how the soil in the “treated DW” diamond lysimeter was able to
reduce concentrations of BOD5, TS and TSS of the treated DW. During Trial 1, (until Spring 2004) the
soil reduced the BOD5 concentrations varying degrees, but on average by 28%. The percentage
reductions in TS were more consistent with an average of 25%. It is also clear that during Trial 2, after
optimisation of the treatment systems (and after the PSP had ceased as a treatment system), the BOD5
of the treated DW that was applied to the lysimeter was already very low (Fig. 3.6.10).
118 of 217
4000
Treated DW lysimeter - BOD concentration
BOD (mg/litre)
3500
3000
BOD applied
2500
BOD effluent
2000
1500
1000
500
0
28-Feb
28-Jun
26-Oct
23-Feb
22-Jun
20-Oct
17-Feb
17-Jun
Fig. 3.6.10 BOD5 concentrations in treated DW applied to and in effluent draining
from the “treated DW” diamond lysimeter
Total solids (mg/litre)
7000
6000
TS applied
5000
TS effluent
4000
3000
2000
1000
0
28-Feb 28-Jun
26-Oct 23-Feb
22-Jun
20-Oct 17-Feb 17-Jun
% reduction in BOD and TS
Fig. 3.6.11 TS concentrations in treated DW applied to and in effluent draining from
the “treated DW” diamond lysimeter
120
100
80
60
40
20
0
-20
28-Feb
-40
-60
-80
-100
BOD % redctn.
TS % redctn.
28-Jun
26-Oct
23-Feb
22-Jun
20-Oct
17-Feb
17-Jun
Fig. 3.6.12 BOD5 and TS removal efficiency from treated DW applied to the “treated
DW” diamond lysimeter.
119 of 217
SS (mg / l)
7000
6000
SS applied
5000
SS effluent
4000
3000
2000
1000
0
28-Feb 28-Jun 26-Oct 23-Feb 22-Jun
20-Oct 17-Feb 17-Jun
Fig. 3.6.13 TSS concentrations in treated DW applied to and in effluent draining from
the “treated DW” diamond lysimeter
3.6.1.5
Treated DW
lysimeter: removal of COD, TN, TAN and P
During those weeks when treated DW was applied to the “treated DW” diamond lysimeter on
consecutive days, other potential contaminants were analysed. Figure 3.6.14 shows that the TN
concentrations of the treated DW applied to the lysimeter were reduced. The TAN and total P
concentrations were also reduced. However, the NO3-N concentrations in the treated DW were greater
than in the raw DW applied to the raw diamond lysimeter (compare Figs 3.6.6 and 3.6.15), suggesting
that the treatment systems were nitrifying the TAN in the DW to NO3-N.
g
1.0
y
Total N applied
0.8
(g / l)
Total N leachate
0.6
0.4
0.2
0.0
03-Jun-03
02-Sep
09-Mar-04
07-Dec-04
15-Feb-05
Fig. 3.6.14 A comparison of TN concentrations in treated DW applied to and effluent
draining from the “treated DW” diamond lysimeter.
120 of 217
g
800
y
700
NH4-N applied
NH4-N leachate
NO3-N applied
NO3-N leachate
Total P applied
Total P leachate
(mg / l)
600
500
400
300
200
100
0
03-Jun-03
02-Sep
09-Mar-04
07-Dec-04
15-Feb-05
Fig. 3.6.15 A comparison of TAN, NO3-N and total P concentrations in treated DW
applied to and effluent draining from the “treated DW” diamond lysimeter
3.6.2 Laboratory-scale column lysimeters
The laboratory-scale experiment determined the ability of five different soils to reduce concentrations
of BOD5, total P and TAN in raw and treated DW taken from Pallinghurst Farm. There were two
phases to the experiment. The first phase comprised 10 weeks of DW applications at 5 mm/day for one
day per week, followed by 2 simulated rainfall events. The second phase comprised of 7 weeks of DW
applications at 10mm/day for one day per week, followed by the same simulated rainfall events as
used in the first phase. Drainage samples were collected once per week. The BOD5 concentrations of
the raw and treated DW are shown in Figs 3.6.16 and 3.6.17 The initial BOD5 of the raw DW was
>2000 mg/l, whilst that for the treated DW was c. 250 mg/l.
BOD of Untreated DW
3000
BO D (m g/
2500
2000
1500
1000
500
ep
23
-S
ep
-S
08
ug
26
-A
ug
-A
11
-J
un
09
ay
27
-M
ay
-M
12
pr
-A
28
14
-A
pr
0
Fig. 3.6.16 Changes in the BOD5 concentration of the stored raw DW (collected from
Pallinghurst Farm) used to apply to the different intact soil cores at IGER.
121 of 217
BOD (mg/l)
300
250
200
150
100
50
23
-S
ep
08
-S
ep
26
-A
ug
11
-A
ug
09
-J
un
27
-M
ay
12
-M
ay
28
-A
pr
14
-A
pr
0
Fig. 3.6.17 Changes in the BOD5 concentration of the stored treated DW (collected
from Pallinghurst Farm) used to apply to the different intact soil cores at IGER.
During the storage of the DW for the first phase of the experiment, the BOD5 concentrations decreased
markedly, such that at the end of the first phase, the BOD5 concentrations of the raw and treated DW
applied to the soils were 266 and 13 mg/l respectively. This significant decrease in BOD5 during
storage is not normally observed in DW tanks on farms because effluent rich in organic matter enters
such tanks on a daily basis. The observed decrease occurred because micro-organisms in the DW
utilised the readily available organic matter (which gives rise to high BOD5 concentrations) releasing it
as carbon dioxide and methane. For this to operate as a ‘treatment’ system on a commercial farm, the
DW would have to be batch stored over a period of months. Such a treatment system would require a
minimum of two tanks, hence the capital costs may be large, but operational costs could be low. In
addition, on a large scale, the deposition of settled solid matter in such tanks could present substantial
mixing problems when the tanks need to be emptied.
The BOD5 of the deionised water applied to the control soil cores and used to simulate rainfall was
always < 2 mg/l.
The average BOD5 concentration in the effluent draining from the base of the different soils can be
seen in Fig. 3.6.18. It is clear that during the first phase (i.e. 5 mm applications once per week) all the
soils reduced the BOD5 concentrations to below 20 mg/litre. There were indications in the first phase
that there were greater BOD5 concentrations in leachate following applications of raw DW from the
Andover and Halstow soils (i.e. those with a greater clay content), than the more textured soils,
possibly a result of macro-pore flow.
122 of 217
Crediton Series BOD (Coarse sandy loam)
60
60
50
50
BOD (mg/l)
BOD (mg/l)
Bridgnorth Series BOD (Loamy sand)
40
30
20
40
30
20
10
10
0
0
1 2
3 4 5 6
7 8 9 10
1
1 2
2 3 4 5 6 7
3 4
5
6 7
Bicton untreat
DBX treat
Bicton con
3
4 5
6 7
DBX untreat
DBX con
Halstow Series BOD (Clay loam)
Andover Series BOD (Silty clay loam)
60
140
50
120
40
100
BOD (mg/l)
BOD (mg/l)
1 2
Week
Week
Bicton treat
8 9 10
30
20
10
80
60
40
20
0
0
1
2
3
4
5 6
7
8
9 10
1 2
3
4
5
6
7
1 2 3 4 5 6 7 8 9 10
Week
Dorset treat
1 2 3 4 5 6 7
Week
Dorset untreat
Dorset con
Great Field treat
Great Field untreat
Great Field con
Denbigh Series BOD (Silty clay)
60
BOD (mg/l)
50
40
30
20
10
0
1
2
3
4
5 6
7
8
9 10
1
2
3
4
5
6
7
Week
Duchy treat
Duchy untreat
Duchy con
Fig. 3.6.18 The BOD5 concentrations in drainage from the five soils
In the second phase, when raw DW was applied at an application rate of 10 mm, elevated BOD5
concentrations were found in leachates from the Andover and Halstow soils as well as the Crediton
soil. This suggests that 10 mm applications should be avoided and that DW should be applied little and
often. Similar information has been found in the past (Chadwick and Pain, 1997).
3.6.3 Modelling
3.6.3.1
Overview of model
A model was constructed to determine the ability of ‘native’ soil to remove BOD5 following
application of DW under a range of climatic, edaphic and site conditions. This information was
required to inform the treatment systems of the degree of DW treatment needed prior to application to
land. This layer-cascade model is similar in principal to the SLIM model (Addiscot et al., 1991) and
the multi-layered ‘tipping bucket’ water availability model of Verdoodt et al (2005). It predicts the
proportion of applied liquor that infiltrates the soil and the interaction of this with soil pores. Effects
123 of 217
on subsequent infiltration rates, path lengths and residence times are determined, together with the
degree of particle retention from the DW and thus removal of BOD5. It accounts on a sub-hourly basis
for soil type, preferential flow and aerobicity.
The DW SOIL model comprises two sub-models:
•
the hydrology sub-model, and
•
the BOD5 decay sub-model.
The hydrology sub-model describes the volume of water/effluent that either ponds on the soil surface,
travels over the soil surface as runoff, or passes through the soil matrix from one soil layer to another
or via macro-pore flow.
The BOD5 decay sub-model describes how the BOD5 in the effluent is either entrained by the soil or
decays as it passes through the soil matrix. The outputs of the two sub-models (effluent volume and
BOD5 concentrations) are used to determine how much effluent passes from one soil layer to another
and at what concentration. The ultimate output is the BOD5 concentration of leachate and runoff
following a DW application.
The DW SOIL model operates with a <1 hour time step on a soil with three 10 cm layers. The texture
of each soil layer can be specified. More detailed explanations of the two sub-models are given below.
3.6.3.2
Hydrology sub-model
The hydrology sub-model is a cascade model, similar to that described by Addiscott et al (1991).
Effluent is applied to the soil surface where the first equations describe the proportion of effluent that
travels over the soil surface or has the potential to infiltrate the soil or flow through macro-pores. This
ratio is a function of: slope angle, surface roughness, infiltration rate and the soil moisture content.
The inputs to this sub-model therefore include slope angle and soil texture. The soil texture ranges
from loamy sand through to clay, and literature values of infiltration rate, porosity and % of WFPS at
field capacity for each soil texture are used in calculations.
The effluent can only enter the first layer of soil if there is sufficient capacity within the layer, i.e.
sufficient air filled pore space. If there is capacity, then the effluent infiltrates according to the typical
hydraulic conductivity for the soil type. Once the capacity of the soil in this layer has been reached,
i.e. at field capacity, the effluent can cascade to the next soil layer or pond on the soil surface where it
may runoff depending on slope angle. Whether the effluent does cascade to the next soil layer depends
on the capacity of that soil layer to accept effluent. Again, this depends on the volume of air filled pore
space available. Thus, in this way the effluent either passes through the soil, via the three layers, or
over the soil surface. At the base of the third soil layer there is a hypothetical drain which collects the
leachate. The model predicts the volume of leachate and runoff generated following DW application.
The Hydrology sub-model inputs are:
•
Effluent: DW delivery rate (mm/h), the total time (h) that effluent is applied, the time step (h).
•
Soil: the soil type of each layer (sand, loamy sand, sandy loam, loam, clay loam, clay), the water
filled pore space (WFPS) of each layer.
Land area: length of spread area (m), width of spread area (m), slope angle (none, gentle,
•
moderate, steep).
The Hydrology sub-model outputs are:
•
Volume of effluent leached at the base of the three layers (m3) , and
•
Volume of runoff (m3).
3.6.3.3
BOD5 decay sub-model
The BOD5 of the DW is divided into 2 portions; that corresponding to the solids fraction and that
contained in the ‘soluble’ fraction. This allows soil entrainment of particles to remove a proportion of
124 of 217
the total BOD5. An assumption is made of the contribution of the solids fraction to the total BOD5 of
the DW. The default value for this is 28%, based on data from a previous project (MAFF, 1996). The
amount of entrainment is dependant on soil texture (pore size) and presence of macro-pores at the time
of application. Effluent that runs off will still lose some of its total solids content due to filtering and
therefore a proportion of the BOD5 will be removed.
So some of the BOD5 is entrained according to soil type and presence of macro-pores, and BOD5 is
removed accordingly. The remaining ‘solids’ not entrained that pass into the matrix of soil layer 1 mix
with ‘soluble’ fraction in first layer in first time step and decay according to a 1st order rate (similar to
that described in Fig. 3.5.10 for the OFS). This decay rate is modified by residence time within the soil
layer, soil temperature and oxygen (O2) availability. Oxygen availability is a function of soil type and
modified by WFPS. Temperature regulates BOD5 decay according to a Q10 of 2. Soil temperature is
therefore an input to this sub-model. However, O2 availability is estimated from soil texture and
WFPS at the time of application.
After BOD5 decay has been calculated in the first layer for first time step, the overall BOD5
concentration is calculated for first layer as function of total soil water content and effluent volume,
i.e. by a mixing ratio (Scholefield, 2005).
If the effluent cascades to the next layer (according to the rules in the hydrology sub-model), then this
calculated BOD5 from layer 1 is the input to layer 2. This same set of rules operates in layers 2 and 3.
However, at the base of layer 3, the BOD concentration of the leachate can be combined with the BOD
concentration of the runoff to provide an overall BOD concentration leaving the applied area. It is this
combined concentration that is the final output for comparison with measured values.
The BOD5 decay sub-model inputs are:
•
Effluent: BOD5 concentration, total solids content, and
•
Soil: soil texture, WFPS, temperature.
The BOD5 decay sub-model outputs are:
•
BOD5 concentration in leachate at the base of layer three ,
•
BOD5 concentration in runoff, and
•
Overall BOD5 concentration in combined runoff and leachate.
3.6.4 Model validation
3.6.4.1
Method
The DW-STOP model was made available elsewhere (Cumby, et al 2005), and was used to predict the
BOD5 of the effluent leaving the raw DW and treated DW lysimeters. Inputs to the model were the
application rate of the DW and the dimensions of the area receiving the application, i.e. for the
lysimeters this was 5 m x 5 m x 0.3 m (deep). Other inputs were the BOD5 of the DW being applied,
the initial soil moisture conditions and soil temperature.
The integrity of the lysimeter soil was maintained throughout the project, therefore the initial soil
conditions were estimated from the previous week’s total rainfall. The starting soil water conditions
for each of the 34 application dates were categorised as, very wet, wet, moist and dry. As the
Pallinghurst lysimeter soil was a clay loam, the initial starting water filled pore space (WFPS) was set
at 70%, 70%, 50% and 30%, respectively. In addition, the proportion of effluent infiltrating into the
soil rather than running off the soil surface was moderated using the ‘roughness’ input. This was
adjusted to 5% for the very wet conditions, 80% for the wet conditions, 85% for the moist and 99% for
dry conditions. These ‘roughness’ values were used as a result of validating the hydrological output of
the model (predicted volumes leaving the lysimeters) with the volumes measured using the tipping
buckets.
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Table 3.6.2 summarises the hydrological input values for roughness and WFPS and the average
measured volumes of effluent leaving the lysimeters for the four categories of initial soil water
conditions.
Table 3.6.2. Hydrological input values for WFPS and Roughness. Roughness values based on
calibration by measured volumes of effluent leaving the lysimeters.
Staring soil
water
conditions
Estimated water
filled pore space
(%)
Roughness (proportion of
applied DW that is lost as
surface runoff)
Average volume of effluent
leaving the lysimeters (litres)
Very wet
70
95
230 (none)
Wet
70
20
80 (8)
Moist
50
15
55 (4)
Dry
30
1
15 (3)
Values in parentheses = n. More than 50% of the volume measurements could not be used due to
missing data. It was assumed that almost the same volume of effluent applied to the lysimeters would
leave them under very wet conditions.
3.6.4.2
Results of the model validation
modelled BOD (mg/l)
Initially, the model validation was summarised separately for the raw and treated DW lysimeters.
Figures 3.6.19 and 3.6.20 demonstrate the goodness of fit between the modelled and measured BOD5
concentrations of effluent leaving the two lysimeters for all 34 applications. The trend line has been
forced through the origin.
y
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
y = 1.6586x
R2 = 0.5873
0
1000
2000
3000
4000
5000
measured BOD (mg/l)
Fig. 3.6.19 The relationship between modelled and measured BOD5 concentration in leachate
leaving the raw DW lysimeter
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water
modelled BOD
(mg/l)
2500
2000
y = 1.6714x
R2 = 0.8798
1500
1000
500
0
0
200
400
600
measured BOD (mg/l)
800
1000
Fig. 3.6.20 The relationship between modelled and measured BOD5 concentration in leachate
leaving the treated DW lysimeter.
The main points that can be drawn from these relationships are that:
•
•
•
•
in the majority of cases, the model over-predicts the BOD5 concentration,
the slope of the line is greater than 1,
there is a high degree of variance explained by the model, the explanation being greater for the
treated DW (88%) than the raw DW (59%),
the two figures illustrate that the model is better at predicting BOD5 concentration when the
initial BOD5 concentration is low, i.e.< 1000 mg/l
modelled BOD
(mg/l)
Since the two lysimeters were constructed in the same manner with the same soil, and applications
were made at the same rate, the raw DW and treated DW data were pooled to determine the ability of
the model to predict the effluent concentration from a wider range of BOD5 concentration inputs. This
relationship is shown in Fig. 3.6.21.
dirty water applications
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
y = 1.6588x
2
R = 0.7128
0
1000
2000
3000
measured BOD (mg/l)
4000
5000
Fig. 3.6.21 The relationship between modelled and measured BOD5 concentration
using the pooled data from the raw and treated DW lysimeters..
As expected, the r2 of this relationship (0.71) using the pooled dataset lies between the 0.88 and 0.59
shown in Figures 3.19 and 3.6.20, and the slope of the line is still greater than 1.
In order to improve the model, certain relationships and functions need to be calibrated. For the
hydrological part of the model, the proportional volumes of effluent that travel via leaching and runoff
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needs to be verified as a function of soil texture and slope. The pathway that the effluent takes
markedly effects the degree of BOD5 removal, with greater residence time resulting in greater
removal. The relationships shown above suggest that perhaps too great a proportion is travelling via
runoff in the current model, resulting in less opportunity for BOD5 removal to take place.
128 of 217
CHAPTER 4 OPTIMISATION OF DW MANAGEMENT
Chapter authors: Trevor Cumby, Andrew Barker, Colin Burton, David Chadwick, Marc Dresser, Gari
Fernandez, John Gregory, Peter Leeds-Harrison, Ian Muir, Elia Nigro, Ken Smith and Joe Wood
4.1
INTEGRATION OF MATHEMATICAL MODELS (TC, CoB, PL-H, DCh, JW))
4.1.1 Basic concepts underlying the approaches to modelling
In order to define the most appropriate DW management system under a given set of farm conditions,
it was first necessary to establish clear definitions of the boundaries of the system. Considering the
management of DW on dairy farms, within the context of the DW-STOP project, it was determined
that the system boundary on the input side would coincide with the point at which DW is collected and
made ready for spreading on land. On the output side, the boundary was taken to be the points at
which DW entered either a watercourse or a body of clean ground water. By adopting these
boundaries, the system thus encompassed two separate but linked processes:
(a) A treatment process (or set of processes) that transforms the properties of the DW to achieved
a desired condition, and
(b) A set of natural processes that occur in soils after the DW has been spread on land, which also
changes the characteristics of the DW before it reaches the defined output boundary of the
system.
Hence, having defined the system boundaries and the major processes that comprise the system, the
next requirement was to define the inputs to the system and the desired outputs from it. These
definitions of inputs and outputs included a number of data sets, as described in Section 2.1, but
amongst the most important input values are the volumetric flows of DW and the associated
concentrations of BOD5, TS and Ammoniacal nitrogen. Equally, the required output concentrations of
BOD5, TS and Ammoniacal nitrogen are necessary to define the required change to be achieved.
Given that, as defined, the DW management system comprises a managed process followed by a
natural process, it was also necessary to establish a definition of the DW that flows from the first to the
second. In this way, with the input, intermediate and output properties of the DW defined, the overall
performance of the system became a function of the changes in the properties created as the DW
passes through each of the two processes (Fig. 4.1.1).
The main goals of the DW-STOP project were to examine a range of treatment technologies that could
be used either singly or in combination to maximise the efficiency of the managed process. Where
treatment technologies are considered for use in combination, it is convenient to label each technology
as a “module”. Figure 4.1.1 lists a number of potential modules that could be incorporated within the
managed process part of the DW management systems. Most of these were assessed by experiment
within DW-STOP.
129 of 217
) BOD , TS, NH -N
5
input
flow
➠
process
Possible process modules:
■
■
■
■
■
■
■
Aeration (CSTR)
Aeration (trickling filter)
Coagulation/flocculation
Overland flow soil filter
Percolating flow soil filter
Reed beds
Sedimentation
3
output
flow
) BOD , TS, NH -N
5
3
reduced
pollution risk
prom
run-off and
drain flow
Fig. 4.1.1 Integration of mathematical models: basic concept
Having described the general principles that were used to produce and collate the DW-STOP
mathematical models, the next step is to illustrate how the component modules were combined.
4.1.2 Development of the modules to produce the collated DW-STOP mathematical model
(“DW-MODEL”)
Given that the system described by the DW-MODEL comprises two processes, and that one of these,
the managed process, could include one or more modules, it follows that a strict protocol was required
to guide their use when determining the most efficient DW management systems for a given farm.
Based on the results shown in Chapter 3, five mathematical models were produced, as listed in Table
4.1.1.
Table 4.1.1 Description and purpose of the five constitute parts of DW-MODEL
Model
number
Project Partner
responsible for
this model
1
IGER
2
NSRI
3
SRI
4
UoB
5
SRI
Purpose of model
To predict, and thus specify, the maximum BOD5 concentration, and
other properties, for the DW applied to the available land, e.g. the
required BOD5 of the treated DW (BOD5 treated), so that the “field
flow” DW leaving the output system boundary meets the standard
defined by the required system outputs.
To predict the dimensions of an OFP treatment facility to achieve the
properties specified by model 1 for the treated DW, and to estimate
the investment and running costs.
To predict the dimensions of an intensive aeration (CSTR) treatment
facility to achieve the properties specified by model 1 for the treated
DW, and to estimate the investment and running costs.
To predict the dimensions of a reed-bed treatment facility to achieve
the properties specified by model 1 for the treated DW, and to
estimate the investment and running costs.
To predict the dimensions of an intensive aeration (HRTF) treatment
facility to achieve the properties specified by model 1 for the treated
DW, and to estimate the investment and running costs.
130 of 217
Figure 4.1.2 shows how the five models described in Table 4.1.1 could be used in combination so
that the performance and cost predictions for each combination could be compared to establish the
most appropriate DW management system. Other combinations of treatment modules are possible in
addition to those shown in Fig. 4.1.2, although the results and observations described in Chapter 3
suggested that other possible arrangements would probably be less efficient.
discharge
flow properties
input
flow properties
Field Flow Model
output
flow properties
discharge
flow properties
=
output
flow properties
*
) BOD5, TS,
NH3-N Field 1
Models 1 - 5: Optional modular Treatment Processes
Option (A)
OFP only
output
flow properties
=
Option (B)
CSTR only
output
flow properties
=
Option (C)
OFP + RB
output
flow properties
=
Option (D)
IAP + RB
output
flow properties
=
input
flow properties
input
flow properties
input
flow properties
input
flow properties
*
) BOD5, TS,
NH3-N Mod 2
*
) BOD5, TS,
NH3-N Mod 3
*
) BOD5, TS,
NH3-N Mod 4
*
) BOD5, TS,
NH3-N Mod 2
*
) BOD5, TS,
NH3-N Mod 4
*
) BOD5, TS,
NH3-N Mod 3
*
) BOD5, TS,
NH3-N Mod 5
Fig. 4.1.2 Possible options for using mathematical models: basic concepts
4.1.3 “What if?” questions and scenarios
Inevitably, a commitment to adopt DW treatment as a means to minimise the risks of water pollution
associated with the land spreading of DW will be a medium to long-term investment decision.
Therefore, it is important to assess, in advance, the likely consequences of changes that may impinge
either on the production of DW or on the operation of the DW treatment process. This leads to a
number of “What if?” questions and scenarios; some of the most likely and significant changes are
summarised in Table 4.1.2, together with quantitative indications of the extent to these changes. The
effects of these changes on possible treatment strategies for DW are addressed below in Sections 4.2
to 4.5 , in connection with the two Case Studies.
131 of 217
Table 4.1.2 “What if?” questions and scenarios
Change factor
Extent
of
change
Comments
No of cows in milk
Up to
+50%
Reduction of fixed costs per litre of milk produced will
remain an essential requirement to maintain profits, and
under some circumstances, this may lead to increased herd
sizes.
Electricity cost
Up to
+50%
Alternative treatment processes will consume different
amounts of energy per unit volume of DW treated, and so it
is important to compare their sensitivities to rising energy
costs.
Cost / availability of potable
mains water
Up to
+50%
Depending on the extent of the treatment, re-use of treated
DW is possible for some purposes, such as:
washing/flushing yard areas, buildings and some
equipment; crop irrigation and use in disinfectant wheel
dips/boot baths. In some instances, supply restrictions may
be applied to potable mains water, necessitating re-use for
some purposes.
Tighter target specifications
for the discharge to land of
treated DW, expressed as a
reduced BOD5 concentration
Down to
-50%
Tighter specifications may be needed if treated DW is to be
recycled, or they may become necessary to meet changes in
legislative requirements for land-spreading DW.
4.2 CASE STUDY 1 - PALLINGHURST FARM
ARRANGEMENTS (TC, CoB, PL-H, DCh, JW, KS, IM, JG))
(>400
COWS):
EXISTING
4.2.1 Description of Pallinghurst Farm
4.2.1.1
Location and general features
Pallinghurst Farm is located near Horsham in West Sussex (Figs 2.1.1 and 2.1.2), and is between 15
and 46 m above sea level. Annual rainfall is about 810 mm, and most of this falls in winter, with
drought-prone periods in July and August. The farm is one of three in the area, under joint
management. Together, these cover approximately 730 ha, of which 567 ha are owned and 162 ha are
tenanted. These farms support three dairy herds comprising about 900 cows in total, with 5.6 million
litres of milk quota (i.e. about 6200 l cow-1 year-1).
Pallinghurst Farm has a herd of about 430 cows and covers approximately 162 ha (400 acres) of
mainly weald clay. The cropping pattern includes approximately 32 ha of maize, plus 89 ha of grazing
areas.
4.2.1.2
Herd management: calving patterns, grazing and herd replacement
The milking herd at Pallinghurst Farm is managed as one group throughout the year, by thee
stockmen. All the cows calve between 14th August and lst November. Therefore, since they are all
dried off in the first two weeks of July, the whole herd is dry for a period of about four weeks during
each summer. The cows have access to straw during this period and magnesium chloride is added to
all the water troughs. The annual replacement rate of cows within the herd is approximately 20%.
132 of 217
The cows are grazed as one group throughout the grazing season. Normally, the herd can be turned out
for 2 hours a day from the beginning of March onto Italian Ryegrass, which is usually strip grazed for
between 4 and 6 weeks. During the main part of the grazing season, the herd grazes 2 ha paddocks in
rotation.
Annual nitrogen fertiliser use is approximately 300 kg / ha (270 units / acre), depending on the
requirements of the season. Soil P and K indices are between 0 and 1.
All replacements are home reared. Calves are housed in groups of 8 and are reared on fresh or
fermented colostrum and waste milk, at a rate of 6 l calf-1 day-1. They are introduced to weaner pellets
and weaned at 6 weeks of age. From 6 - 8 weeks, they are introduced to self-feed silage and are also
gradually transferred onto a supplementary diet of wheat and rape meal, plus straw. Target bulling
weight is 320 - 360 kg and the target calving age is 24 months
4.2.1.3
Buildings and housing management for in-milk cows
The in-milk cows at Pallinghurst Farm are housed in 450 cubicles, which were erected in 2002 at cost
of £70k. This installation was concurrent with the enlargement of the slurry lagoon. The cubicles are
bedded with chopped straw 2-3 times each week and the cubicle design enables rapid scraping of
manure into the lagoon via drive-over ramps. A separate reservoir was built in 2002 to hold all roof
water and to provide trough water.
When housed, the cows have outdoor access to self-feed grass and maize silage over an electric tape.
Maize and grass are side by side in the same pit giving the cows a free choice. The width of the feed
face is approximately 85 m (i.e. about 0.2 m/cow).
The cows are milked in a 60 point rotary parlour, which was commissioned in September 2001,
enabling milking at a rate of 300 cows per hour. All cows receive the same ration of concentrates in
the parlour from an automatic feeder, up to a maximum of 6.5 kg/day. This is a farm-produced blend
of wheat, soya, rape meal and minerals Minimum concentrates are fed after turnout.
4.2.1.4
Management of slurry, DW and other farm operations
Substantial amounts of washing water are used in the milking parlour. The DW collected from the
adjacent three-tank settlement (Fig. 2.1.17) has been estimated by Mr Jonathan Harrison to be
approximately 12 m3/day. Dairy hygiene also requires the daily use of 4 l of "TC86" (Sodium
hypochlorite/Sodium Hydroxide mixture), diluted in 850 l water (morning wash); plus 4 l of
"Acidbright" (Phosphoric acid solution), diluted in 850 l water (afternoon wash). Both products are
supplied by Diversey Lever Ltd, Weston Favell Centre, Northampton, Northamptonshire NN3 4PD
Tel: +44 (0) 1604 405311 / Fax: +44 (0) 1604 783506 All spent wash water from the dairy parlour is
collected in the combined slurry/DW system. Further details of slurry and DW management are
described in Section 2.1.3.
Apart from management of the dairy herd, all other farm operations (e.g. cultivations, drilling, silage
harvesting, cereal harvesting, and manure and DW
management) are undertaken by other staff,
shared with the two other farms (Woodsomes Farm and Dedisham Farm).
4.2.2 Engineering appraisal of the existing systems for slurry and DW management at
Pallinghurst Farm (Case Study 1)
4.2.2.1
Volumes and flows of DW
133 of 217
During the milking period, approximately 12 m3 / d of DW are derived from the slurry lagoon strainer
box plus a further 12 m3/d from the dairy parlour. Hence, the total DW production is approximately
((12+12) *(365 – (8 * 7)) = 7400 m3 (i.e. with the equivalent of about 8 weeks down time in summer,
when the cows are dried off). All of this DW is generated in and around a closely grouped set of
farm buildings, thus leading, in effect, to a very intensive point source of DW production. Table 4.2.1
provides further details of the key user-defined input properties needed to describe the production of
DW at Pallinghurst Farm, on a month-by-month basis.
Table 4.2.1 Key user-defined input properties to describe the production of DW at Pallinghurst
Farm
Property to be defined
Userdefined
Value
Units
Number of cows on concrete
430
Manure per cow per day
53
kg
BOD5 per cow per day
0.5
kg
5
kg
Parlour washings per year
2800
tonnes (m3)
Yard rainfall per year
900
tonnes (m3)
Manure store rainfall per year
450
tonnes (m3)
Amount of water to store retained by solid manure*
20
%
Amount of TS to store retained by solid manure
90
%
Amount of BOD5 to store retained by solid manure
50
%
DW store rainfall per year
0
tonnes (m3)
TS per cow per day
* including rainfall to yard and store and parlour washings
The data in Table 4.2.1 were used in DW-MODEL to define the volumetric loads used to specify the
various options for DW treatment at the Farm, as summarised in Tables 4.2.2 and 4.2.3, and as
described further in Section 4.3.
Table 4.2.2 Prediction of DW production from manure during a typical year at Pallinghurst
Farm, based on inputs defined in Table 4.2.1
Month
Occupancy
Equivalent
Total
Water content
BOD5
Total solids
%
cows
manure
Total
in DW
Total
in DW
Total
in DW
tonnes
tonnes
tonnes
tonnes
tonnes
tonnes
tonnes
Jan
100
430
693
627
502
6.54
3.3
65.4
6.9
Feb
100
430
693
627
502
6.54
3.3
65.4
6.9
Mar
100
430
693
627
502
6.54
3.3
65.4
6.9
Apr
100
430
693
627
502
6.54
3.3
65.4
6.9
May
10
43
69
63
50
0.65
0.3
6.5
0.7
Jun
10
43
69
63
50
0.65
0.3
6.5
0.7
Jul
10
43
69
63
50
0.65
0.3
6.5
0.7
Aug
10
43
69
63
50
0.65
0.3
6.5
0.7
Sep
100
430
693
627
502
6.54
3.3
65.4
6.9
Oct
100
430
693
627
502
6.54
3.3
65.4
6.9
134 of 217
Month
Occupancy
Equivalent
Total
Water content
BOD5
Total solids
%
cows
manure
Total
in DW
Total
in DW
Total
in DW
tonnes
tonnes
tonnes
tonnes
tonnes
tonnes
tonnes
Nov
100
430
693
627
502
6.54
3.3
65.4
6.9
Dec
100
430
693
627
502
6.54
3.3
65.4
6.9
Mean values
301
485
439
351
4.6
2.3
45.8
4.8
Total values
n/a
5820
5271
4217
55
27.45
549
58
Table 4.2.3 Prediction of total DW production from manure and other sources during a typical
year at Pallinghurst Farm, based on Table 4.2.2 and on inputs defined in Table 4.2.1
Month
Parlour
Rain %
Rain fall
Rain fall
DW
Rain fall
washings
of annual
yard
on store
to spread
on tank
tonnes
tonnes
tonnes
tonnes
tonnes
4.2.2.2
Jan
233
9
83
41.7
795
na
Feb
233
7
64
32.2
773
na
Mar
233
6
58
29.0
765
na
Apr
233
6
57
28.4
764
na
May
233
7
64
32.2
315
na
Jun
233
7
59
29.7
309
na
Jul
233
8
73
36.6
325
na
Aug
233
9
83
41.7
338
na
Sep
233
9
78
39.1
789
na
Oct
233
10
86
42.9
798
na
Nov
233
12
104
51.8
820
na
Dec
233
10
90
44.8
803
na
Mean values
233
8
75
38
633
na
Total values
2800
100
900
450
7594
na
Properties of the DW at Pallinghurst Farm
Although the properties of the DW at Pallinghurst Farm are described fully in Section 3.1, DWMODEL was required to predict such properties for other farm situations, on a week-by-week basis,
where detailed results from biochemical analysis were not available. Therefore, DW-MODEL
included appropriate predictive calculations to achieve this, based on the data shown in Tables 4.2.1 –
4.2.3. The results of these predictions for BOD5 and TS concentrations are shown in Figs 4.2.1 and
4.2.2, including for comparison, the corresponding data previously shown in Section 3.1. Generally,
the comparisons show good agreement between the predicted and measured average values and
between the corresponding upper 95% confidence limit values. However, the predictions do not show
the other periodic changes that were observed in the measured values.
135 of 217
BOD5 mg/l
15000
10000
5000
0
1-Apr-03
18-Oct-03
5-May-04
date
21-Nov-04
9-Jun-05
F a rm D W - B O D 5 T ria l 1 2 0 0 3
F ie ld D W - B O D 5 T ria l 1 2 0 0 3
F a rm D W - B O D 5 T ria l 1 2 0 0 4
F ie ld D W - B O D 5 T ria l 1 2 0 0 4
F a rm D W - B O D 5 T ria l 2 2 0 0 4
F ie ld D W - B O D 5 T ria l 2 2 0 0 4
E s ti m a te d B O D
E s ti m a te d B O D
E s ti m a te d B O D
Est m ean BOD
c o n c m g /l
c o n c m g /l
Meas m ean BOD
Est 95% BOD
c o n c m g /l
c o n c m g /l
Meas 95% BOD
c o n c m g /l
Est m ean BOD
c o n c m g /l
c o n c m g /l
Meas m ean BOD
Est 95% BOD
c o n c m g /l
c o n c m g /l
Meas 95% BOD
c o n c m g /l
Est m ean BOD
c o n c m g /l
c o n c m g /l
Meas m ean BOD
Est 95% BOD
c o n c m g /l
c o n c m g /l
Meas 95% BOD
c o n c m g /l
Fig. 4.2.1 Serial data comparison of predicted and measured raw DW characteristics at
Pallinghurst Farm
25000
TS mg/l
20000
15000
10000
5000
0
1-Apr-03
18-Oct-03
5-May-04
21-Nov-04
9-Jun-05
date
Fig. 4.2.2 Serial data comparison of predicted and measured raw DW TS
concentrations at Pallinghurst Farm
4.2.2.3
Current problems to be solved
The key challenges associated with the management of DW at Pallinghurst Farm arise from the need
to spread large volumes during the winter housing period in ways that do not lead to water pollution
through contaminated run-off and drain flow. The main factors that increase the risk of pollution are
the heavy soil, substantial winter rainfall and the intensity of DW production.
136 of 217
The relatively heavy burden of suspended solids of the DW at Pallinghurst Farm (Fig. 4.2.2) means
that if it were stored during in the winter, instead of being spread, the store would be at risk of
accumulating settled solid matter. This would progressively reduce the effective storage volume, and
depending upon the type of store used, its eventual removal by mechanical means could be costly and
difficult. Therefore, whilst winter storage followed by spring spreading of DW may be a valid option
to reduce the risk of water pollution by run-off during the winter, the solids removal aspects of the
treatment options investigated by the DW-STOP project are also relevant in this context to maximise
storage efficiency.
Application of untreated DW at any time of year will incur some risks of water pollution, although the
magnitude of these risks are likely to increase substantially when the soil that received the DW is at or
near field capacity, or in summer, if it is cracked. Furthermore, phosphorus accumulation will remain
a potential issue throughout the year. Consequently, the field-scale and laboratory-scale aspects of
DW-STOP concerned with the reductions in the concentration of DW following land-spreading are
also relevant to these possible problems arising from the discharge of untreated DW, including spring
discharge following storage.
4.3 CASE STUDY 1 - PALLINGHURST FARM (>400 COWS): SPECIFICATIONS AND
COSTS OF ALTERNATIVE DW SYSTEMS (TC, COB, PL-H, DCH, JW, KS, IM, JG)
4.3.1 Application of DW-MODEL: overview of approaches
In order to define the costs and specification of alterative treatment systems for case study 1, a
modelling protocol is needed, as shown in Fig. 4.3.1.. This defines the functional relationships
required to investigate the system options defined in Fig. 4.1.2.
Input data
•
•
•
•
•
•
•
•
Volumes of dirty water
Composition, (BOD, TS etc)
Seasonal trends
Target composition for discharge to surface waters
(BOD, TS etc)
Area of land receiving effluents (hectares)
Distances to local streams/rivers (metres)
Soil conditions
Land available for treatment system
Plant too large or
too costly or not
achieving
targets?
Model 1 (IGER)
Full data set
Prediction of maximum BOD
concentration for applied water
to the available land that
produces a discharge quality
meeting the set standard
As well as input data, this includes
the required quality of the treated
dirty water that is to be irrigated to
the specified grassland.
Model 2 (NSRI)
Model 3 (SRI)
This predicts (a) the dimensions of
the overland flow treatment facility
to meet the constraints and (b) the
estimated investment and running
costs
This predicts (a) the dimensions of
the intensive aeration (CSTR)
treatment facility to meet the
constraints and (b) the estimated
investment and running costs
YES
Evaluate the best
composition of
intermediate treated
effluent achieved
YES
NO
Plant too large or
too costly or not
achieving
targets?
NO
Model 4 (Birmingham)
Model 5 (SRI)
This predicts (a) the dimensions of
the reed-bed treatment facility to
meet the constraints and (b) the
estimated investment and running
costs
This predicts (a) the dimensions of
the HRTF treatment facility to meet
the constraints and (b) the estimated
investment and running costs
Output information
Final design - selection of best option; specification of system
Fig. 4.3.1 Procedures for using mathematical models
137 of 217
The following sections describe the application and results of using Models 1- 5 in connection with the
circumstances prevailing at Pallinghurst Farm (Case study 1).
4.3.2 Application of DW-STOP Model 1: Field flow
4.3.2.1
Methods
The soil model was used to investigate the ability of the ‘native’ soil at this case study farms to reduce
BOD5 concentrations following the application of DW either prior to treatment or after treatment by
various options, e.g. IAP (CSTR only) or OFP and DRB in combination, thus achieving different
levels of treatment. The model was run on the current herd and 10% increased herd size scenarios.
Table 4.3.1 summarises the BOD5 and TS concentrations of the DW for these different scenarios for
Pallinghurst Farm.
Table 4.3.1 DW BOD5 and TS concentrations for the model scenarios runs with different herd
sizes and treatment systems for Pallinghurst Farm.
Treatment system
Current herd
+10% herd size
BOD5 (mg/l)
3300
3400
TS (mg/l)
7000
7200
90% reduction BOD5 (mg/l)
330
340
90% reduction TS (mg/l)
700
720
92% reduction BOD5 (mg/l)
264
272
92% reduction TS (mg/l)
560
576
95% reduction BOD5 (mg/l)
165
170
95% reduction TS (mg/l)
350
360
BOD5 (mg/l)
33
34
TS (mg/l)
70
72
No treatment
IAP 90% reduction
OFP+DRB 99% reduction
Pallinghurst Farm operates on clay loam soil types. Therefore, this soil input to the model was used.
The model was run under four different initial soil moisture conditions, very wet, wet, moist and dry.
The model was not designed to predict the removal of TS from DW applications. However, from the
empirical data collected from the diamond lysimeters at the Pallinghurst site, we can use the average
reduction in TS concentration to estimate the TS concentrations leaving the applied area. The average
percentage TS reductions for the raw DW and treated DW were 40% and 25%, respectively.
The temperature was set at 10oC for all the model runs. The model is not particularly sensitive to
temperature variations in its present form. The slope of the fields receiving DW at Pallinghurst Farm
was assumed to be 1%.
4.3.2.2
Results
Tables 4.3.2 and 4.3.3 summarise the outputs of the model for the range of soil moisture scenarios for
case study 1 under the current herd and 10% increased herd size, respectively. Although the model
138 of 217
tended to overestimate the predicted BOD5 concentration leaving the applied area, it was apparent that
application of untreated DW from both herd size scenarios could result in significant concentrations of
BOD5 reaching watercourses. Intrinsically, these model runs only provided concentrations of BOD5 in
effluent at the edge of the applied area, and, in accordance with the Code of Good Agricultural
Practice for the Protection of Water (Defra, 2001), there would be at least a 10 m strip between the
applied area and any water course, ditch etc. This would provide greater potential for BOD5 removal
before reaching the watercourse.
Table 4.3.2 Model prediction of BOD5 concentrations of effluent leaving the applied area
(combined runoff and leachate concentration) from the current herd scenario.
Treatment system
Current
herd
Model prediction
Very wet
Wet
Moist
Dry
No treatment
BOD5 (mg/l)
3300
3131
2545
2596
1784
TS (mg/l)
7000
4200
4200
4200
4200
90% reduction BOD5 (mg/l)
330
313
255
259
178
90% reduction TS (mg/l)
700
525
525
525
525
92% reduction BOD5 (mg/l)
264
250
204
208
143
92% reduction TS (mg/l)
560
420
420
420
420
95% reduction BOD5 (mg/l)
165
157
127
130
89
95% reduction TS (mg/l)
350
262
262
262
262
BOD5 (mg/l)
33
31
25
26
18
TS (mg/l)
70
52
IAP 90% reduction
OFP+DRB 99% reduction
Application of untreated of treated DW under dry conditions results in lower concentrations of BOD5.
However, if there is considerable cracking of the soil, macropore flow may be greater than assumed in
the model in its present form, hence the BOD5 concentration could be greater in the effluent leaving
(combined runoff and vertical movement through the soil) the applied area. The BOD5 concentrations
of the combined effluent between the wet and moist soil conditions were largely similar because the
model simulates similar proportions of infiltration under both sets of conditions in its present form.
Table 4.3.3. Model prediction of BOD5 concentrations of effluent leaving the applied area
(combined runoff and leachate concentration) from the 10% increased herd size scenario.
Treatment system
10% greater
herd output
Model prediction
Very wet
Wet
Moist
Dry
No treatment
BOD5 (mg/l)
3400
3226
2623
2675
1838
TS (mg/l)
7200
4320
4320
4320
4320
139 of 217
Treatment system
10% greater
herd output
Model prediction
Very wet
Wet
Moist
Dry
IAP 90% reduction
90% reduction BOD5 (mg/l)
340
324
262
267
184
90% reduction TS (mg/l)
720
540
540
540
540
92% reduction BOD5 (mg/l)
272
258
210
214
147
92% reduction TS (mg/l)
576
432
432
432
432
95% reduction BOD5 (mg/l)
170
161
131
134
92
95% reduction TS (mg/l)
360
270
270
270
270
BOD5 (mg/l)
34
32
62
27
18
TS (mg/l)
72
54
54
54
54
OFP+DRB 99% reduction
Obviously, the treatment systems reduced the BOD5 concentration of the DW applied to the soil.
Hence, the concentrations were much lower in the effluent leaving the edge of the applied area when
treated DW was applied. The reductions in TS appear to be the same irrespective of the soil moisture
conditions. This is because there was no apparent effect of soil moisture conditions on the reduction in
the TS as measured using the diamond lysimeter data.
4.3.3 Application of DW-STOP Model 2: Overland Flow treatment system
4.3.3.1
Methods
Because the treatment plane does not receive exactly the same amount of DW per week and the
concentration of the BOD5 is variable, it is useful to be able to estimate the performance of the system
based on mass removal of pollutant and inflow concentration. In addition, account can be taken of the
evapotranspiration from the grass and the rainfall received. The data developed in this study indicated
that the mass removal is always a fixed proportion of the influent load, typically greater than 90% as
seen in Fig 3.5.3.
The equation describing the relationship between influent and effluent concentrations is:




1

Co = (1 − s )Ci 
1 + A( R − E ) 


Vi
Equation 4.3.1
Where:
Co is the effluent BOD5 concentration
Ci is the influent BOD5 concentration
S is the proportion of BOD5 mass removed
A is the total area of the treatment plane
R is the rainfall during a treatment week
E is the evapotranspiration during a treatment week
Vi is the volume of influent DW
140 of 217
4.3.3.2
Results: design of a full-scale system
Using Equation 4.3.1 with the known data from the Pallinghurst Farm trials over a 52 week cycle, as
reported in Section 3.1, the variations in influent and effluent BOD5 concentrations were determined,
as shown in Fig. 4.3.2. This analysis was based on a series of one-week treatment cycles using two
treatment planes alternately. It was found that under these circumstances, the required area of each
treatment plane was equivalent to 200 m2 m-3 day-1 of DW treated, with a average reduction in BOD5
concentration of 92%
12000
BOD5 (mg/l)
10000
discharge conc.
Input conc.
8000
6000
4000
2000
0
0
10
20
30
40
50
60
week
Fig. 4.3.2 Modelled variation in BOD5 over a 52 week period of one
week treatment cycles.
As expected, the predicted concentration of the treated effluent has some high peaks but because the
influent concentration is often below 2000 mg/l, the effluent is often at quite low predicted
concentrations. Ideally, such a model would be used where the influent DW concentrations are known
on a week-by-week basis. However, these data are not likely to be available in practice so this model
then needs to be used in conjunction with the complementary model described in Section 4.2.2 to
predict the composition of the influent DW.
In practice, an OFP system comprising two treatment planes requires at least one reservoir associated
with each of the planes. Where a further treatment process is used as well, such as a DRB, a third
reservoir is needed to provide a feedstock for this additional treatment stage. This arrangement is
illustrated in Fig. 4.3.3. If additional treatment in a DRB is not needed, reservoir 3 provides the
source for the treated DW to be spread on other areas of land.
141 of 217
Source of DW
Treatment plane 1
Treatment plane 2
Reservoir 1
Reservoir 2
Contents of
Reservoir 1
recirculated to
Treatment Plane 1
to achieve treatment
Raw DW directed to
Reservoir 2 whilst
Treatment Plane 1 is in
operation
Reservoir 3
Reservoir 3 receives treated DW
from Reservoirs 1 and 2
alternately and thus provides a
continuous supply for the DRB.
DRB
Treated DW from
the DRB for landspreading
Fig. 4.3.3 Schematic layout of an OFP system, including two treatment planes and two reservoirs,
plus a third reservoir to supply an optional downstream DRB.
Thus, the concentration of the third reservoir receiving water from the treatment planes as a weekly
batch is described by the mixing model:
Cr (t ) =
CoVi + Cr (t − 1)Vr
Vr + Vi
Equation 4.3.2
Where:
Cr(t) and Cr(t-1) are the concentration of the reservoir before and after mixing with the treated DW,
Vr is the volume of the reservoir and other terms are as defined above.
Equations 4.3.1 and 4.3.2 therefore formed the basis of a model to estimate the likely performance of
an OFP treatment system. Hence, the present version of Model 2 was produced in spreadsheet form,
based on the parameters listed above in connection with Fig 4.3.2. This was used to determine the key
system dimensions, (i.e. total treatment plane area and minimum volume per reservoir). These
calculations were based on:
•
the DW production data presented in Table 4.2.3,
•
the DW properties predicted in Section 4.2.2.2,
•
the input data shown in Table 4.3.4, and
•
the effects of the net balance of rainfall and evaporation/evapotranspiration on the reservoirs
and treatment planes.
The minimum volume necessary for either Reservoir 1 or Reservoir 2 was defined as the minimum
capacity needed when maximum DW production occurred, added to the user-defined initial (and
residual) liquid volume in each reservoir. This latter volume enabled the user to represent the amount
of liquid that could be allowed to remain in each reservoir. Increasing this value tended to increase the
buffering effect of each reservoir, thus reducing the week-to-week changes in the concentration of the
142 of 217
treated DW released from the treatment planes. The model allowed the user to define a larger total
volume for each of reservoirs 1 and 2, if extra capacity was required for other reasons, e.g. to provide
additional short-term storage capacity for periods of very cold weather when freezing of the Treatment
planes would prevent operation until thawed. The key output values produced by Model 2 are listed in
Table 4.3.4.
Table 4.3.4 Summary of OFP specification to treat DW at Pallinghurst Farm
using a one-week batch treatment
System properties
Value
Volume of buffer reservoir (m3)
600
3
Minimum volume of buffer reservoir per treatment plane (m )
438
Depth of buffer reservoir (m)
1.5
BOD5 concentration of reservoir at start (mg/l)
100
TS of buffer reservoir at start (mg/l)
1000
Fraction of BOD5 mass removed per week
0.92
Fraction of TS removed per week
0.8
Initial (and residual) liquid volumes in Reservoir 1 and 2 (m3)
50
3
Volume discharge to DRB per week (m )
all OFP output
Combined area of two treatment planes (ha)
1.09
Total area of treatment plane based on 200m2 m-3 day-1 of DW
for one week
User input data are shown in shaded cells
The Model 2 calculations predicted the week-by-week input and output flows from Reservoirs 1 and 2,
as shown in Fig. 4.3.4. As shown by Fig. 4.3.3, these output flows were equal to the input flows to
Reservoir 3, and thus to a downstream DRB, if used.
800
Vol. aplied to OFP, m3 / wk
DW volume added to reservoirs at start of week, m3
600
Vol, m3
R - ET, m3
400
200
0
-200
0
10
20
30
40
50
60
70
80
week
Fig. 4.3.4 Input and output flows to and from the OFP, plus net rainfall minus evapotranspiration
over a 68 week period at Pallinghurst Farm
143 of 217
The corresponding week-by-week predictions of output BOD5 and TS concentrations from Model 2
are shown in Figs 4.3.5 and 4.3.6 respectively, based on the initial conditions defined in Table 4.3.4.
The data show that when rainfall dominates the weather, so that the value of (R-ET) is positive, the
dilution effect on the treatment plane is significant, and with a relatively small residual volume in each
Reservoir (50 m3 in this example), the concentrations in the reservoirs follow closely those in the
effluents from the treatment planes. When (R-ET) is negative, and DW production is at a low level, no
water is discharged from the treatment planes to either reservoir 1 or 2. Hence, the BOD5 and TS
concentrations in the residual liquid in reservoirs 1 and 2 rise sharply.
5000
4000
BOD, mg/l
BOD of DW to be treated, mg/l
3000
BOD5 conc of treated DW, mg/l
Reservoir 1 running BOD conc
(full), kg
Reservoir 2 running BOD conc
(full), kg
2000
1000
0
0
10
20
30
40
week
50
60
70
80
Fig. 4.3.5 Input and output BOD5 concentrations to and from the OFP, plus changes in BOD5
concentrations in Reservoirs 1 and 2 over a 68 week period at Pallinghurst Farm
10000
8000
TS, mg/l
TS of DW to be treated, mg/l
6000
TS conc of treated DW, mg/l
Reservoir 1 running TS conc
(full), kg
Reservoir 2 running TS conc
(full), kg
4000
2000
0
0
10
20
30
40
week
50
60
70
80
Fig. 4.3.6 Input and output TS concentrations to and from the OFP, plus changes in TS
concentrations in Reservoirs 1 and 2 over a 68 week period at Pallinghurst Farm
Model 2 predicted that the average BOD5 and TS concentrations in reservoirs 1 and 2 were 275 mg/l
and 1449 mg/l respectively. Table 4.3.2 indicates that spreading of DW with this BOD5 concentration
on other land at Pallinghurst farm would lead to concentrations of between about 140 and 260 mg/l in
the resulting combined run-off and leachate. Alternatively, the predicted average BOD5 concentration
of 275 mg/l would be suitable for tertiary treatment in a DRB or HRTF.
In its present form, Model 2 provides the basis for estimating the reservoir requirements for other
climatic areas with different daily DW amounts.
144 of 217
Figure 4.3.7 shows the likely configuration for a full scale system at Pallinghurst Farm, constructed in
accordance with the previous example design specification. The system can be automated in that the
single pump re-circulates the DW during treatment and a diverter valve is used to transfer water from
the treatment plane reservoir (i.e. reservoir 1 or 2) to Reservoir 3. Figure 4.3.7 shows redistribution
channels to prevent preferential down-slope flow. These may be simple gravel-filled surface drains .
redistribution
channels
1% slope
20 m
liner
pump
diverter
valve
switch
135 m
Plan view
80 m
Treatment plane
reservoir
(reservoir 1 or 2)
reedbed
DRBrecirculation
reservoir
reservoir 3)
(reservoir
Fig. 4.3.7 Schematic of a proposed full scale system based on a re-circulating OFP
treatment system delivering to a DRB at Pallinghurst Farm.
4.3.3.3
Results: estimated costs of a full-scale system
Costs of the example system described above were estimated based on the need to line an area of land
and to create the necessary lagoons. Details are listed in Table 4.3.5. These refer to a system with a
design life of 5 years, which is based on the expected rate of phosphorus accumulation in the soil in
the treatment plane, as described in Chapter 3.
Table 4.3.5 Summary of OFP costs to treat DW at Pallinghurst Farm (Case study 1), using two
treatment planes, each providing a one-week batch treatment
Cost item
Land area required
Labour
+
constructions
digger
for
Pumps + local power supply
Liner for OFP
Units
1.1
ha
Comments
No value associated with this
£5,500
Farm labour plus excavator and
driver for 25 days
£3,000
Excludes provision for power
supplies in remote locations
HDPE liner at £15/m2
£165,000
Number of lagoons needed
3
Volume of each lagoon
Lagoons
Value
m3
600
£
16,650
145 of 217
Costing details are illustrated in
Fig. 4.3.8
Cost item
Value
Units
Total capital cost including
OFP liner
£ 190,150
Total capital cost excluding
OFP liner
£ 25,150
Running costs
£360
Comments
3.3 kW pump running for 2 hours
per day against a 5 m head.
Electricity charged at £0.08/kWh
£/year
Notes:
The estimated cost exclude the following:
Labour charges for supervising the OFP installation, costs of transporting the DW to the OFP site
and of spreading the treated DW.
The costs of the optional DRB are detailed separately in Section 4.3.5
£6,000
£5,000
Cost, £
£4,000
£3,000
£2,000
£1,000
£0
100
200
300
400
500
600
700
800
3
Working volume, m
Fig. 4.3.8 Relationship between lagoon volume and total cost based on 1.5 m working
depth and membrane cost of £15/m3 (Weatherhead and Knox, 2000)
Total costs with the HDPE liner for the treatment planes are therefore in the region of £190,000, thus
equating to a capital cost of about £440 per cow. The HDPE liner is a major cost in this scheme, and
considerable savings would be possible if this could be avoided. Since the system is designed for used
on heavy clay soils, these savings would be possible if the appropriate authority allows the use of
compacted subsoil clay instead of a liner.
4.3.4 Application of DW-STOP Model 3: Intensive Aeration: Continuous Stirred Tank Reactor
4.3.4.1
Methods
The performance of the IAP was modelled by completing a mass balance analysis of the 16 separate
flows that comprised the complete system, as shown in Fig 4.3.9. For convenience, these were
grouped according to the four key sub-sections of the system. With reference to Fig. 2.2.1, IAP
Module 1 comprised Sections 1 and 2, whilst IAP Module 2 comprised Sections 3 and 4. In practice,
the run-down screen included in section 1 was not used and therefore stream 3 was not produced, and
therefore stream 2 was identical to stream 1. The mass balance analysis was completed on week-byweek basis, corresponding with the weekly measurements of DW properties undertaken during Trials
1 and 2. Hence, this analysis was based on the assumption that, during each week of operation, the
characteristics of the DW treated by the IAP did not change. This assumption was justified by the
146 of 217
design of the 10 m3 steel tank and the associated mixing systems used to supply DW to the IAP (Fig.
2.1.12). The weekly DW sampling regime meant that the DW sampled from the steel tank in any
week was compared with the characteristics of the treated DW sampled during the following week.
Section 1
Section 2
SC
RAW
FEED
1
44
2
Section 3
6
T2
7
T1
Section 4
14
T5
T3
T
T6
Key:
OUTPUT
SC
T1
T2
T3
T4
T5
T6
11
3
10
13
7
8
12
5
15
Primary Run-Down Screen
Primary Settling Tank
Continuous Stirred Tank Reactor
Secondary Settling Tank
Tower Sump
High Rate Trickling Filter
Tertiary Settling Tank
T4
9
16
PURGE
Fig. 4.3.9 Schematic representation of the 16 flows used to model the performance of the IAP
Note: all references in Chapter 4 to IAP tank numbers (i.e. T1- T6) refer to Fig. 4.3.9
The key stages of the overall treatment process were completed in Tanks T1 – T6, as shown in Fig.
4.3.9. Of these, T1, T3 and T6 allowed solid matter to settle from the DW. These settlement
processes were characterised by functions in which values for the percentage removal of TSS in each
tank were prescribed manually, based on previous experience, and adjusted using a manual iteration
technique to achieve a satisfactory empirical representation of the settling performance measured
during Trials 1 and 2. This was established by comparing the predicted and measured values of TS
and TSS measured in three streams:
•
•
•
Stream 1 (Raw DW),
Stream 7 (CSTR output), and
Stream 14 (HRTF output).
All of the biochemical reactions that took place in the IAP were assumed to occur in T2 (CSTR), and
(T4 + T5), which together comprised the HRTF. In each case, these transformations were represented
using a set of equations previously reviewed by Burton, 1992, based on Monod kinetics. Reductions
of COD, TS and TSS concentrations were all represented by equations of the following form:
 A 

S = S o 
 + C
 1 + BR 

Equation 4.3.3
where So and S represent concentrations in the raw and treated DW respectively, R is the mean
hydraulic residence time, expressed in days and A, B and C are constants with the values given in
Table 4.3.6 as originally reported by Evans et al l(1983).
Table 4.3.6 Values of constants for the general equations to predict the reductions in the
concentrations of COD, TS and TSS during continuous aerobic treatment of slurries
3
COD, kg/m
3
TS, kg/m
3
TSS, kg/m
A
B
C
0.33
0.4
0.535
0.262
0.4
0.744
0.282
0.4
0.696
Reductions in the concentrations of BOD5 during continuous aerobic treatment were described by a
modified form of the general equations such that:
147 of 217
B
R
S = AS o +
Equation 4.3.4
where S, So and R have the same definition as above, and A and B are constants, as defined in Table
4.3.7, although in this case, two sets of values are included, as determined from two different studies:
Table 4.3.7 Values of constants for the general equations to predict the reductions in the
concentrations of BOD5 during continuous aerobic treatment of slurries
Source
A
B
Evans at al (1983)
0.15
1.57
Williams et al (1989)
0.23
1.30
A spreadsheet model was created based on the above analysis and equations and was first used to
determine values for the necessary coefficients and then to provide system specifications to meet the
treatment requirements described in Section 4.2.2.
Since the IAP comprised both the CSTR and HRTF stages, the model was constructed so that the each
part could be considered separately. Thus, the combined representations of the transformations that
took place between stream 1 and stream 7 provided “Model 3” as defined in Fig. 4.1.3, whilst those
that took place between stream 7 and stream 14 provided “Model 5”.
4.3.4.2
Model validation: Model 3
The values of the coefficients for removal of BOD5, to achieve the “best fit” between the predicted
and measured concentrations in Stream 7 are listed in Table 4.3.8, which shows that the values of
coefficients during Trial 1 were very close to those listed in Table 4.3.7. However, the improved
system performance during Trial 2 resulted in the elimination of coefficient B, and a value for A that
was substantially smaller than that found during Trial 1. Indeed, the effects of the changes in
coefficient values for Trial 2 meant that the process was represented, in effect, by a simple 95%
reduction in BOD5 concentration.
Table 4.3.8 Values of constants for the general equations to predict the reductions in the
concentrations of BOD5 during continuous aerobic treatment of slurries
Source
A
B
Trial 1
0.2
1.5
Trial 2
0.05
0
The results of applying the coefficients and equations described above are illustrated in Figs. 4.3.10
and 4.3.11. Although these show that the models were able to represent general trends, some errors
remained during the observed week-to-week fluctuations.
148 of 217
predicted BOD5 from CSTR
1200
1000
800
600
y = 0.9521x + 62.479
2
R = 0.5225
400
200
0
0
200
400
600
800
1000
measured BOD5 from CSTR
Fig. 4.3.10 Comparison of predicted and measured BOD5 concentrations in the treated
effluent from the CSTR, (i.e. Stream 7)
1200
1000
Measured BOD5
BOD, mg/l
800
Predicted BOD5
600
400
200
0
28-Jun-03
26-Oct-03
23-Feb-04
22-Jun-04
20-Oct-04
17-Feb-05
17-Jun-05
Date
Fig. 4.3.11 Serial data comparison of predicted and measured BOD5 concentrations in
the treated effluent from the CSTR, (i.e. Stream 7)
A similar procedure for fitting coefficients was adopted to achieve predictive equations for both TS
and TSS. The coefficients for TS listed in Table 4.3.9 provided a satisfactory representation of the
general trends observed in Stream 7, although Figs 4.3.12 and 4.3.13 indicate, that to some extent, the
predicted TS and TSS values appeared to pre-empt the measured values by between one and two
weeks, i.e. the model was predicting changes too early. However, this was expected because Sections
1 and 2 of the IAP comprised two tanks in series, T1 and T2, with respective mean hydraulic residence
times of approximately 2 days and 4 days, this meant that it would take significantly more than one
week to replace the entire contents of these tanks. Thus, the measured concentrations in Stream 7
would include, in effect, some material from previous weeks of operation. Whilst this effect was
relatively unimportant during periods when the concentration of the DW to be treated did not change
greatly from week-to-week, it could lead to deviations between predicted and measured values during
periods of greater change, as shown in Fig. 4.3.11.
149 of 217
12000
10000
Measured TS
Predicted TS
TS, mg/l
8000
6000
4000
2000
0
22-Jun-04
20-Oct-04
17-Feb-05
17-Jun-05
Date
Fig. 4.3.12 Serial data comparison of predicted and measured TS concentrations in the
treated effluent from the CSTR, (i.e. Stream 7)
2400
Measured TSS
2000
Predicted TSS
TS, mg/l
1600
1200
800
400
0
22-Jun-04
20-Oct-04
17-Feb-05
17-Jun-05
Date
Fig. 4.3.13 Serial data comparison of predicted and measured TSS concentrations in
the treated effluent from the CSTR, (i.e. Stream 7)
4.3.4.3
Results: design of a full-scale system
The present version of Model 3 was produced in spreadsheet form, based on the parameters listed
above in Section 4.3.4.2. This was used to determine the key system dimensions were based on the
DW production data presented in Table 4.2.3 and the DW properties predicted in Section 4.2.2. The
calculations also required a number of user-defined inputs to indicate the required dimensions and
capacities of certain key components of the CSTR. These are detailed in Table 4.3.9, together with the
system specifications calculated by Model 3. Given the predicted tank sizes, it was estimated that a
CSTR system for Pallinghurst Farm would be relatively compact, occupying between 300 m2 and 400
m2 of land area, including space for the feed tank. The total installed power requirement would be
about 17 kW (although mostly operating intermittently).
Table 4.3.9 CSTR Specification for Case Study 1, Pallinghurst Farm, including both userdefined inputs and specifications calculated by Model 3
System properties
Feed tank mean hydraulic residence
time, days at maximum flow
Feed tank volume, m3
Value
14
378.3
System properties
Foam
breaker
power
requirement, kW
Foam
breaker
power
150 of 217
Value
2.0
48
System properties
Working volume of CSTR, m3
CSTR diameter, m
Value
108.1
5.5
Mean hydraulic residence time in
CSTR, days
4
CSTR freeboard, m
1
CSTR overall height, m
5.55
Total volume of CSTR, m3
131.9
Average
fraction
of
BOD5
concentration removed
Average fraction of TS concentration
removed
Sludge removal in settling tank 1 (%
of input flow)
Sludge removal in settling tank 2 (%
of CSTR output flow)
0.93
0.31
5%
5%
Tank 1 feed pump flow rate, l/min
30
Tank 1 feed pump power, kW
1.5
Tank 1 feed pump average duty, h/day
12.3
Tank 1 feed pump average energy
use, kWh/d
18.4
Feed tank stirrer power, kW
3.0
Feed tank stirrer duty cycle, % of time
50%
Feed tank stirrer energy use, kW/d
36.0
System properties
requirement, kWh/d
Aeration efficiency kgO2 / kWhe
Maximum CSTR aerator duty
cycle, % of time
Value
Max aerator power required, kW
9.0
Average
aeration
energy
required, kWh/d
First
Settling
tank
mean
hydraulic residence time, days
Required First Settling tank
volume, m3
Second Settling tank mean
hydraulic residence time, days
Required second Settling tank
volume, m3
Tank 1 sludge pump flow rate,
l/min
Tank 1 sludge pump power, kW
1.5
75%
123.7
2.0
54.0
2.0
51.3
5.0
0.75
Tank 1 sludge pump average
duty, h/day
Tank 1 sludge pump average
energy use, kWh/d
Tank 3 sludge pump flow rate,
l/min
2.8
Tank 3 sludge pump power, kW
0.75
Tank 3 sludge pump average
duty, h/day
Tank 3 sludge pump average
energy use, kWh/d
3.7
5.0
3.5
2.6
User input data are shown in shaded cells
The Model 3 calculations predicted the week-by-week input and output flows from the CSTR, as
shown in Fig. 4.3.14. Unlike the OFP, these input and output flows were independent of rainfall and
evaporation effects because the CSTR was covered and because the other tanks associated with it
provided only very small rainfall catchment areas. The predicted differences between the input an
output flows in the CSTR were due to the removal of sludge from the two associated settlement tanks.
151 of 217
200
Vol, m3
150
100
50
Vol. supplied to CSTR, m3 / wk
Weekly output volume from CSTR, m3
0
0
10
20
30
40
50
60
70
80
week
Fig. 4.3.14 Weekly input and output flows to and from the IAP CSTR, over a 68
week period at Pallinghurst Farm
The corresponding week-by-week predictions of output BOD5 and TS concentrations from Model 3
are shown in Figs 4.3.15 and 4.3.16 respectively, based on the system specifications defined in Table
4.3.9. Model 3 predicted that the average BOD5 and TS concentrations in the treated DW from the
CSTR would be 232 mg/l and 4796 mg/l respectively. Applying this BOD5 concentration to Model 1
indicated that spreading of this DW on other land at Pallinghurst farm would lead to concentrations of
between about 120 and 220 mg/l in the resulting combined run-off and leachate. Alternatively, as with
the OFP, the predicted average BOD5 concentration of 232 mg/l would be suitable for tertiary
treatment in a DRB or HRTF.
5000
BOD, mg/l
4000
3000
BOD of DW to be
treated, mg/l
2000
BOD5 conc of
treated DW, mg/l
1000
0
0
10
20
30
40
week
50
60
70
80
Fig. 4.3.4.7 Input and output BOD5 concentrations to and from the IAP CSTR, over a
68 week period at Pallinghurst Farm
152 of 217
12000
TS of DW to be treated, mg/l
TS conc of treated DW, mg/l
TS, mg/l
8000
4000
0
0
10
20
30
40
week
50
60
70
80
Fig. 4.3.4.8 Input and output TS concentrations to and from the IAP CSTR, over a 68
week period at Pallinghurst Farm
4.3.4.4
Results: estimated costs of a full-scale system
Costs of the example system described above are listed in Table 4.3.10, which was based on the
following key assumptions:
•
Land costs are excluded,
•
Labour charges for supervising the process are not included,
•
Costs of transporting the DW to the CSTR site and of spreading the treated DW are not
included,
•
The provision of additional power supplies is excluded, and
•
The costs of the optional DRB are detailed separately in Section 4.3.5
The values given in Table 4.3.10 refer to a system with a design life of 15 years, which is based on the
anticipated life expectancy of the CSTR and settling tanks. It is anticipated that the pumps, mixers
and blowers would require replacement at least once during this period, although these costs are not
included in the values given.
Table 4.3.10 Summary of costs to treat DW at Pallinghurst Farm (Case study 1), using a CSTR
Tanks
Feed tank volume, m3
Required First Settling tank
volume, m3
Total volume of CSTR, m3
Required second Settling tank
volume, m3
TOTAL CAP COST - tanks
Pumps
Feed tank stirrer power, kW
Tank 1 feed pump power, kW
Tank 1 sludge pump power, kW
Tank 2 sludge pump power, kW
Foam breaker power requirement,
kW
Capacity,
m3
378
54
Capital
cost
£126,108
£18,015
132
51
£43,950
£17,115
£
Power, kW
3.0
1.5
0.75
0.75
2.0
205,188
Capital
cost
£1,993
£1,295
£945
£945
£3,055
153 of 217
Comments
All settling tank and CSTR costs
are based on an average erection
cost of £333/ m3
The capital costs of the stirrers,
pumps and blowers are all based
on the relationship shown in Fig.
4.3.17
Max aerator power required, kW
(inc diffusers)
TOTAL CAP COST - pumps,
etc
Energy costs
9.0
£14,332
£ 22,565
Energy
required,
kWh/d
36.0
48.0
Feed tank stirrer energy use, kW/d
Foam
breaker
energy
requirement, kWh/d
Average aeration energy required,
kWh/d
Tank 1 feed pump average energy
use, kWh/d
Tank 1 sludge pump average
energy use, kWh/d
Tank 2 sludge pump average
energy use, kWh/d
TOTAL energy use, kWh/d
Electricity cost, £/kWh
TOTAL energy cost, £/year
Installation cost factor
(factor times all other cap costs)
Running
costs
Comments
Electricity charged at £0.08/kWh
The effects of changes in
electricity prices can be assessed
pro-rata.
123.7
18.4
2.8
2.6
231.5
0.08
£6,759
Comments
This factor includes an allowance
for specialist services and process
commissioning
1.3
Total investment cost
£ 296,080
5000
cost, £
4000
cost= 466*power + 596
3000
2000
1000
0
0
1
2
3
4
5
6
7
8
power kW
Fig. 4.3.17 Relationship used to estimate costs of pumps, mixers and blowers
Total capital costs of the CSTR system are therefore in the region of £296,000, thus equating to a
capital cost of about £688 per cow. Although this appears to be more expensive that the OFP with a
lined treatment plane, the 15-year life expectancy of the CSTR, compared with 5 years in the case of
the OFP means that in some circumstances, the CSTR would be competitive. However, compared
with an OFP without a liner, the CSTR is expensive. Compared with either version of the OFP, the
running costs of the CSTR, are about 20 times higher.
154 of 217
4.3.5 Application of DW-STOP Model 4: Reed beds for secondary / tertiary treatment
4.3.5.1
Methods
Mathematical modelling was applied to the DRB system in order to calculate the size of the system
required for a given application and to predict the effluent concentration for a given influent
composition and DRB design. Previous models of DRB operation vary in complexity from simple
plug flow models to complex sub-surface flow simulations solved using techniques such as finite
element analysis. Sun, et al (1998) developed a mathematical model of a tidal flow DRB, which
predicts outlet concentration from the DRB using the equation:
C1 =
C 0V f
V f + 0.09175 Ah h(1 − exp(− 0.272t 0 ))
Equation 4.3.5
where C0 and C1 are the BOD5 concentrations of the influent and effluent respectively; Vf is the volume
of flush (m3); Ah is the bed area (m2); h is the bed depth (m) and t0 is the time interval between flushes
(hours). The equation is applicable to a single DRB and is based on first order removal kinetics with
respect to BOD5 of the waste being treated. It was derived from data for the treatment of pig slurry in
a tidal flow DRB. Since the bed is flooded, this predicts the average outlet concentration over a flush
rather than the dynamics of the outlet as a function of time.
During DW-STOP, a dynamic model of DRB operation was also developed. The tidal nature of the
DRB means that outlet concentration varies as a function of elapsed time since the water pulse was
applied. This is because the wastewater flow has a distribution of residence time. The cycles of filling
and draining the bed represent unsteady operation. Also, the system of tanks associated with the
DRBs introduce a significant time lag between the influent and effluent, as well as dilution of the
influent feed with recycle. Therefore, the initial performance of the system depends on the quantity
and concentration of water in the tanks as well as the properties of the influent water. If the holding
tanks initially contain tap water, the initial outlet concentration will differ significantly from the case
where the holding tanks contain water with an appreciable BOD5 concentration. However, the
performance should converge to the same result after a number of cycles of operation.
Regarding the flow through the beds, percolation of water through a porous medium suggests an
approximation to plug flow behaviour. However, the complete flooding of the bed and relatively long
drain time, may lead to significant backmixing. Laboratory residence time distribution tests were
carried out in order to determine the dispersion coefficient. In some respects, tidal operation of a DRB
resembles periodic operation of trickle bed catalytic reactors, whereby liquid reactant is supplied in a
pulsed fashion onto a bed of catalyst pellets. The DRB model reported here is therefore based on the
trickle bed reactor model of Stegasov et al (1994). It assumes that air is drawn in to the bed only by
the drainage of water rather than from a pressurised supply. The relevant differential equation is:
∂C
∂C
∂ 2C
α
− akC
= −αVZ
+ αεDZ
∂z
∂t
∂z 2
Equation 4.3.6
where: α is the total liquid hold up (α = βs + βd); C is BOD5 (mg l-1); z is distance through the bed
(m); VZ is liquid velocity (ms-1); ε is porosity; DZ is dispersion coefficient (m2s-1); a is specific surface
area of the gravel (m2m-3) and k is BOD5 removal rate constant (m s-1). As noted above, the total hold
up is the sum of static and dynamic hold ups. Dynamic hold up, βd, is calculated from the equations of
Spechia and Baldi (1977), whilst static hold up is assumed constant (βs = 0.102), following Stegasov et
al (1994). The rate constant was assumed to follow the equation (CH2M Hill and Payne Engineering,
1997):
155 of 217
k = k 20θ (T − 20 )
Equation 4.3.7
where θ = 1.03 and k20 is rate constant at 20°C and T is temperature (°C). A simplifying assumption
was made regarding the velocity of water flow through the bed; it was assumed that this is Ql/Ah,
where Ql is the rate at which water is applied to the bed during a pulse and Ah is the cross sectional
area of the bed.
When a pulse of water is first introduced to the bed, a liquid front moves through it with this velocity.
When liquid flow stops, an air-water front moves through the bed with the same velocity. Clearly, this
approximates the complex drainage process that will occur in practice.
The model equation for the DRB was solved, together with mass balances for the mixing tanks in the
DRB system using gProms software. An advantage of this software is that sequential operations can
be modelled, such that the DRB timing sequence can be simulated, including filling and draining of
the tanks and application of water pulses to the bed.
The operation of the plant was simulated using the parameter values shown in Table 4.3.11. The
dispersion coefficient was determined by carrying out a residence time distribution test on a laboratory
gravel bed using potassium chloride as a salt tracer. The gravel bed was constructed in a cylinder to
0.6 m depth to represent the structure of the DRB matrix. The salt was injected to the water flowing in
the bed in the form of a short pulse. The concentration of salt at the outlet was monitored using a
conductivity probe. A dispersion model was fitted to the recorded residence time distribution curve in
order to obtain a numerical value of the dispersion coefficient shown in Table 4.3.11. The rate
constant was estimated from an experiment in which the standard BOD5 test was modified, and
dissolved oxygen of the samples recorded on a daily basis from 1 to 14 days. A first order rate
equation was fitted to the data in order to obtain the numerical value of the rate constant shown in
Table 4.3.11.
Table 4.3.11. Input parameters for simulation of DRB behaviour.
Bulk density
1300 kg m-3
Static hold up βs
Fluid density
1000 kg m--3
Dynamic hold up βd 0.15
Rate Const
3 × 10-6 s-1
Gas viscosity
Dispersion coefficient 3.5 × 10-5 m2s-1 Liquid viscosity
4.3.5.2
Voidage
0.35
Bed area
1 m2
Gravel particle size
0.102
1 × 10-5 Pa s
1 × 10-3 Pa s
5 mm
Model validation: Model 4
Figure 4.3.18 shows the predicted BOD5 levels in each tank and at the DRB outlet, plotted as a
function of time. Initial conditions were assumed such that each tank in the DRB system contained
water with a BOD5 of 20 mg/l and that the DRBs themselves contained clean water, because of resting
whilst not in use. It is observed from Fig. 4.3.18 that the predicted concentrations in the tanks
fluctuate significantly, particularly in tank 2, as pulses of DW and recycled water are introduced. The
raw DW introduced has a BOD5 of around 15 mg/l, which is lower than the BOD5 of water initially in
tank 2. This situation is intended to represent the start up conditions of a DRB in mode 1 of operation,
when the effluent from the IAP-HRTF had a low BOD5. Consequently, the simulation shows how the
DRB responded dynamically during the start up period. As shown from Fig. 4.3.18, it takes about 2
156 of 217
days for the BOD5 in tank 2 to stabilise within steady limits, though it continues to cycle within these
limits on a daily basis depending on the stage of the operating cycle.
The BOD5 concentrations of the water in tanks 3, 4 and 5 decrease with time, the decrease in tanks 4
and 5 being progressively slower than in tank 3 as a result of the longer time taken for treated water to
reach these tanks as it flows through the system. At first, the BOD5 at the DRB outlets increases with
time as clean water from the resting period is flushed out; thereafter, a peak is reached as the stronger
DW initially held in the storage tanks is flushed through. Finally, the outlet concentration from the
beds stabilises within limits. Even so, the concentration of the outlet water from the beds is dependent
upon the stage of tidal operation. Careful consideration must be given to the times at which samples
are collected from the tanks and DRB outlets, due to the unsteady nature of the process. Ideally,
samples should be collected at the same time after the operating sequence commences. Simulations for
greater influent BOD5 concentrations typically observed in this project were also carried out and the
results are reported below.
Fig. 4.3.18 Model predictions of BOD5 concentrations at the outlets of the DRBs and tanks
Figure 4.3.19 shows the simulated and experimental BOD5 concentrations of the outlet wastewater
from the DRB system for the entire period of plant operation. It is observed that the predicted outlet
BOD5 from the dynamic model agrees reasonably well with the experimental BOD5 values recorded
by Direct Laboratories whilst operating in Modes 1, 3 and 4, (i.e. with pre-treatment of the DRB input
flow). For operation in Mode 2, with feed of raw DW, the model predicts a much larger BOD5
removal than occurred in practice. One reason for the difference may be that the removal rate constant
decreases during the winter months because of cold weather and die-back of the reeds.
157 of 217
1000
900
Experiment
Model
800
Model (reduced k)
BOD5, mg/l
700
Model (reduced k)
600
500
400
300
200
100
0
01-May-03
09-Aug-03
17-Nov-03
25-Feb-04
04-Jun-04
12-Sep-04
21-Dec-04
31-Mar-05
Date
Fig. 4.3.19 Model predictions of BOD5 concentrations in the treated effluent from the DRB
Simulations were carried out with reduced values of the rate constant for a selected period of Mode 2
data, and it is noted that the BOD5 removal is very sensitive to the value of the removal rate constant
(k). It is observed from Fig. 4.3.19 that the predictions are improved when k is decreased to 20 % of
its value in summer, that is k = 6 × 10-7 s-1. However, caution must be applied in adopting this
approach, as during the winter period the DRBs also suffered from the blockage problems mentioned
earlier. The model of Sun et al (1998) was also applied to the Pallinghurst Farm data, and as seen in
Fig. 4.3.20, predicts the BOD5 removal reasonably accurately over the whole period of operation.
Therefore, although a dynamic model is useful for studying the response of the DRBs to process
disturbances, a simpler time averaged model such as the one developed by Sun et al (1998) is more
suitable for the design and sizing of DRBs.
1000
900
Experimental
800
Sun et al model
BOD5 , mg/l
700
600
500
400
300
200
100
0
01-May-03
09-Aug-03
17-Nov-03
25-Feb-04
04-Jun-04
12-Sep-04
21-Dec-04
31-Mar-05
Date
Fig. 4.3.20 Predictions of BOD5 concentrations in the treated effluent from the DRB, using the
model of Sun et al (1998)
158 of 217
4.3.5.3
Results: design and estimated costs of a full-scale DRB system
The results shown above suggest that both the dynamic model and that of Sun et al (1998) could be
used to estimate the total DRB area required to achieve a given reduction in BOD5 concentration,
provided that pre-treatment of the DW achieved an average BOD5 of 500 mg/l or less in the effluent
supplied to the DRB. In the interests of convenient use of these calculations, the present version of
Model 4 was produced as a spreadsheet and therefore it was more appropriate to use the model of Sun
et al (1998) because this required les sophisticated mathematical techniques to complete the design
calculations.
Table 4.3.12 shows the results of these calculations to specify a full scale, single-stage DRB system to
deal with pre-treated DW at Pallinghurst Farm. The calculations were applied conditions typical of
those observed at Pallinghurst Farm, representing the range of BOD5 and TAN concentrations
expected in effluent from either the OFP or the CSTR, as described above. In each case, the flow of
treated DW was assumed to be 25 m3/d and each DRB design was specified to achieve BOD5 and
TAN concentrations in the treated effluent of 20 mg/l and 20 mg/l respectively. The values for “total
DRB area” calculated using the model of Sun et al, 1998 are compared with equivalent areas
estimated independently by ARM Ltd, based on the required oxygen transfer rate in the system. It is
notable that the two calculations agree to within 5 %. Cost estimates of the DRB systems were
provided by ARM Ltd, and are also shown in Table 4.3.12. These cost estimates do not include pipe
work, pumps and removal of spoil from the site. As an approximate guide, these costs may be
estimated as 20 % of the costs of the DRB (Cooper, 2005), and are included in the installed costs in
the final row of Table 4.3.12
Table 4.3.12 Estimated areas and costs for full scale (25m3/d) DRB systems appropriate to
Pallinghurst Farm.
Assumed inlet BOD5
500 mg/l
250 mg/l
100 mg/l
Assumed inlet TAN
500 mg/l
250 mg/l
100 mg/l
Calculated total DRB area (Sun et al, 1998)
2029 m2
972 m2
338 m2
Calculated total DRB area (Cooper, 2005)
2120 m2
1020 m2
350 m2
Cost of DRB (Cooper, 2005)
£250,000
£125,000
£55,000
Installed cost to include pipe work & pumps (assumed to be
20% of cost of DRB)
£300,000
£150,000
£66,000
As shown in Section 3.3, the experimental DRB used at Pallinghurst Farm performed best as a twostage process. In this case, the overall performance is represented by including a squared term, as
follows:

Vf
C2 
=

C 0 V f + 0.09175 Ah h(1 − exp(− 0.272t 0 )) 
2
Equation 4.3.8
where C2 is the concentration of effluent from the second stage. The above equation can be rearranged
to find the area required as:
159 of 217

C2 

V f 1 −

C
0


Ah =
C
0.09175h 2 (1 − exp(− 0.272t 0 ))
C0
Equation 4.3.9
Hence, Model 4 used the above equation to determine the required dual-stage DRB surface area and
costs for tertiary treatment of the predicted effluent from the CSTR, as shown previously in Figs
4.3.14 and 4.3.15. The resulting week-by-week predictions showed that the material had an annually
averaged BOD5 concentration of 232 mg/l with average and peak flow rates of 21 and 25.7 m3/day
respectively. In this case, the specification was designed to achieve an output BOD5 concentration of
30 mg/l. Although TS removal was not modelled explicitly, comparison with Table 3.3.1 implied that
50% removal should be possible, leading to a predicted average TS concentration of about 2400 mg/l
in the treated effluent from the second stage of the DRB .
The resulting specification included two DRB stages, each of 177 m2 surface area, with 117 flushes
per week, giving a period of 1.4 hours between flushes. Model 4 predicted that between September
and April, each flush would average about 1.5 m3 in volume, whilst during the summer period, this
would fall to 0.6 m3. It was also predicted that the annually averaged BOD5 concentration produced
from the second stage of this system would be in the region of 22 mg/l, which was based on a values
of approximately 30 mg/l from September to April, and of 2 mg/l during the summer. Comparing
these values with those listed in Table 4.3.12 shows that the dual-stage DRB system was able to
achieve an equivalent performance using a smaller total bed area.
In order to calculate the overall treatment costs of the dual-stage DRB, the capital costs were estimated
by interpolation of the data presented in Table 4.3.12, as shown in Fig. 4.3.21, plus the 20% allowance
for the additional costs of installation. The following assumptions were made in estimating the costs
of the DRB:
•
Land costs are excluded,
•
Labour charges for supervising the process are not included,
•
Costs of transporting the DW to the DRB site and of spreading the treated DW are not included,
•
The provision of additional power supplies is excluded, and
•
The costs of a supply reservoir for the DRB (necessary for use with an OFP system) are
excluded. Use with a CSTR would not require a reservoir due to the continuous operation of
the CSTR.
Hence, the total capital cost of the 2 * 177 m2 DRB system was estimated to be £61,500, i.e.
approximately £143 per cow. The total land area needed for a dual DRB system of this capacity at
Pallinghurst Farm would be in the region of 400 to 500 m2.
160 of 217
£300,000
capital cost, £
£250,000
£200,000
y = 111x + 12053
£150,000
£100,000
£50,000
£0
500
1000
1500
2
DRB area, m
2000
2500
Fig. 4.3.21 Estimated of the capital costs of DRB installations, as detailed in Table 4.3.12
The running costs of DRB systems are expected to be much lower that the CSTR. As the pumps only
have to supply a head of approximately 2 m, the required power ratings of the pumps are typically
only 0.5 kW. These pumps operate for short periods to supply the feed pulses on to the DRBs, and are
not in continuous operation. Table 4.3.13 summarises the calculations of running costs included in
Model 4, leading to an estimated annual running cost for the 2 * 177 m2 DRB system of £134 per year.
Table 4.3.13 Summary of data used by Model 4 to calculate DRB running costs for tertiary
treatment of effluent from the CSTR – predictions for Pallinghurst Farm
System properties
Value
Pulse duration, minutes
10
Number of pulses per day
24
Pressure head, m
2
3
Density of DW, kg/m
1000
Pump efficiency, %
50%
Electricity cost, £/kWh
0.08
Number of pumps used
5
Recycle ratio, factor of input flow, (i.e. a recycle ratio of 4
means that the total flow is 5 times the input flow)
4
Yearly cost per pump, £
26.7
Yearly cost for all pumps, £
134
User input data are shown in shaded cells
4.3.6 Application of DW-STOP Model 5: Intensive Aeration: High Rate Trickling Filter for
secondary / tertiary treatment
4.3.6.1
Methods
As for the CSTR part of the IAP, the performance of the HRTF was modelled by a week-by-week
mass balance analysis of the 7 separate flows that comprised this part of the system. This approach
was also based on the equations, assumptions and coefficients described in Section 4.3.4.1. The
161 of 217
necessary coefficients to describe the performance of the HRTF were determined by comparison of the
measured characteristics of the effluent supplied to the HRTF (Stream 7) with those of the final treated
effluent (Stream 14).
4.3.6.2
Model validation: Model 5
The values of the coefficients for removal of BOD5, to achieve the “best fit” between the predicted and
measured concentrations in Stream 14 were: A: 0.25 and B: -0.1. Compared with the coefficients
determined for the CSTR (Section 4.3.4), this larger value of A reflected the smaller percentage
reduction in BOD5 concentration that was achieved in this stage of the process. As previously noted,
this was due largely to the effectiveness of the CSTR, which often left only small concentrations in the
supply to the HRTF (Stream 7). The negative value determined for B resulted from the numerical
procedure used to establish a “best fit” between the predicted and measured data, but in effect,
introduced a term in the model that represented a constant degree of BOD5 removal that was
independent of the concentration in Stream 7. As such, it may represent, to some extent, the “filter”
characteristics of the HRTF.
The results of applying the coefficients and equations described above are illustrated in Figs. 4.3.22
and 4.3.23. Overall, the predicative accuracy of the model was disappointing, possibly suggesting
that other factors need to be included such as temperature, as described in Section 3.2.3.
Measured BOD5 in HRTF outlet, mg/l
600
y = 0.7875x
500
2
R = -0.0496
400
300
200
100
0
0
100
200
300
400
500
600
Measured BOD5 in outlet from HRTF, mg/l
Fig. 4.3.22 Comparison of predicted and measured BOD5 concentrations in the treated
effluent from the HRTF, (i.e. Stream 14)
162 of 217
300
Measured BOD5
Predicted BOD5
BOD, mg/l
200
100
0
28-Feb-03
28-Jun-03
26-Oct-03
23-Feb-04
22-Jun-04
20-Oct-04
17-Feb-05
17-Jun-05
Date
Fig. 4.3.23 Serial data comparison of predicted and measured BOD5 concentrations in
the treated effluent from the HRTF, (i.e. Stream 14)
A similar procedure for fitting coefficients was adopted to achieve predictive equations for both TS
and TSS. As for the CSTR, the coefficients for TS listed in Table 4.3.6 described the general trends
observed in Stream 14, although, as before the predicted TS and TSS values appeared, at times, to
pre-empt the measured values by between one and two weeks (Figs 4.3.24 and 4.3.25). The predicted
values for both TS and TSS feature a number of “spikes” where predictions greatly exceeded
measured values. These arose where high values were recorded in Stream 7, and, by proportion, these
were reflected in the subsequent predictions.
10000
TS, mg/l
8000
6000
4000
2000
Measured TS
0
22-Jun-04
20-Oct-04
Predicted TS
17-Feb-05
17-Jun-05
Date
Fig. 4.3.24 Serial data comparison of predicted and measured TS concentrations in the
treated effluent from the HRTF, (i.e. Stream 14)
163 of 217
4000
TS, mg/l
3000
Measured TSS
2000
Predicted TSS
1000
0
22-Jun-04
20-Oct-04
Date
17-Feb-05
17-Jun-05
Fig. 4.3.25 Serial data comparison of predicted and measured TSS concentrations in
the treated effluent from the HRTF, (i.e. Stream 14)
4.3.6.3
Results: design of a full-scale HRTF system
As for Model 3, the present version of Model 5 was produced in spreadsheet form, based on the
parameters listed above in Section 4.3.6.2. This was used to determine the key system dimensions,
assuming that the HRTF would treated effluent from the CSTR. Hence these dimensions were based
on the CSTR effluent production data presented in Fig. 4.3.14, and the DW properties predicted in
Figs 4.3.15 and 4.3.16. The calculations also required a number of user-defined inputs to indicate the
required dimensions and capacities of certain key components of the HRTF. These are detailed in
Table 4.3.14, together with the system specifications calculated by Model 5. Overall, a HRTF system
suitable for Pallinghurst Farm would require between 200 and 300 m2 of land area, and the installed
power requirement would be approximately 6 kW.
Table 4.3.14 HRTF Specification for Case Study 1, Pallinghurst Farm, including both userdefined inputs and specifications calculated by Model 5
System properties
Residence time in HRTF sump, days
Value
0.5
HRTF sump freeboard, m
0.40
HRTF and sump diameter, m
Working volume of HRTF, sump m3
Total volume of HRTF sump, m3
10.0
12.2
43.6
HRTF sump overall height, m
HRTF volume: m3 of tank capacity
per unit flow of DW (m3/d)
HRTF packed column freeboard, m
0.56
10.0
1.0
HRTF packed column working
volume, m3
HRTF packed column overall height,
m
HRTF packed column total volume,
m3
System properties
HRTF
packed
column
circulation time, min
HRTF
packed
column
circulation rate, m3/d
HRTF
packed
column
circulation rate, l/min
HRTF
packed
column
circulation head, m
HRTF
packed
column
circulation hydraulic energy, W
HRTF packed column pump
efficiency
HRTF
packed
column
circulation input energy, kW
HRTF
packed
column
circulation energy use, kWh/d
Value
5.0
3512
2439
4.7
1858
50%
3.7
89.2
244
4.11
Tank 9 feed pump flow rate,
l/min
Tank 9 feed pump power, kW
322
164 of 217
30
1.5
System properties
HRTF packed column + sump overall
height, m
Av Fraction of BOD5 concentration
removed by HRTF
Av Fraction of TS concentration
removed by HRTF
Value
4.7
0.6
System properties
Tank 9 feed pump average duty,
h/day
Tank 9 feed pump average
energy use, kWh/d
Tank 9 residence time, days
0.01
Value
10.7
16.0
2.0
Required Tank 9 volume, m3
48.8
Sludge removal in tank 9 (% of
HRTF output flow)
Tank 9 sludge pump flow rate,
l/min
Tank 9 sludge pump power, kW
5%
5.0
0.75
Tank 9 sludge pump average
duty, h/day
Tank 9 sludge pump average
energy use, kWh
3.3
2.5
User input data are shown in shaded
cells
As described in Section 3.2.1, the output flows from the HRTF and its subsequent settlement stage
were expected to be approximately 5% lower than the input flows to the HRTF, i.e. 5% lower than the
CSTR output flows, as shown previously in Fig. 4.3.14. Based on these flows, the Model 5
calculations predicted the week-by-week BOD5 and TS concentrations in the output flows from the
HRTF. Hence, it was predicted that the annually averaged BOD5 concentration produced from the
HRTF would be in the region of 93 mg/l, which was based on a values of between 110 and 120 mg/l
from September to April, and of 27 - 29 mg/l during the summer. Applying this average BOD5
concentration to Model 1 indicated that spreading of this DW on other land at Pallinghurst farm would
lead to concentrations of between about 50 and 90 mg/l in the resulting combined run-off and leachate.
The predicted TS values were largely unchanged from those in the CSTR output flows, although the
predicted TN and TAN concentrations were reduced by approximately 65% and 89% respectively,
reflecting the particular capability of this system to remove surplus nitrogen from DW.
4.3.6.4
Results: estimated costs of a full-scale HRTF system
Costs of the example system described above are listed in Table 4.3.15, which was based on the
following key assumptions:
•
Land costs are excluded,
•
Labour charges for supervising the process are not included,
•
Costs of transporting the DW to the HRTF site and of spreading the treated DW are not
included, and
•
The provision of additional power supplies is excluded.
As for the CSTR, the values given in Table 4.3.15 for the HRTF are based on a design life of 15 years.
However, it is anticipated that the pumps, would require replacement at least once during this period,
although these costs are not included in the values given.
165 of 217
Table 4.3.15
Summary of costs for tertiary treatment of effluent from the CSTR at
Pallinghurst Farm (Case study 1), using a HRTF
Tanks
HRTF sump volume, m3
HRTF packed column total
volume, m3
Third settling tank volume (tank
9), m3
Packing materials, m3
TOTAL CAP COST - tanks
Pumps
HRTF packed column circulation
pump, kW
Tank 9 feed pump power, kW
Tank 9 sludge pump power, kW
TOTAL CAP COST - pumps,
etc
Energy costs
HRTF packed column circulation
pump, kWh/d
Tank 9 feed pump power, kWh/d
Tank 9 sludge pump power,
kWh/d
TOTAL energy use, kWh/d
Electricity cost, £/kWh
TOTAL energy cost, £/year
Installation cost factor
(factor times all other cap costs)
Total investment cost
Capacity,
m3
12.2
322
Capital
cost
£ 4,065
£ 64,485
48.8
3.7
£ 16,259
£ 15,837
£ 100,645
Capital
cost
£ 2,327
1.5
0.75
£ 1,295
£ 945
244
Power, kW
Comments
Sump costs are based on an
average erection cost of £333/
m3, whilst packed column costs
are based on £200/m3. Packing
materials are based on £65/m3
The capital costs of the stirrers,
pumps and blowers are all based
on the relationship shown in Fig.
4.3.17
£ 4,567
Energy
required,
kWh/d
89.2
Running
costs
Comments
Electricity charged at £0.08/kWh
The effects of changes in
electricity prices can be assessed
pro-rata.
16.0
2.5
107.7
0.08
£ 3,146
Comments
This factor includes an allowance
for specialist services and process
commissioning
1.3
£ 136,775
Total capital costs of the HRTF system are therefore in the region of £137,000, thus equating to a
capital cost of about £319 per cow, plus annual running costs of over £3000. Hence, the HRTF is
clearly more expensive than the equivalent DRB system described in Section 4.3.5.
4.3.7 Application of the DW-STOP Models: comparison of results
4.3.7.1
Comparisons of benefits
Based on the results and model production presented in Sections 4.3.2 – 4.3.6, it is clear that there are
two treatment options able to deal effectively with raw DW at Pallinghurst Farm: the CSTR or the
OFP. Their key operational differences are summarised in Table 4.3.16.
166 of 217
Table 4.3.16 Summary comparison of alternative systems to treat raw DW at Pallinghurst Farm
Aspect of
performance
Land area required.
Mode of operation
Expected average
BOD5 concentration in
treated effluent, mg/l
Expected average TS
concentration in
treated effluent, mg/l
Total installed power
requirement, kW
Cold weather
operation
Maintenance and
supervision
Life expectancy
Other impacts
OFP
CSTR
200 to 300 m2
At least 1.1 ha, plus additional areas
for buffer reservoirs
One-week batch treatment
275 i.e. suitable for subsequent
tertiary treatment
Continuous operation
232 i.e. suitable for subsequent
tertiary treatment
1449
4796
3.3
17.0
Operations would cease in freezing
conditions to avoid damage to the
treatment planes. Extra capacity
would be required in the buffer
reservoirs to store DW until
operations resume.
Daily supervision by farm staff, plus
weekly attention to refill the
reservoirs and to detect any leakage
or spillage. Periodic cutting of
vegetation on the treatment planes
5 years (depending on rate of
phosphorus accumulation)
Pipes and pumps can be protected
with trace heating and insulation
enabling continued treatment in
freezing conditions. However,
land spreading of treated DW may
be curtailed
Daily supervision by farm staff,
plus either remote twice-weekly
monitoring or monthly service
visits by specialist technicians.
Low profile, but lagoon-style
reservoirs would require safety
fencing, etc. Quiet operation
15 years (depending on life of
tanks). Intermediate replacement
of pumps, etc would be necessary
Security fencing required to
prevent unauthorised access to
automatic machinery. Noise levels
from blowers could be a problem
to neighbours
Similarly, either the DRB or the HRTF were found to be suitable for tertiary treatment of DW from
either the OFP or the CSTR. The properties of these alternative tertiary treatment systems are
summarised in Table 4.3.17.
Table 4.3.17 Summary comparison of alternative systems to treat raw DW at Pallinghurst Farm,
both treating effluent from the CSTR
Aspect of
performance
Land area required
Mode of operation
Expected average
BOD5 concentration in
treated effluent, mg/l
Expected average TS
concentration in
treated effluent, mg/l
Total installed power
DRB
HRTF
400 to 500 m2
Continuous operation with recycle
22 i.e. suitable for subsequent
tertiary treatment
200 to 300 m2
Continuous operation
93 i.e. suitable for subsequent
tertiary treatment
2400 (estimate)
4748
2.5
6.0
167 of 217
Aspect of
performance
requirement, kW
Cold weather
operation
Maintenance and
supervision
Life expectancy
Other impacts
4.3.7.2
DRB
Operations would cease in
prolonged freezing conditions.
Daily supervision by farm staff,
plus weekly attention to detect any
leakage or spillage and to maintain
percolation of DW through the
DRBs.
15 years. Intermediate replacement
of pumps, etc would be necessary
Low profile with some visual
impact due to reed growth. Quiet
operation.
HRTF
Pipes and pumps can be protected
with trace heating and insulation
enabling continued treatment in
moderate freezing conditions.
However, prolonged freezing could
prevent operation and land
spreading of treated DW may be
curtailed.
Daily supervision by farm staff,
plus either remote twice-weekly
monitoring, or monthly service
visits by specialist technicians.
15 years (depending on life of
tanks). Intermediate replacement of
pumps, etc would be necessary
Security fencing required to prevent
unauthorised access to automatic
machinery. Some noise levels from
pumps and discharge of DW
Comparisons of costs
Tables 4.3.18 and 4.3.19 summarise the costs of treating DW at Pallinghurst Farm as discussed and
presented in Sections 4.3.2 – 4.3.6. These costs are in addition to those of handling and spreading
DW, and include illustrations of the sensitivities of these costs to an increase of 10% in the size of the
milking herd at Pallinghurst Farm. It was assumed that this herd increase in the herd size would be
accommodated without increasing the size of existing buildings and therefore rainfall amounts are
unchanged, leading to slightly higher concentrations in the raw DW. Whereas Table 4.3.18 addresses
reductions in BOD5 concentration of about 90%, Table 4.3.19 considers ways to meet tighter target
specifications for the discharge to land of treated DW, and/or opportunities for re-use of treated DW,
as described in Table 4.1.2.
Single process treatment options are included in Table 4.3.18, in which the CSTR is compared with
the OFP. This comparison shows that, with a herd of 430 cows, if the OFP option includes plastic
membrane liners to isolate the treatment planes hydrologically, the simple annual overall treatment
cost of the OFP will be greater than the CSTR by about £42 cow-1 year-1. However, if the appropriate
authority allowed the use of compacted subsoil clay instead of a liner, the OFP option would become
cheaper than the CSTR by approximately £35 cow-1 year-1. Herd expansion by 10% would reduced
the cost per cow of this cheapest option by about 6%.
Table 4.3.18 Example comparative output from the DW-STOP mathematical models: Case
study 1: specification of least cost systems to reduce pollutant concentration by 90%
System Properties
Current herd
No of cows in milking herd
Annual milk production, million litres
430
2.7
168 of 217
“What if” the
milking herd
were 10%
larger?
473
2.9
System Properties
Current herd
7600
3300
7000
IAP(CSTR only)
“What if” the
milking herd
were 10%
larger?
8000
3400
7200
IAP(CSTR only)
% reduction in BOD5 before spreading
% reduction in TS before spreading
Capital cost, £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
93%
31%
£296,000
15
£6,759
£20,184
£47
£2.66
OFP
93%
31%
£313,630
15
£7,160
£21,386
£45
£2.67
OFP
% reduction in BOD5 before spreading
% reduction in TS before spreading
Capital cost, £
Capital cost (without OFP membrane liner), £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Data relevant to an OPF constructed without a plastic
membrane liner are shown in shaded cells
92%
79%
£190,000
£25,000
5
£360
£38,072
£89
£5.01
£5,072
£11.80
£0.67
92%
79%
£206,000
£26,000
5
£380
£41,276
£87
£5.16
£5,276
£11.15
£0.66
Annual DW applied to land, m3
Average input BOD5
Average input TS
Dual process treatment options are included in Table 4.3.19, in which use of DRB systems are
compared downstream of both a CSTR and an OFP. As in Table 4.3.18, this comparison shows that,
with a herd of 430 cows, inclusion of plastic membrane liners in the OFP option makes this approach
expensive, with a simple annual overall treatment cost of £101 cow-1 year-1, for the combined OFP +
DRB system. This compares with £57 cow-1 year-1 for the combined CSTR + DRB system. However,
if the OFP liner can be replaced by indigenous compacted clay, the treatment cost of the OFP +DRB
option would fall to approximately £24 cow-1 year-1. Herd expansion by 10% would reduce the cost
per cow of this cheapest option by about 4%.
169 of 217
Table 4.3.19 Example comparative output from the DW-STOP mathematical models: Case study 1: Specification of least cost systems to allow re-use of
DW for yard and machine washing: pollutant concentration reduced by 99%
System Properties
% reduction in BOD5 before re-use
% reduction in TS before re-use
Capital cost, £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Annual volume of treated DW collected, m3/yr
Simple annual total cost per m3 DW recycled, £/yr
% reduction in BOD5 before re-use
% reduction in TS before re-use
Capital cost, £
Capital cost (without OFP membrane liner), £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Annual volume of treated DW collected, m3/yr
Simple annual total cost per m3 DW recycled, £/yr
Current herd
PAL IAP(CSTR) + DRB.
IAP(CSTR)
DRB
TOTAL
93%
90%
99%
31%
20%
45%
£296,000
£61,500
£357,500
15
15
15
£6,760
£350
£7,110
£24,307
£57
£3.20
7600
£3.20
PAL OFP+ DRB.
OFP
DRB
TOTAL
92%
93%
99%
79%
20%
83%
£190,000
£80,000
£650,000
£25,000
£80,000
£155,000
5
15
15
£360
£510
£870
£43,391
£101
£5.71
£10,391
£24
£1.37
11700
£0.89
170 of 217
“What if” the milking herd were 10% larger?
PAL +10% IAP(CSTR) + DRB.
IAP(CSTR)
DRB
TOTAL
93%
91%
99%
31%
20%
45%
£313,630
£65,600
£379,230
15
15
15
£7,160
£370
£7,530
£25,784
£55
£3.22
8100
£3.18
PAL+10% OFP+ DRB.
OFP
DRB
TOTAL
92%
93%
99%
79%
20%
83%
£206,000
£85,600
£703,600
£26,000
£85,600
£163,600
5
15
15
£380
£540
£920
£46,968
£99
£5.87
£10,968
£23
£1.37
12400
£0.88
4.4 CASE STUDY 2 - NORTH BREAZLE FARM (~ 150 COWS) EXISTING
ARRANGEMENTS (TC, CoB, PL-H, DCh, JW, KS, IM, JG))
4.4.1 Description of North Breazle Farm
4.4.1.1
Location and general features
North Breazle Farm is located in Devon, between Okehampton and Launceston, on the eastern side of
Roadford Reservoir (Fig. 4.4.1), and is between 150 and 170 m above sea level. Annual rainfall is
about 1250 mm, and typically, about two-thirds of this falls between October and May.
Fig. 4.4.1 Regional location of North Breazle Farm
The farm is owned and run by Mr C Gaden and family. It covers approximately 81 ha of heavy, culm
measure soil. Approximately 90% of the farm is in permanent pasture. No maize or other forage
crops are grown. The farm supports a dairy herd comprising, on average, 120 cows in milk plus 35
dry cows, with 960,000 litres of milk quota (i.e. about 6200 l cow-1 year-1). In addition, the herd
includes about 50 calves and 60 dairy herd replacements. Up to 40 beef animals are also kept on the
farm. All dairy and beef animals are reared on the farm (i.e. no animals are reared on other sites).
4.4.1.2
Herd management: calving patterns, grazing and herd replacement
A year-round calving pattern is followed at North Breazle Farm and so, unlike Pallinghurst Farm,
there is no “dry period” without milk production.
Based on the above herd statistics, the annual
replacement rate of cows within the herd is between 25% and 30% (i.e. 60 young stock, assumed to be
between 6 months to 24 months in age, in a total herd of 155). None of the livestock is out-wintered;
the typical housing period is from mid October until early April.
4.4.1.3
Buildings and housing management for in-milk cows
Almost all of the in-milk cows at North Breazle Farm are housed in cubicles fitted with automatic
scrapers that deliver slurry to a system of underground channels and pits (Fig. 4.4.2). These convey
the slurry by gravity, to a weeping wall store of 1478 m3 capacity (Fig. 4.4.3). To assist this flow of
slurry, DW is stored in a 1179 m3 cylindrical tank (Fig. 4.4.4) and is discharged by gravity, when
necessary, into the channels and pits to flush them clear. The design of this system means that DW
171 of 217
may be collected and spread from either the cylindrical tank (via an adjacent reception pit), or from a
leachate collection channel surrounding the weeping wall store. This discharges leachate (which is
regarded as DW) to a “conventional” three-tank settlement system, with a centrifugal pump located in
the third of these tanks, which is covered by a concrete pump house. This pump delivers the DW to a
system of underground pipelines leading to hydrants for connection of static sprinklers. If required,
the centrifugal pump can deliver DW to the cylindrical tank, although this is rarely done.
Above-ground steel store erected
Weeping-wall slurry store
1986, estimated by Mr Gaden to
provide 6-7 weeks storage for DW
New building including
dairy parlour and
covered collecting yards
Three-tank DW
system plus
centrifugal pump
and pipeline with
static sprinklers
Collection tank
for weeping-wall
leachate
Silage store
Cubicle building
100 m
Fig. 4.4.2 Annotated aerial view of the principal farm
buildings at North Breazle Farm
Fig. 4.4.3 Weeping wall slurry store at North Breazle Farm
At the time of the visit to the farm, June 2005, Mr Gaden noted that the cylindrical tank had
accumulated a substantial amount of settled solid matter, such that the remaining useable volume
provided only sufficient storage capacity for 6-7 weeks’ production of DW during the winter housing
period.
172 of 217
The cows are milked in a herringbone parlour, which was commissioned in 2004. Substantial amounts
of washing water are used in this area; in 2004, this was estimated to be approximately 2.7 m3/day, and
thus almost equalled the estimated volume of slurry produced by the in-milk cows. Dairy hygiene
also requires the daily use of 2.0 l of sodium hypochlorite, 2.0 l of phosphoric acid and 1.4 l of
formalin (used in a footbath). All wash water from the dairy parlour is collected in the combined
slurry/DW system.
Calves and young stock are housed in strawed yards. The cubicles and yards are bedded with chopped
straw during the housing period. Straw use in the cubicles amounts to about 2 tonnes / week and the
total rate of straw use is between 10 and 12.5 tonnes / wk
Fig. 4.4.4 Above-ground steel store for DW next to the dairy parlour and cow cubicle
buildings at North Breazle Farm
4.4.1.4
Management of slurry, DW and other farm operations
Most farm operations (e.g., cultivations, drilling, silage harvesting, cereal harvesting, and manure and
DW management) are undertaken by members of the family. Contractors are sometimes used to
assists with removal of residual solids from the weeping wall store.
4.4.2 Engineering appraisal of the existing systems for slurry and DW management at North
Breazle Farm
4.4.2.1
Volumes and flows of DW
The typical annual pattern of production and management of solid manure, slurry and DW is
summarised in Fig. 4.4.5, indicating that the volume of DW discharged annual is approximately 3000
m3. The month-by-month distribution of this DW production is detailed in Table 4.4.1.
173 of 217
Cows on bedding
600 tonne/y of
solid manure to
land
North Breazle Farm
Average herd statistics:
120 cows in milk plus 35
dry cows
In building – 169 days
996 tonne/y of liquid manure
(assumed TS 10%)
Rain water from yard
drainage:
1602 m3/year
Grazing 196
days
Rain on lagoon
800 m3/year
Weeping wall lagoon
1478 m3 capacity
Parlour washings:
977 m3/year
Weeping wall lagoon
solid contents: Solids plus
20% of the added water,
i.e. 996 + 676 = 1672
m3/year to land
Average number of in-milk
cows on slats: 111.
Housing period: mid-Oct
to early April
Rain on store
351 m3/year
Some drained liquid for flushing
returned to above ground DW
store: 1179 m3 capacity.
DW discharged to land: 80%
of the water added to the
lagoon, plus rain on the store,
i.e. 2703 + 351 = 3054 m3/year
Fig. 4.4.5 Schematic diagram of the slurry and DW flows at North Breazle Farm
Table 4.4.1 Month-by-month details of slurry and DW management at North Breazle Farm
Property to be defined
Userdefined
Value
111
Units
Manure per cow per day
53
kg
BOD5 per cow per day
1
kg
TS per cow per day
5
kg
Parlour washings per year
977
tonnes (m3)
Yard rainfall per year
1602
tonnes (m3)
Manure store rainfall per year
800
tonnes (m3)
Amount of water to store retained by solid manure*
20
%
Amount of TS to store retained by solid manure
60
%
Amount of BOD5 to store retained by solid manure
10
%
DW store rainfall per year
351
tonnes (m3)
Number of cows on concrete
* including rainfall to yard and store and parlour washings
The data in Table 4.4.1 were used in DW-MODEL to define the volumetric loads used to specify the
various options for DW treatment at North Breazle Farm, as summarised in Tables 4.4.2 and 4.4.3, and
as described further in Section 4.5.
174 of 217
Table 4.4.2 Prediction of DW production from manure during a typical year at North Breazle
Farm, based on inputs defined in Table 4.4.1
Month
Occupancy
Equivalent
Total
Water content
BOD5
Total solids
%
cows
manure
Total
in DW
Total
in DW
Total
in DW
tonnes
tonnes
tonnes
tonnes
tonnes
tonnes
tonnes
Jan
100
111
176
160
128
3.3
3.0
16.7
3.3
Feb
100
111
176
160
128
3.3
3.0
16.7
3.3
Mar
100
111
176
160
128
3.3
3.0
16.7
3.3
Apr
10
11
18
16
13
0.3
0.3
1.7
0.3
May
10
11
18
16
13
0.3
0.3
1.7
0.3
Jun
10
11
18
16
13
0.3
0.3
1.7
0.3
Jul
10
11
18
16
13
0.3
0.3
1.7
0.3
Aug
10
11
18
16
13
0.3
0.3
1.7
0.3
Sep
10
11
18
16
13
0.3
0.3
1.7
0.3
Oct
50
56
88
80
64
1.7
1.5
8.3
1.7
Nov
100
111
176
160
128
3.3
3.0
16.7
3.3
Dec
100
111
176
160
128
3.3
3.0
16.7
3.3
Mean values
56
90
81
65
1.7
1.5
8.5
1.7
Total values
n/a
1077
975
780
20
18
102
20
Table 4.4.3 Prediction of total DW production from manure and other sources during a typical
year at Pallinghurst Farm, based on Table 4.4.2 and on inputs defined in Table 4.41
Month
Parlour
Rain %
Rain fall
Rain fall
DW
Rain fall
washings
of annual
yard
on store
to spread
on tank
tonnes
tonnes
tonnes
tonnes
tonnes
4.4.2.2
Jan
81
16
256
128
504
56
Feb
81
14
224
112
465
49
Mar
81
10
160
80
388
35
Apr
81
8
128
64
232
28
May
81
6
96
48
194
21
Jun
81
4
64
32
155
14
Jul
81
2
32
16
117
7
Aug
81
4
64
32
155
14
Sep
81
6
96
48
194
21
Oct
81
8
128
64
284
28
Nov
81
10
160
80
388
35
Dec
81
12
192
96
427
42
Mean values
81
8
134
67
292
29
Total values
977
100
1602
800
3504
351
Properties of the DW at North Breazle Farm
175 of 217
Unlike Pallinghurst Farm, which was closely monitored for almost 2 years, the properties of the DW at
North Breazle Farm remain largely unknown. Therefore, North Breazle is typical if the situation that
would prevail on most UK dairy farms seeking to evaluate alternative options for the management of
DW. Naturally, it would be impossible to devise specifications for DW Treatment systems with no
indications of the characteristics of the material to be treated, and therefore the DW at North Breazle
Farm was sampled on 9/6/05 from the first chamber of the three-tank system that received the leachate
from the weeping wall lagoon. The results are included in Figs 4.4.6 and 4.4.7.
In addition, other information was collated concerning the production of DW at North Breazle Farm so
that DW-MODEL was able to predict the DW properties on a week-by-week basis, based on the data
summarised in Tables 4.4.1 – 4.4.3. The results of these predictions for BOD5 and TS concentrations
are shown in Figs 4.4.6 and 4.4.7, including for comparison, synchronised data from Case Study 1.
Generally, the comparisons show good agreement between the predicted and measured average values
and between the corresponding upper 95% confidence limit values. The predicted BOD5
concentration is also within the error band of the samples taken on 9/6/05, although the predicted TS
value is low compared with the sample.
15000
BOD5 mg/l
10000
5000
0
1-Apr-03
18-Oct-03
5-May-04
Date
21-Nov-04
9-Jun-05
Fig. 4.4.6 Serial data comparison of predicted BOD5 concentrations at North Breazle
Farm, compared with those recorded at North Breazle Farm
25000
TS mg/l
20000
15000
10000
5000
0
1-Apr-03
18-Oct-03
5-May-04
21-Nov-04
9-Jun-05
Date
Fig. 4.4.7 Serial data comparison of predicted TS concentrations at North Breazle
Farm, compared with those recorded at North Breazle Farm
176 of 217
4.4.2.3
Current problems to be solved
The key challenge arising from the data presented in Fig 4.4.5 and Table 4.4.1 is to manage the landspreading of the DW produced within the constraints of high rainfall, heavy soil and sloping ground
that characterise North Breazle Farm. These constraints have been noted within the farm’s “Farm
Waste Management Plan”, which is summarised in Fig. 4.4.8, indicating that all of its land is
classified as “High Risk”, and that a significant proportion is unsuitable for spreading solid manure,
slurry, or DW under any circumstances.
Fig. 4.4.8 Summary of the Farm Waste Management Plan for North Breazle Farm
Figure 4.4.9 provides a detailed aerial view of and area of approximately 20 ha surrounding the farm
buildings described above. In particular, it highlights the location of the pump house near to the
weeping wall manure store, and the approximate routes and locations of the underground pipelines and
hydrants. The shaded rectangular area of pasture immediately to the east of the farm buildings defines
an area measuring approximately 140 m * 40 m, that could be used, in principle, to accommodate a
DW treatment facility. However, two key aspects of this area require careful consideration:
•
•
Proximity of a stream along the south-east edge of the area (as denoted by the adjacent nospreading area), and
The substantial slope of the area, as illustrated in Fig 4.4.10.
177 of 217
Fig. 4.4.9 Annotated aerial view of the farm and existing DW distribution system at
North Breazle Farm, including, to the East of the Buildings, an area identified by Mr
Gaden for possible installation of a soil-based DW treatment system
Fig. 4.4.10 View in a south-easterly direction, across an area identified by Mr Gaden
for possible installation of a soil-based DW treatment system at North Breazle Farm
4.4.2.4
“What if?” questions and scenarios
Like Pallinghurst Farm, consideration of future options is also essential in the case of North Breazle
Farm. Therefore, the “What if?” questions and scenarios presented in Table 4.2.1, also apply in this
case.
178 of 217
4.5 CASE STUDY 2 - NORTH BREAZLE FARM (~ 150 COWS)): SPECIFICATIONS AND
COSTS OF ALTERNATIVE DW SYSTEMS (TC, COB, PL-H, DCH, JW, KS, IM, JG)
4.5.1 Application of the DW-STOP Models: overview of approaches
Although the Case Study 2 involves a herd of less than one-third the size of that in Case Study 1, the
Modelling protocol previously defined by Fig. 4.3.1.1, and including models 1-5 remains valid.
Hence, the following sections describe the application and results of using models 1- 5 in connection
with the circumstances prevailing at North Breazle Farm (Case Study 2). Except where key points
differed from the approaches used in connection with Case Study 1, the methods used for Case Study
2 are not described in detail.
4.5.2 Application of DW-STOP Model 1: Field flow
4.5.2.1
Methods
The soil model was used to investigate the ability of the ‘native’ soil at this case study farms to reduce
BOD5 concentrations following the application of DW either prior to treatment or after treatment by
the IAP (CSTR only) or by the OFP + DRB. The model was run on the current herd and 10%
increased herd size scenarios. Table 4.5.1 summarises the BOD5 and TS concentrations of the DW for
these different scenarios for this case study farm.
Table 4.5.1 DW BOD5 and TS concentration for the model scenarios runs with different herd
sizes and treatment systems for case study 2 (North Breazle Farm).
Treatment system
Current herd
+10% herd size
BOD5 (mg/l)
4200
4500
TS (mg/l)
9300
10000
90% reduction BOD5 (mg/l)
420
450
90% reduction TS (mg/l)
930
1000
92% reduction BOD5 (mg/l)
336
360
92% reduction TS (mg/l)
744
800
95% reduction BOD5 (mg/l)
210
225
95% reduction TS (mg/l)
465
500
BOD5 (mg/l)
42
54
TS (mg/l)
93
100
No treatment
IAP 90% reduction
OFP+DRB 99% reduction
Case study 2 (North Breazle Farm) operates on clay loam soil types. Therefore, this soil input to the
model was used. The model was run under four different initial soil moisture conditions, very wet,
wet, moist and dry.
The model has not been designed to predict the removal of TS from DW applications. However, from
the empirical data collected from the diamond lysimeters at the Pallinghurst site, it was possible to use
179 of 217
the average reduction in TS concentration for to estimate the TS concentrations leaving the applied
area. Because the soil textures are the same, it was assumed that the soils reduce the TS to the same
extent at both sites, then these reductions can be used in both case studies. The average percentage TS
reductions for the raw and treated DW were 40% and 25%, respectively.
The temperature was set at 10oC for all the model runs. The model is not particularly sensitive to
temperature variations in its present form. The slope of the fields receiving DW at North Breazle was
assumed to be 5%.
4.5.2.2
Results
Tables 4.5.2 and 4.5.3 summarise the outputs of the model for the range of soil moisture scenarios for
case study 2 farm under the current herd and 10% increased herd size, respectively. Although the
model still overestimates the predicted BOD5 concentration leaving the applied area, it is clearly
apparent that application of untreated DW from both herd size scenarios could result in significant
concentrations of BOD5 reaching watercourses. As for case study 1, these model runs only provide
concentrations of BOD5 in effluent at the edge of the applied area, and, compliance with the Code of
Good Agricultural Practice for the Protection of Water (Defra, 2001), would provide a 10 m strip of at
least 10 m between the applied area and any water course, ditch etc. This would provide greater
potential for BOD5 removal before reaching the watercourse.
Table 4.5.2 Model prediction of BOD5 concentrations of effluent leaving the applied area
(combined runoff and leachate concentration) from the current herd scenario.
Treatment system
Current
herd output
Model prediction
Very wet
Wet
Moist
Dry
No treatment
BOD5 (mg/l)
4200
4035
3270
3336
2275
TS (mg/l)
9300
5580
5580
5580
5580
90% reduction BOD5 (mg/l)
420
403
327
334
228
90% reduction TS (mg/l)
930
698
698
698
698
92% reduction BOD5 (mg/l)
336
323
262
267
182
92% reduction TS (mg/l)
744
558
558
558
558
95% reduction BOD5 (mg/l)
210
202
163
167
114
95% reduction TS (mg/l)
465
349
349
349
349
BOD5 (mg/l)
42
40
33
33
23
TS (mg/l)
93
70
70
70
70
IAP 90% reduction
OFP+DRB 99% reduction
Table 4.5.3 Model prediction of BOD5 concentrations of effluent leaving the applied area
(combined runoff and leachate concentration) from the 10% increased herd scenario.
Treatment system
10% greater
herd output
180 of 217
Model prediction
Very wet
Wet
Moist
Dry
No treatment
BOD5 (mg/l)
4500
4323
3503
3575
2438
TS (mg/l)
10000
6000
6000
6000
6000
90% reduction BOD5 (mg/l)
450
432
350
358
344
90% reduction TS (mg/l)
1000
750
750
750
750
92% reduction BOD5 (mg/l)
360
346
280
286
195
92% reduction TS (mg/l)
800
600
600
600
600
95% reduction BOD5 (mg/l)
225
216
175
179
122
95% reduction TS (mg/l)
500
375
375
375
375
BOD5 (mg/l)
54
52
42
43
29
TS (mg/l)
100
75
75
75
75
IAP 90% reduction
OFP+DRB 99% reduction
Application under dry conditions results in lower concentrations of BOD5, although it should be noted
that if there is considerable cracking of the soil, macropore flow may be greater than assumed in the
model in its present form, hence the BOD5 concentration could be greater in the effluent leaving
(combined runoff and vertical movement through the soil) the applied area. There is little difference
in the BOD5 concentration of the combined effluent between the wet and moist soil conditions because
the model simulates similar proportions of infiltration under both sets of conditions in its present form.
Again (as for case study 1), the treatment systems reduced the BOD5 concentration of the DW applied
to the soil. Hence the concentrations were much lower in the effluent leaving the edge of the applied
area when treated DW was applied. The reductions in TS appear to be the same irrespective of the soil
moisture conditions. This is because there was no apparent effect of soil moisture conditions on the
reduction in the TS as measured using the diamond lysimeter data.
4.5.3 Application of DW-STOP Model 2: Overland Flow treatment system
4.5.3.1
Methods
For the North Breazle farm site where the soil has been identified as suitable for an OFP system, but
where rainfall is much higher than the site of Case Study 1 and the land slope is greater the following
design specification was produced.
4.5.3.2
Results: design of a full-scale system
The present version of Model 2 was used to determine the key system dimensions, (i.e. total treatment
plane area and minimum volume per reservoir) based on the following:
•
the DW production data presented in Table 4.4.3,
•
the DW properties predicted in Section 4.4.2.2,
•
the input data shown in Table 4.5.4, and
•
the effects of the net balance of rainfall and evaporation/evapotranspiration on the reservoirs
and treatment planes.
Table 4.5.4 Summary of OFP specification to treat DW at North Breazle Farm
181 of 217
using a one-week batch treatment
System properties
Value
Volume of buffer reservoir (m3)
400
3
Minimum volume of buffer reservoir per treatment plane (m )
341
Depth of buffer reservoir (m)
1.5
BOD5 conc of reservoir at start (mg/l)
100
TS of buffer reservoir at start (mg/l)
1000
Fraction of BOD5 mass removed per week
0.92
Fraction of TS removed per week
0.8
3
Initial (and residual) liquid volumes in Reservoir 1 and 2 (m )
50
3
Volume discharge to DRB per week (m )
all OFP output
Combined area of two treatment planes (ha)
0.75
2
-3
-1
Total area of treatment plane based on 200m m day of DW
for one week
User input data are shown in shaded cells
The Model 2 calculations predicted the week-by-week input and output flows from Reservoirs 1 and 2,
as shown in Fig. 4.5.1. These output flows were equal to the input flows to Reservoir 3, and thus to a
downstream DRB, if used.
400
Vol, m3
200
Vol. aplied to OFP, m3 / wk
0
DW volume added to reservoirs
at start of week, m3
R - ET, m3
-200
0
10
20
30
40
week
50
60
70
80
Fig. 4.5.1 Input and output flows to and from the OFP, plus net rainfall minus evapotranspiration
over a 68 week period at North Breazle Farm
The corresponding week-by-week predictions of output BOD5 and TS concentrations from Model 2
are shown in Figs 4.5.2 and 4.5.3 respectively, based on the initial conditions defined in Table 4.3.4.
As noted for Case Study 1, the data show that when the value of (R-ET) is positive, dilution effects on
the treatment plane are significant, and that when (R-ET) is negative, evapotranspiration effects
predominate. The impacts of both effects can be reduced by selecting an appropriate residual volume
in each Reservoir (50 m3 in this example).
182 of 217
8000
BOD of DW to be
treated, mg/l
BOD5 conc of
treated DW, mg/l
BOD, mg/l
6000
Reservoir 1 running
BOD conc (full), kg
4000
Reservoir 2 running
BOD conc (full), kg
2000
0
0
10
20
30
40
50
60
70
80
week
Fig. 4.5.2 Input and output BOD5 concentrations to and from the OFP, plus changes in BOD5
concentrations in Reservoirs 1 and 2 over a 68 week period at North Breazle Farm
20000
TS of DW to be
treated, mg/l
TS, mg/l
16000
TS conc of treated
DW, mg/l
12000
Reservoir 1 running
TS conc (full), kg
Reservoir 2 running
TS conc (full), kg
8000
4000
0
0
10
20
30
40
50
60
70
80
week
Fig. 4.5.3 Input and output TS concentrations to and from the OFP, plus changes in TS
concentrations in Reservoirs 1 and 2 over a 68 week period at North Breazle Farm
Model 2 predicted that the average BOD5 and TS concentrations in reservoirs 1 and 2 were 193 mg/l
and 1083 mg/l respectively. Model 1 indicates that spreading of DW with this BOD5 concentration on
other land at North Breazle Farm would lead to concentrations of between about 100 and 180 mg/l in
the resulting combined run-off and leachate. Alternatively, the predicted average BOD5 concentration
of 193 mg/l would be suitable for tertiary treatment in a DRB or HRTF.
Figure 4.5.4 shows an example configuration for a full scale system at North Breazle Farm,
constructed in accordance with the design specification given in connection with Case Study 1. The
system can be automated in that the single pump re-circulates the DW during treatment and a diverter
valve is used to transfer water from the treatment plane reservoir (i.e Reservoir 1 or 2) to Reservoir 3.
Figure 4.5.4 shows how gabions can be used to avoid excessive slopes, and thus prevent preferential
down-slope flow.
183 of 217
terraces
gabion
Resultant
10% slope
1% slope
15 m
liner
pump
reservoir
diverter
valve
125 m
Plan view
Treatment plane
reservoir
(reservoir 1 or 2)
switch
60 m
DRB or spreading
reservoir (reservoir 3)
Plan view
Fig. 4.5.4 Schematic of a proposed full scale system based on a re-circulating OFP
treatment system delivering to a DRB at North Breazle Farm.
4.5.3.3
Results: estimated costs of a full-scale system
Costs were estimated based on the need to line an area of land and to create a lagoon for the
wastewater. Details are listed in Table 4.5.5. These refer to a system with a design life of 5 years,
which is based on the expected rate of phosphorus accumulation in the soil in the treatment plane, as
described in Chapter 3.
Table 4.5.5 Summary of OFP costs to treat DW at North Breazle Farm (Case study 2), using
two treatment planes, each providing a one-week batch treatment
Cost item
Land area required
Labour
+
digger
constructions
Value
0.75
for
Units
ha
£3,750
Pumps + local power supply
£2,800
Liner for OFP
Number of lagoons needed
Volume of each lagoon
£112,500
3
400
Lagoons
£
Comments
No value associated with this
Farm labour plus excavator and
driver for 17 days
Excludes provision for power
supplies in remote locations
HDPE liner at £15/m2
m3
Costing details are illustrated in
Fig. 4.3.8
12,707
Total capital cost including
£ 131,757
OFP liner
Total capital cost excluding
£ 19,257
OFP liner
Running costs
£250
£/year
2.2 kW pump running for 2 hours
per day against a 5 m head.
Electricity charged at £0.08/kWh
Notes:
The estimated cost exclude the following:
Labour charges for supervising the OFP installation, costs of transporting the DW to the OFP site
and of spreading the treated DW.
The costs of the optional DRB are detailed separately in Section 4.5.5
184 of 217
Total costs with the HDPE liner for the treatment planes are therefore approximately £132,000. This
equates too a capital cost of £1190 per cow. As at Pallinghurst Farm (Case Study 1), the HDPE liner is
a major cost item, and considerable savings would be possible if a compacted subsoil clay could be
used instead.
4.5.4 Application of DW-STOP Model 3: Intensive Aeration: Continuous Stirred Tank Reactor
4.5.4.1
Results: design of a full-scale system
The present version of Model 3 was used to determine the key system dimensions were based on the
DW production data presented in Table 4.4.3 and the DW properties predicted in Section 4.4.2. The
calculations also required a number of user-defined inputs to indicate the required dimensions and
capacities of certain key components of the CSTR. These are detailed in Table 4.5.6, together with the
system specifications calculated by Model 3. Given the predicted tank sizes, it was estimated that a
CSTR system for North Breazle Farm would occupy between 250 m2 and 350 m2 of land area,
including space for the feed tank. If the existing above-ground steel store (Fig. 4.4.4) is used for this
purpose, a more compact layout would be possible. The total installed power requirement would be
about 16.3 kW (although mostly operating intermittently).
Table 4.5.6 CSTR Specification for Case Study 1, North Breazle Farm, including both userdefined inputs and specifications calculated by Model 3
System properties
Feed tank mean hydraulic residence
time, days at maximum flow
Value
14
Feed tank volume, m3
261
Working volume of CSTR, m3
75
CSTR diameter, m
5.0
Mean hydraulic residence time in
CSTR, days
4
CSTR freeboard, m
1
CSTR overall height, m
Total volume of CSTR, m3
Average
fraction
of
BOD5
concentration removed
Average
fraction
of
TS
concentration removed
Sludge removal in settling tank 1
(% of input flow)
Sludge removal in settling tank 2
(% of CSTR output flow)
System properties
Foam breaker power requirement,
kW
Foam breaker power requirement,
kWh/d
Aeration efficiency kgO2 / kWhe
Maximum CSTR aerator duty
cycle, % of time
Max aerator power required, kW
4.80
94
0.93
0.31
5%
5%
Tank 1 feed pump flow rate, l/min
30
Tank 1 feed pump power, kW
1.5
Tank 1 feed pump average duty,
h/day
6.1
Average aeration energy required,
kWh/d
First Settling tank mean hydraulic
residence time, days
Required First Settling tank
volume, m3
Second
Settling
tank
mean
hydraulic residence time, days
Required second Settling tank
volume, m3
Tank 1 sludge pump flow rate,
l/min
Tank 1 sludge pump power, kW
Tank 1 sludge pump average duty,
h/day
Tank 1 sludge pump average energy
use, kWh/d
Tank 3 sludge pump flow rate,
l/min
185 of 217
Value
2.0
48
1.5
75%
8.3
82.7
2.0
37.3
2.0
35.4
5.0
0.75
1.8
1.4
5.0
System properties
Tank 1 feed pump average energy
use, kWh/d
Value
9.1
Feed tank stirrer power, kW
3.0
Feed tank stirrer duty cycle, % of
time
50%
Feed tank stirrer energy use, kW/d
36.0
System properties
Value
Tank 3 sludge pump power, kW
0.75
Tank 3 sludge pump average duty,
h/day
Tank 3 sludge pump average energy
use, kWh/d
1.7
1.3
User input data are shown in shaded cells
The Model 3 calculations predicted the week-by-week input and output flows from the CSTR, as
shown in Fig. 4.5.5. As in the case of Pallinghurst Farm, these input and output flows were largely
independent of rainfall and evaporation effects because the CSTR was covered and because the other
tanks associated with it provided only very small rainfall catchment areas. The predicted differences
between the input an output flows in the CSTR were due to the removal of sludge from the two
associated settlement tanks.
150
Vol. supplied to
CSTR, m3 / wk
Weekly output
volume, m3
Vol, m3
100
50
0
0
10
20
30
40
50
60
70
80
week
Fig. 4.5.5 Weekly input and output flows to and from the IAP CSTR, over a 68 week
period at North Breazle Farm
The corresponding week-by-week predictions of output BOD5 and TS concentrations from Model 3
are shown in Figs 4.5.6 and 4.5.7 respectively, based on the system specifications defined in Table
4.5.6. Model 3 predicted that the average BOD5 and TS concentrations in the treated DW from the
CSTR would be 294 mg/l and 6441 mg/l respectively. Applying this BOD5 concentration to Model 1
indicated that spreading of this DW on other land at North Breazle Farm would lead to concentrations
of between about 160 and 290 mg/l in the resulting combined run-off and leachate. Alternatively, as
with the OFP, the predicted average BOD5 concentration of 294 mg/l would be suitable for tertiary
treatment in a DRB or HRTF.
186 of 217
8000
BOD of DW to be
treated, mg/l
BOD, mg/l
6000
BOD5 conc of treated
DW, mg/l
4000
2000
0
0
10
20
30
40
week
50
60
70
80
Fig. 4.5.6 Input and output BOD5 concentrations to and from the IAP CSTR, over a 68
week period at North Breazle Farm
20000
TS of DW to be
treated, mg/l
TS, mg/l
16000
TS conc of treated
DW, mg/l
12000
8000
4000
0
0
10
20
30
40
50
60
70
80
week
Fig. 4.5.7 Input and output TS concentrations to and from the IAP CSTR, over a 68
week period at North Breazle Farm
4.5.4.2
Results: estimated costs of a full-scale system
The costs of the example CSTR system described above are listed in Table 4.5.7, which was based on
the same key assumptions that were applied in connection with case Study 1, as follows:
Land costs are excluded,
•
•
Labour charges for supervising the process are not included,
•
Costs of transporting the DW to the CSTR site and of spreading the treated DW are not
included,
•
The provision of additional power supplies is excluded, and
•
The costs of the optional DRB are detailed separately in Section 4.5.5.
The values given in Table 4.5.7 refer to a system with a design life of 15 years, which is based on the
anticipated life expectancy of the CSTR and settling tanks. As for Case Study 1, It is anticipated that
the pumps, mixers and blowers would require replacement at least once during this period, although
these costs are not included in the values given.
187 of 217
Table 4.5.7 Summary of costs to treat DW at North Breazle Farm (Case study 2), using a CSTR
Tanks
Feed tank volume, m3
Required First Settling tank
volume, m3
Total volume of CSTR, m3
Required second Settling tank
volume, m3
TOTAL CAP COST - tanks
Pumps
Feed tank stirrer power, kW
Tank 1 feed pump power, kW
Tank 1 sludge pump power, kW
Tank 2 sludge pump power, kW
Foam breaker power requirement,
kW
Max aerator power required, kW
(inc diffusers)
TOTAL CAP COST - pumps,
etc
Energy costs
Feed tank stirrer energy use, kW/d
Foam
breaker
energy
requirement, kWh/d
Average aeration energy required,
kWh/d
Tank 1 feed pump average energy
use, kWh/d
Tank 1 sludge pump average
energy use, kWh/d
Tank 2 sludge pump average
energy use, kWh/d
TOTAL energy use, kWh/d
Electricity cost, £/kWh
TOTAL energy cost, £/year
Installation cost factor
(factor times all other cap costs)
Total investment cost
Capacity,
m3
260.8
Capital
cost
£86,935
37.3
£12,419
94.2
£31,383
35.4
£11,798
3.0
1.5
0.75
0.75
£ 142,536
Capital
cost
£1,993
£1,295
£945
£945
2.0
£3,055
8.3
£13,445
Power, kW
Comments
All settling tank and CSTR costs
are based on an average erection
cost of £333/ m3 . The cost of
the feed tank could be saved at
North Breazle farm by using the
existing above-ground steel store
for this purpose. However, in
the interests of fair comparison,
this cost will be left in place.
The capital costs of the stirrers,
pumps and blowers are all based
on the relationship shown in Fig.
4.3.17
£ 21,678
Energy
required,
kWh/d
36.0
Running
costs
Comments
Electricity charged at £0.08/kWh.
The effects of increases in
electricity prices can be assessed
pro-rata.
48.0
82.7
9.1
1.4
1.3
178.5
0.08
£ 5,213
1.3
Comments
This factor includes an allowance
for specialist services and process
commissioning
£ 213,478
188 of 217
Total capital costs of the CSTR system for North Breazle Farm are therefore in the region of
£214,000, thus equating to a capital cost of about £1,928 per cow. However, this could be reduced by
about £87,000 by using an exiting above-ground steel store as the feed tank for the CSTR.
Although the total price including the feed tank appears to be more expensive that the OFP with a
lined treatment plane, the 15-year life expectancy of the CSTR, compared with 5 years in the case of
the OFP means that in some circumstances, the CSTR would be competitive. However, compared
with an OFP without a liner, the CSTR is expensive. Compared with either version of the OFP, the
running costs of the CSTR are about 20 times higher.
4.5.5 Application of DW-STOP Model 4: Reed beds for secondary / tertiary treatment
4.5.5.1
Results: design and estimated costs of a full-scale system
Model 4 was used to determine the required dual-stage DRB surface area and costs for tertiary
treatment of the predicted effluent from the CSTR, as shown previously in Figs 4.5.6 and 4.5.7 This
material had an annually averaged BOD5 concentration of 294 mg/l with average and peak flow rates
of 10.4 and 17.7 m3/day respectively. Thus, as for Case Study 1, this material was suitable for tertiary
treatment using a DRB system. In this case, the DRB specification was designed to achieve an output
BOD5 concentration of 30 mg/l. Although TS removal was not modelled explicitly, comparison with
Table 3.3.1 implied that 50% removal should be possible, leading to a predicted average TS
concentration of about 3200 mg/l in the treated effluent from the second stage of the DRB.
The resulting specification included two DRB stages, each of 151 m2 surface area, with 117 flushes
per week, giving a period of 1.4 hours between flushes. The week-by-week predictions of Model 4
indicated that the volume of each flush would range from 0.2 m3 in summer to 1.1 m3 in winter. It
was also predicted that the annually averaged BOD5 concentration produced from the second stage of
this system would be in the region of 14 mg/l, which was based on a values of between 26 and 30 mg/l
in winter and 1 to 2 mg/l in summer.
In order to calculate the overall treatment costs of the dual-stage DRB, the capital costs were estimated
using the same technique as applied for Case Study 1 (i.e. interpolation of the data presented in Table
4.3.12, as shown in Fig. 4.3.21, plus a 20% allowance for the additional costs of installation). The
following assumptions were made in estimating the costs of the DRB:
Land costs are excluded,
•
•
Labour charges for supervising the process are not included,
•
Costs of transporting the DW to the DRB site and of spreading the treated DW are not included,
•
The provision of additional power supplies is excluded, and
•
The costs of a supply reservoir for the DRB (necessary for use with an OFP system) are
excluded. Use with a CSTR would not require a reservoir due to the continuous operation of
the CSTR.
Hence, the total capital cost of the 2 * 151 m2 DRB system was estimated to be £54,600, i.e.
approximately £492 per cow. The total land area needed for a dual DRB system of this capacity at
Pallinghurst Farm would be in the region of 400 to 500 m2.
As for Case Study 1, the running costs of DRB systems were predicted to be much lower than those of
the CSTR. As the pumps only have to supply a head of approximately 2 m, their required power
ratings are typically only 0.5 kW. These pumps operate for short periods to supply the feed pulses on
to the DRBs, and are not in continuous operation. Table 4.5.8 summarises the calculations of running
costs included in Model 4, leading to an estimated annual running cost for the 2 * 151 m2 DRB system
of £70 per year.
189 of 217
Table 4.5.8 Summary of data used by Model 4 to calculate DRB running costs for tertiary
treatment of effluent from the CSTR – predictions for North Breazle Farm
System properties
Value
Pulse duration, minutes
10
Number of pulses per day
24
Pressure head, m
2
3
Density of DW, kg/m
1000
Pump efficiency, %
50%
Electricity cost, £/kWh
0.08
Number of pumps used
5
Recycle ratio, factor of input flow, (i.e. a recycle ratio of 4
means that the total flow is 5 times the input flow)
4
Yearly cost per pump, £
14
Yearly cost for all pumps, £
70
User input data are shown in shaded cells
4.5.6 Application of DW-STOP Model 5: Intensive Aeration: High Rate Trickling Filter for
secondary / tertiary treatment
4.5.6.1
Results: design of a full-scale system
Model 5 was used to determine the key system dimensions, assuming that the HRTF would treated
effluent from the CSTR. Hence these dimensions were based on the CSTR effluent production data
presented in Fig. 4.5.5, and the DW properties predicted in Figs 4.5.6 and 4.5.7. The calculations also
required a number of user-defined inputs to indicate the required dimensions and capacities of certain
key components of the HRTF. These are detailed in Table 4.5.9, together with the system
specifications calculated by Model 5. Overall, a HRTF system suitable for North Breazle Farm would
require between 200 and 300 m2 of land area, and the installed power requirement would be
approximately 4.3 kW.
Table 4.5.9 HRTF Specification for Case Study 2, North Breazle Farm, including both userdefined inputs and specifications calculated by Model 5
System properties
Residence time in HRTF sump, days
Value
0.5
HRTF sump freeboard, m
0.40
HRTF and sump diameter, m
10.0
Working volume of HRTF, sump m3
8.4
Total volume of HRTF sump, m3
39.8
HRTF sump overall height, m
0.51
HRTF volume: m3 of tank capacity
10.0
System properties
HRTF
packed
column
circulation time, min
HRTF
packed
column
circulation rate, m3/d
HRTF
packed
column
circulation rate, l/min
HRTF
packed
column
circulation head, m
HRTF
packed
column
circulation hydraulic energy, W
HRTF packed column pump
efficiency
HRTF
packed
column
190 of 217
Value
5.0
2421
1681
3.6
1003
50%
2.0
System properties
per unit flow of DW (m3/d)
HRTF packed column freeboard, m
Value
HRTF packed column working
volume, m3
HRTF packed column overall height,
m
HRTF packed column total volume,
m3
HRTF packed column + sump overall
height, m
Av Fraction of BOD5 concentration
removed by HRTF
Av Fraction of TS concentration
removed by HRTF
168
1.0
3.14
247
3.6
0.6
0.01
System properties
circulation input energy, kW
HRTF
packed
column
circulation energy use, kWh/d
Value
Tank 9 feed pump flow rate,
l/min
Tank 9 feed pump power, kW
30
48.1
1.5
Tank 9 feed pump average duty,
h/day
Tank 9 feed pump average
energy use, kWh/d
Tank 9 residence time, days
5.2
Required Tank 9 volume, m3
33.6
Sludge removal in tank 9 (% of
HRTF output flow)
Tank 9 sludge pump flow rate,
l/min
Tank 9 sludge pump power, kW
5%
Tank 9 sludge pump average
duty, h/day
Tank 9 sludge pump average
energy use, kWh
7.8
2.0
5.0
0.75
1.6
1.2
As in case Study 1, the predicted week-by-week output flows from the settlement stage after the
HRTF were approximately 5% lower than the input flows to the HRTF. Hence, Model 5 predicted the
week-by-week BOD5 and TS concentrations in effluent from the HRTF system. These led to an
average annual BOD5 concentration of approximately 118 mg/l, which was based on a range of values
from 30 to 200 mg/l. Applying this average BOD5 concentration to Model 1 indicated that spreading
this DW on other land at North Breazle Farm would lead to concentrations of between about 60 and
110 mg/l in the resulting combined run-off and leachate.
The predicted TS values were largely unchanged from those in the CSTR output flows, although as
for Case Study 1, the predicted TN and TAN concentrations were reduced by approximately 65% and
89% respectively, reflecting the particular capability of this system to remove surplus nitrogen from
DW.
4.5.6.2
Results: estimated costs of a full-scale system
Costs of the example system described above are listed in Table 4.5.10, which was based on the
following key assumptions:
•
Land costs are excluded,
•
Labour charges for supervising the process are not included,
•
Costs of transporting the DW to the HRTF site and of spreading the treated DW are not
included, and
•
The provision of additional power supplies is excluded.
191 of 217
As for the CSTR, the values given in Table 4.5.10 for the HRTF are based on a design life of 15 years.
However, it is anticipated that the pumps, would require replacement at least once during this period,
although these costs are not included in the values given.
Table 4.5.10
Summary of costs for tertiary treatment of effluent from the CSTR at North
Breazle Farm (Case study 2), using a HRTF
Tanks
HRTF sump volume, m3
HRTF packed column total
volume, m3
Third settling tank volume (tank
9), m3
Packing materials, m3
TOTAL CAP COST - tanks
Pumps
HRTF packed column circulation
pump, kW
Tank 9 feed pump power, kW
Tank 9 sludge pump power, kW
TOTAL CAP COST - pumps,
etc
Energy costs
HRTF packed column circulation
pump, kWh/d
Tank 9 feed pump power, kWh/d
Tank 9 sludge pump power,
kWh/d
TOTAL energy use, kWh/d
Electricity cost, £/kWh
TOTAL energy cost, £/year
Installation cost factor
(factor times all other cap costs)
Total investment cost
Capacity,
m3
8.4
247
Capital
cost
£2,802
£49,333
33.6
£11,208
168
£10,917
£ 74,261
Capital
cost
£ 1,530
Power, kW
2.0
1.5
0.75
£ 1,295
£ 945
£ 3,770
Energy
required,
kWh/d
48.1
Running
costs
Comments
Sump costs are based on an
average erection cost of £333/
m3, whilst packed column costs
are based on £200/m3. Packing
materials are based on £65/m3
The capital costs of the stirrers,
pumps and blowers are all based
on the relationship shown in Fig.
4.3.17
Comments
Electricity charged at £0.08/kWh
The effects of changes in
electricity prices can be assessed
pro-rata.
7.8
1.2
57.2
0.08
£ 1,670
Comments
This factor includes an allowance
for specialist services and process
commissioning
1.3
£ 101,440
The total capital costs of the HRTF system for Case Study 2 are therefore in the region of £102,000,
thus equating to a capital cost of about £920 per cow, plus annual running costs of over £1600. Hence,
the HRTF is clearly more expensive than the equivalent DRB system described in Section 4.3.6.
4.5.7 Application of the DW-STOP Models: comparison of results
4.5.7.1
Comparisons of benefits
192 of 217
The benefits provided by the various treatment options in the context of case Study 2, as described in
Sections 4.3.2 – 4.3.6, are similar to those described in connection with Case Study 1, as summarised
in Tables 4.3.16 and 4.1.17.
4.5.7.2
Comparisons of costs
Tables 4.5.11 and 4.5.12 summarise the costs of treating DW at North Breazle Farm as discussed and
presented in Sections 4.3.2 – 4.3.6. These costs are in addition to those of handling and spreading
DW, and include illustrations of the sensitivities of these costs to an increase of 10% in the size of the
milking herd at North Breazle Farm. As for Case Study 1, It was assumed that this herd increase
would be accommodated without enlarging existing buildings and yards. Therefore, rainfall amounts
are unchanged, leading to slightly higher concentrations in the raw DW. Whereas Table 4.5.11
addresses reductions in BOD5 concentration of about 90%, Table 4.5.12 considers ways to meet
tighter target specifications for the discharge to land of treated DW, and/or opportunities for re-use of
treated DW, as described in Table 4.1.2.
Single process treatment options are included in Table 4.5.11, comparing the CSTR with the OFP, and
showing that, with a herd of 111 cows, if the OFP option includes plastic membrane liners to isolate
the treatment planes hydrologically, the simple annual overall treatment cost of the OFP will be
greater than the CSTR by about £106 cow-1 year-1. However, if the appropriate authority allowed the
use of compacted subsoil clay instead of a liner, the OFP option would become cheaper than the CSTR
by approximately £97 cow-1 year-1. Herd expansion by 10% would reduce the cost per cow of this
cheapest option by about 8%.
Table 4.5.11 Example comparative output from the DW-STOP mathematical models: Case
study 2: specification of least cost systems to reduce pollutant concentration by 90%
System Properties
Current herd
111
1.0
3890
4200
9300
IAP(CSTR only)
“What if” the
milking herd
were 10%
larger?
122
1.1
4000
4500
10000
IAP(CSTR only)
% reduction in BOD5 before spreading
% reduction in TS before spreading
Capital cost, £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
93%
31%
£214,000
15
£5,200
£14,613
£132
£3.76
OFP
93%
31%
£219,259
15
£5,462
£14,981
£123
£3.75
OFP
% reduction in BOD5 before spreading
% reduction in TS before spreading
Capital cost, £
Capital cost (without OFP membrane liner), £
Design life, years
95%
88%
£132,000
£19,300
5
95%
88%
£135,000
£19,500
5
No of cows in milking herd
Annual milk production, million litres
Annual DW applied to land, m3
Average input BOD5
Average input TS
193 of 217
System Properties
Current herd
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Data relevant to an OPF constructed without a plastic
membrane liner are shown in shaded cells
£250
£26,450
£238
£6.80
£3,910
£35.23
£1.01
“What if” the
milking herd
were 10%
larger?
£260
£27,052
£222
£6.76
£3,952
£32.39
£0.99
Dual process treatment options are included in Table 4.5.12, in which use of DRB systems are
compared downstream of both a CSTR and an OFP. As in Table 4.5.11, this comparison shows that,
with a herd of 111 cows, inclusion of plastic membrane liners in the OFP option makes this approach
expensive, with a simple annual overall treatment cost of £281 cow-1 year-1, for the combined OFP +
DRB system. This compares with £165 cow-1 year-1 for the combined CSTR + DRB system.
However, if the OFP liner can be replaced by indigenous compacted clay, the treatment cost of the
OFP +DRB option would fall to approximately £78 cow-1 year-1. Herd expansion by 10% would
reduce the cost per cow of this cheapest option by about 5%.
Table 4.5.12 Example comparative output from the DW-STOP mathematical models: Case
study 2: Specification of least cost systems to allow re-use of DW for yard and machine washing:
pollutant concentration reduced by 99%
194 of 217
System Properties
% reduction in BOD5 before re-use
% reduction in TS before re-use
Capital cost, £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Annual volume of treated DW collected, m3/yr
Simple annual total cost per m3 DW recycled, £/yr
% reduction in BOD5 before re-use
% reduction in TS before re-use
Capital cost, £
Capital cost (without OFP membrane liner), £
Design life, years
Running costs, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Simple annual total cost, £/yr
Simple annual total cost per cow, £/yr
Simple annual total cost per m3 DW spread, £/yr
Annual volume of treated DW collected, m3/yr
Simple annual total cost per m3 DW recycled, £/yr
Current herd
NB IAP(CSTR) + DRB.
IAP(CSTR)
DRB
TOTAL
93%
95%
99%
31%
20%
45%
£214,000
£54,600
£268,600
15
15
15
£5,200
£180
£5,380
£18,265
£165
£4.70
3800
£4.81
NB OFP+ DRB.
OFP
DRB
TOTAL
95%
92%
99%
88%
20%
90%
£132,000
£71,400
£467,400
£19,300
£71,400
£129,300
5
15
15
£250
£360
£610
£31,201
£281
£8.02
£8,661
£78
£2.23
8200
£1.06
195 of 217
“What if” the milking herd were 10% larger?
NB +10% IAP(CSTR) + DRB.
IAP(CSTR)
DRB
TOTAL
93%
95%
99%
31%
20%
45%
£219,259
£57,600
£276,859
15
15
15
£5,460
£180
£5,640
£18,833
£154
£4.71
3900
£4.83
NB +10% OFP+ DRB.
OFP
DRB
TOTAL
95%
92%
99%
88%
20%
90%
£135,000
£76,300
£481,300
£19,500
£76,300
£134,800
5
15
15
£260
£370
£630
£32,129
£263
£8.03
£9,029
£74
£2.26
8400
£1.07
4.6
GENERAL RECOMMENDATIONS AND KEY PRACTICAL FINDINGS (TC, CoB,
PL-H, DCh, JW, KS, IM, JG)
4.6.1 General items required in most DW handling and treatment systems
•
•
•
•
•
Primary settlement of solids, grit etc. as in any conventional DW system.
Frost protection of above ground pipework and pumps.
Sampling chamber on discharge point to field irrigation or other final discharge.
Provision for future metering of final discharge volume.
Cover any open tanks to minimise blockages with leaves and prevent access by wildlife.
4.6.2 Specific recommendations for systems using CSTR and/or HRTF processes
•
•
•
•
Redox sensors require regular cleaning.
Use a rundown screen or other type of mechanical separator on inlet where chopped maize
silage is fed.
Need to ensure stable electricity mains supply voltages to avoid process control problems.
Connect computer to mains electricity supply via a Uninterruptible Power Supply (UPS) with
automatic voltage stabilisation.
Computer runs continuously therefore a robust operating system in required. Consider a Unix
based system in preference to Windows.
4.6.3 Specific recommendations for Soil based treatments (OFP and PSP)
•
•
•
•
•
Allow time for soil structure to stabilise before applying DW.
Ensure that viable grass cover is established before applying heavy DW loads
Split plot into a minimum of 2 sections and use in rotation.
Use Gypsum when treating dairy wash water, unless this is diluted substantially with yard
water.
OFP systems require regular cutting and removal of grass and weed growth.
4.6.4 Specific recommendations for DRBs
•
•
•
DRBs provide Tertiary treatment.
DRBs need a supply of water in prolonged dry weather to preserve the reeds.
Where blinding of the surface by organic material is a problem Consider regular cutting and
removal of reeds followed by raking.
196 of 217
CHAPTER 5 DW-STOP EXPLOITATION PLAN
Chapter authors: Trevor Cumby, Andrew Barker, Colin Burton, David Chadwick, Marc Dresser, Gari
Fernandez, John Gregory, Peter Leeds-Harrison, Ian Muir, Elia Nigro, Ken Smith and Joe Wood
5.1
RATIONALE FOR COMMERCIAL EXPLOITATION
5.1.1 Environmental protection
The outputs from DW-STOP will enable the commercial exploitation of a procedure for specifying a
range of defined and proven processes for the cost effective management of dairy farm DW. This will
enable the application of DW to land to be better-understood and will thus reduce environmental
impacts, whilst making the most of beneficial effects, such as the recycling of plant nutrients.
Currently, protecting the environment is not perceived to have a direct and immediate link with dairy
farm profitability. Indeed, it might be argued that fines are cheap compared with the investment
required to avoid pollution, but this ignores the much greater harm that may be done to the consumer
and retailer perceptions of UK dairy farming, which are essential for the long-term sustainability of the
industry. Hence, by identifying and illustrating optimal approaches to DW management, the outputs
from DW-STOP can do much to ensure that future investment decisions by UK dairy farmers will
achieve the necessary results and will give good value for money. In turn, consumers and retailers can
be assured that milk production is not a threat to the environment, and that the beneficial effects of
recycling DW to land are maximised.
5.1.2 Water recycling to secure future water supplies
The treatment technologies developed through DW-STOP also offer potential benefits for re-use of
dairy farm DW. In some instances, were future restrictions may be applied to potable mains water,
such re-use of water will become increasingly important. Examples of ways in which the DW-STOP
outputs might facilitate this include the following:
•
•
•
washing/flushing yard areas, buildings and some equipment,
use in disinfectant wheel dips/boot baths, and
crop irrigation.
Although restriction of water supplies are likely to affect relatively few dairy farmers in the UK,
many more are likely to face increasing water costs as water supply companies seek extra revenue to
meet the rising costs of increased public demand together with the need to repair or replace aged or
leaking distribution networks. It is conceivable that water re-use, based on the low-cost DW treatment
technologies developed through DW-STOP, could lead to direct cost savings in such circumstances.
5.1.3 Other applications (non-dairy)
Although some further development would be required, the outputs from DW-STOP could also
underpin application of the effluent treatment techniques developed in other sectors beyond dairy
farming. For example, these include: safer discharge (or re-use) of vegetable wash water; treatment of
leachate from landfill sites and alternative means of sewage treatment for isolated rural communities.
5.2
EXPLOITABLE OUTPUTS FROM DW-STOP
5.2.1 Mathematical models and case studies: the DW-STOP Data Set
197 of 217
The project has delivered the means to specify and to assess the costs of sufficient (but not excessive)
DW treatment strategies and techniques to meet defined targets in managing dairy farm DW. This
was achieved via an integrated set of spreadsheet-based mathematical models. These include three
parts to predict treatment system performance on a week-by-week basis and hence to produce
specifications for full-scale versions (i.e. OFP, IAP and DRB). In addition, the DW-SOIL model
predicts likely reductions in key concentrations during field flow after land spreading, but before the
DW reaches a watercourse.
The integrated OFP, IAP, DRB and DW-SOIL models enable competent users to design and operate
treatment systems based on the validated results of the project. The aspects of these models that
estimate the costs of treatment also give these users a set of useful tools to compare DW treatment
strategies with alternative approaches such as winter storage and spring application of DW. Indeed,
the DW-SOIL model is also relevant to this alternative strategy because it can indicate the likely
pollution risks of spreading stored, untreated DW.
However, the models are not an Expert System, so users, such as waste treatment engineers and soil
scientists, are expected to have relevant background knowledge.
5.2.2 Case studies
The two DW-STOP case studies illustrate, in detail, the application of the four models to real farm
situations, where DW management is recognised as a serious issue. These show how DW-STOP’s
findings can be used to compare the costs and effectiveness of alternative DW systems. Moreover,
they include investigations of a number of “what-if” scenarios to illustrate the technical,
environmental and financial effects of farm policy changes, including the following:
•
•
•
•
Increased herd size - reduction of fixed costs per litre of milk produced will remain an essential
requirement to maintain profits, and under some circumstances, this may lead to increased herd
sizes.
Electricity and other energy costs - alternative treatment processes will consume different
amounts of energy per unit volume of DW treated, and so it is important to compare their
sensitivities to rising energy costs.
Cost / availability of potable mains water - Depending on the extent of the treatment, re-use of
treated DW is possible for some purposes, such as: washing/flushing yard areas, buildings and
some equipment; crop irrigation and use in disinfectant wheel dips/boot baths. In some
instances, supply restrictions may be applied to potable mains water, necessitating re-use for
some purposes.
Tighter target specifications for the discharge to land of treated DW, expressed as a reduced
BOD5 concentration - Tighter specifications may be needed if treated DW is to be re-used, or
they may become necessary to meet changes in legislative requirements for land-spreading DW.
5.2.3 The DW-STOP data set
DW-STOP has produced a comprehensive data set that uniquely defines the properties of DW, and
their seasonal variations, on a large commercial dairy farm in the UK. This significant and useful
resource provides a detailed insight into the impacts of herd management and weather factors on DW
production and composition. As such, it has further implications for the design and management of
DW systems, beyond the DW-STOP treatment systems.
Dairy advisers, consultants, environmentalists and waste management engineers will value the insights
into the properties of DW provided by the DW-STOP Data Set.
5.3
EXPLOITATION ACTIVITIES
198 of 217
5.3.1 Commercial exploitation of results
5.3.1.1
ARM Ltd
ARM Ltd intends to add the intellectual property evolved by the DW-STOP project to its already
comprehensive knowledge base, which enables the company to offer solutions for treating a wide
range of wastewaters. ARM has been designing and selling products to farmers for more than 50 years
and is conversant with this market. It has built up a reputation and expertise in the application of
wetland technology to the extent that it is now recognised by most people in the wastewater treatment
industry. The Company has an effective Marketing and Sales function that will be used to exploit new
applications where this intellectual property is relevant. ARM Ltd has an easy to use, informative
website and features on all reputable lists of DRB suppliers. The Company already receives numerous
enquiries from farmers and food processors. It is expected that notification of ARM’s involvement in
the DW – STOP project will accelerate the number of enquiries.
5.3.1.2
Pallinghurst Farm Partners
Pallinghurst Farm Partners’ strategy is to achieve a high farm net profit by maintaining a simple
system with low overhead costs and to expand the business, allowing the next generation to take over
a viable operation that can support several families. The target of current developments is to reduce
production costs by limiting labour time to 10 seconds per litre of milk produced, (including feeding,
milking, bedding and cleaning), and to keep the whole system as simple as possible. DW-STOP has
provided information necessary to ensure that the imperatives of environmental protection can be
maintained within these strategic plans.
5.3.1.3
Carier Pollution Control Ltd and others
Since the project began, another of the original Industrial Partners, Carier Pollution Control Ltd, has
ceased trading. Therefore, it is unable to participate further in exploitation of the Intellectual Property
generated in the project. However, other project partners with, hitherto, predominantly research-based
interests (or their former employees) are exploring the opportunities to use the results from DW-STOP
in a commercial context.
5.3.2 Technology transfer
5.3.2.1
The Milk Development Council
The third Industrial Partner, the Milk Development Council, will assess how the information in the
final report fits into their communication strategy and will develop the relevant messages to present to
farmers from within this body of work. Commercial reality means that the science will have to be
expressed in a more user-friendly fashion with the beneficial points being possibly demonstrated via
the use of case studies or farm walks. Detailed planning of this communications strategy remains to
be completed.
5.3.2.2
Demonstration projects: answering ten key questions
All of the DW-STOP Project Partners have acknowledged that full-scale demonstrations activities are
amongst the most convincing ways to communicate new ideas and research results to a wide farming
community. However, it is also recognised that such endeavours are not cheap and that careful
evaluation of the costs and benefits is an essential pre-requisite. In order to illustrate the value of the
results from DW-STOP, such a demonstration project would need to need to answer the ten key
questions to be asked by dairy farmers, each with respect to their own businesses, as follows:
•
•
•
What benefits can be gained by treating DW that are not available by other means?
What does it cost per litre of milk produced?
Is it the cheapest option?
199 of 217
•
•
•
•
•
•
•
What are the other options and how are the results of DW-STOP relevant to these alternatives?
Will investment in the treatment of DW meet current and future environmental regulations and
legislation?
Are treatment processes reliable?
Who is using these processes already, and what do they think of them?
Would they re-invest in the same system?
Will DW treatment systems provide the necessary flexibility to meet the changing demands on
UK dairy farmers?
Who can supply more information/design specifications?
The two case studies reported in Chapter 4 provide examples of the first steps that are necessary to
produce a full-scale demonstration of a DW management system, incorporating treatment. Therefore,
it is anticipated that development of a demonstration project of this type would involve at least some
of the current partners in DW-STOP. Accordingly, the DW-STOP Partners intend to remain in contact
during 2005 and 2006 in order to assess the costs and benefits of a possible demonstration project, and
then, if appropriate, to seek the necessary resources and location(s) to construct a full-scale system.
Such endeavours will build upon the basis established in April 2004 when a discussion group of about
40 leading farmers, consultants, academics and others, (“The Haymakers”), visited the DW-STOP
experiments at Pallinghurst Farm, at the end of Trial 1. The aims and approaches of the project were
presented, and led to useful discussion, with particular emphasis on estimating the costs of alternative
DW management options. Subsequent feedback from The Haymakers Chairman, confirmed that the
project was well received by many attendees.
5.3.2.3
Other means of technology transfer
DW-STOP has delivered significant scientific findings and it is anticipated that these will reported in
at least three refereed papers, to be published in relevant journals such as Bioresource Technology,
Biosystems Engineering and/or Environmental Technology. Presentations will also be made at one or
more conferences, workshops or symposia.
Whilst the importance of peer-reviewed publications is fully recognised, it is also important to ensure
that the key messages from the project reach a wider readership. Therefore, it is expected that popular
articles will be published in the farming press, and may feature at a relevant agricultural
show/exhibition.
Further dissemination of the project’s results to the industry is to be proposed via the MDC and Defra,
possibly including the production of a generic booklet on DW Management. It is envisaged that this
booklet could be compatible with the “Managing Livestock Manures” series, which already includes
four other titles.
5.3.3 Strategic issues: interaction with the UK environmental regulatory authorities
During the final 8 months of the DW-STOP project, Defra sought seeking public consultation on the
draft Waste Management (England and Wales) Regulations 2005, i.e. “Agricultural Waste Regulations
Consultation” (Defra, 2005).
A detailed response was submitted on behalf of DW-STOP in March 2005 (Appendix 2). This
response was essential because the proposed regulations could have far-reaching effects on the
possible future use of treatment techniques for DW on UK dairy farms. In particular, the proposals
might have the undesirable effect of inhibiting the exploitation of the results from DW-STOP, and
therefore the dangers of unintended consequences required clear identification. The main issues raised
were as follows:
•
Implications of “treatment” - any notion that the “treatment” of “livestock manure, slurry or
effluent” automatically means that the material is a “waste”, (requiring either a waste
200 of 217
•
•
•
•
•
management licence or a licence exemption), is illogical and shows a profound
misunderstanding of the role of relevant treatment technologies. Implementation of the
Statutory Instrument (SI) should be based on assessments of the source, nature and application
of the materials involved.
Limits on application rates - the statutory guidance to be provided by Defra needs to clarify
how the proposed limits on the annual application rate should be applied on individual farms.
Use of the whole farm area would provide the simplest approach, provided that localised
pollution is avoided. The findings from DW-STOP and related studies would help to support
this approach. For example, this research has shown that dairy farm DW typically contains
0.5 - 1 kg/t total nitrogen, N, (including 0.5 kg/t as ammoniacal nitrogen), 0.5 kg/t of P2O5 and
1-1.5 kg/t of K2O. Thus, the minimum value is approximately £0.6 /t so an application of 50
t/ha has a potential value of £30!
What is DW ? - Defra’s statutory guidance also needs clarify the circumstances when DW is
regarded as a sub-set either of “slurry” or of “effluent”, as mentioned in Paragraph 49 of the
draft SI. Further clarification is needed to indicate the maximum proportion of waste milk that
can be included in DW
for licence exemption subject to “Paragraph 49”, rather than
“Paragraph 47”.
Benefits to the land - the criteria to determine whether any spreading operation is “beneficial to
the land” must be objective, clear and practical. Subjective judgements, introduced through the
processes of interpretation and implementation of the SI would be inappropriate.
The role of treatment in managing manures, slurries and DW - treatment technologies,
including those used in DW-STOP, can help farmers to meet nutrient application limits, (e.g.
nitrogen and phosphorus). Therefore, implementation of the SI must accept such measures as
part and parcel of manure/slurry management, where returning these materials to land, within
the prescribed limits, is outside of the Waste Regulations.
Soil-based treatment systems - where hydrologically isolated volumes of soil form part of a
treatment process, application of “livestock manure, slurry or effluent” to this soil, and its
collection there from, must be recognised as being outside the controls that relate to
conventional field applications. Where “used” soil is removed from such systems after a period
of use, this material should be regarded as a mixture of soil plus “livestock manure, slurry or
effluent”. Accordingly, it should be outside of the Waste Regulations.
At the time of completing this report, the full ramifications of this communication with Defra have yet
to become apparent.
5.3.4 Further research
5.3.4.1
Experimental and other data acquisition studies
The observations and occasional problems encountered during completion of DW-STOP indicated that
certain assumptions and practices concerning the production and management of DW in the UK
should be re-assessed in the interests of safe and efficient dairy production, and of practical
environmental protection. For example, this includes the interactions between certain soils and
sodium, e.g. from dairy cleaning chemicals, in DW. Potential projects to further investigate and
moderate this effect include:
•
•
•
•
Gathering information on the incidence, level and persistence of sodium and other be biocidal
dairy farm chemicals in the UK;
Developing strategies for dealing with dairy chemicals in DW, e.g. limiting sodium use or
adding calcium salts to reduce the Sodium Adsorption Ratio (SAR);
Measuring the effects of adding calcium hydroxide on precipitation of solid matter in DW;
Measuring the performance of dairy cleaning agents, e.g. via new bioassays to minimise
chemical usage.
The research partners within DW-STOP will continue to assess future opportunities to develop new
research project embracing these topics.
201 of 217
5.3.4.2
Mathematical modelling
The predictive performance of the mathematical model to represent the ability of ‘native’ soil to
remove BOD5 could be improved by further work in the following areas:
•
•
•
•
•
The inclusion of the ability of effluent to run laterally at the surface of layer 1 and layer 2 when
capacity has been exceeded in those layers. Currently, if there is overcapacity and the effluent
cannot cascade from one layer to another it pools at the soil surface and can be subject to runoff.
The runoff : infiltration ratio requires further validation, particularly with reference to slope and
soil type.
Validation of the proportional flow of effluent via macropores and matrix flow.
Validation of the BOD5 removal as effluent flows as runoff and in macropores.
A function should be included to moderate infiltration into the soil following repeated
applications of DW due to blinding/blocking of pores by solids material
202 of 217
CHAPTER 6 CONCLUSIONS
Chapter authors: Trevor Cumby, Andrew Barker, Colin Burton, David Chadwick, Marc Dresser, Gari
Fernandez, John Gregory, Peter Leeds-Harrison, Ian Muir, Elia Nigro, Ken Smith and Joe Wood
6.1
SCIENTIFIC FINDINGS
6.1.1 Trial 1 - Overview
6.1.1.1
Overall Performance
During Trial 1, all four systems treated this effluent effectively with average reductions of over 80 per
cent in BOD5 and 40 per cent in TS. Occasional BOD5 and TS reductions of 99% and 90% were also
observed. Comparable results were found with: chemical oxygen demand, ammoniacal nitrogen,
nitrates, nitrites, phosphorus and thermotolerant coliforms. However, the most valuable results from
Trial 1 were those that defined the boundaries of efficient and reliable operation, as described below.
6.1.1.2
Overall Performance of the PSP
Sustained DW treatment was found to require coarse textured, free draining soil to avoid waterlogging. Since such soils are uncommon on dairy farms with recognised DW pollution problems, this
approach was withdrawn from Trial 2, enabling use of the plot for detailed investigations of DW
purification after land spreading.
6.1.1.3
Overall Performance of the OFP
The overland flow DW treatment system at Pallinghurst Farm demonstrated that such a system can
remove large amounts of the DW pollution load. In the case of BOD5 and TSS, which were key target
pollutants, the average reductions were >90% and >80% respectively. Other pollutants, notably total
phosphorus and TAN, were also treated with observed reductions in concentrations of approximately
80%. However, the system did not treat the DW to concentrations that are likely to be suitable for
direct discharge to a watercourse. This requires that a post OFP treatment system will be needed
before discharge to the wider environment. The use of a DRB system as a tertiary treatment presents
an attractive solution as the DRB is efficient at treating low BOD5 and low suspended solids influent
wastewater but is often damaged by high levels of these pollutants.
The indigenous soil at Pallinghurst Farm worked well at first in this system, but its structure gradually
deteriorated due to accumulation of sodium, which was derived from dairy cleaning chemicals
regularly discharged in the DW. Alternative remediation strategies were assessed, eventually leading
to an effective technique using a shallow gypsum-filled (CaSO4) lateral trench, plus other DWredistribution trenches filled with gravel.
The costs of a full scale overland flow scheme at Pallinghurst Farm is estimated to be approximately
£370 per cow. The high cost results from the need for a liner for the grassed plots. If compacted
subsoil clay can be used costs can be greatly reduced.
6.1.1.4
Overall Performance of the DRB
The high TS concentration in the DW led to periodic blockages in the gravel matrix of the DRBs.
Treated DW from the outlet of the beds was recycled using various strategies to alleviate the problem,
but since this did not provide a reliable solution, the system was subsequently limited to accepting
only pre-treated DW.
203 of 217
6.1.1.5
Overall Performance of the IAP
Despite some electrical and mechanical malfunctions, this system was robust and did not accumulate
any matter. However, it required an increased aeration capacity following Trial 1 to cope better with
the highest BOD5 concentrations and to allow an increased flow to supply the DRB with pre-treated
DW during Trial 2.
6.1.2 Trial 2 - Overview
The results from Trial 2 confirmed the expected improvements in performance following the changes
implemented after Trial 1 (Figs 4 and 5). Both the OFP and IAP worked reliably with untreated DW
and the DRB operated consistently in completing the treatment of partially treated DW from the first
biological treatment stage of the IAP.
As planned, Trial 2 provided the data required for development and validation of an integrated set of
three spreadsheet-based mathematical models (OFP, IAP and DRB) to specify full-scale DW
treatment systems and to compare their costs and effectiveness. The data also supported the DW-SOIL
model, which helps to avoid the inefficiencies of treating DW beyond environmental needs. The data
included herd statistics, dairy chemicals, slurry management, weather and soil type. The results of the
laboratory studies also supported model development, for example:
•
•
the IGER experiments showed that sandy soils gave the best reductions in BOD5, whilst clays
were less consistent; and
the CUS studies led to better predictions of the rates of phosphorus accumulation, plus risk of
phosphorus leaching.
6.1.3 Case studies
The models facilitated the specification and cost estimation for full-scale processes to meet the
anticipated need for DW treatment on two UK dairy farms. Through these models, the project reached
its goal of providing the means to specify efficient full-scale DW management systems and to estimate
the associated costs.
As expected, the project showed that cost-effective management of DW on dairy farms must take
account of individual farm circumstances; a “one size fits all” approach is entirely inappropriate and
inefficient. Hence, the DW-STOP models were developed to process relevant multi-factorial data and
to compare the technical and financial consequences of using a range of management options, either
singly or in combination. This was demonstrated using two full-scale case studies based on data from
two contrasting farms where DW management is already a recognised problem (Table 6.1). These
case studies included a number of “what if" questions, showing how the models could compare the
effects of future changes. These questions included issues such as: herd size, water costs, water re-use,
energy costs and tighter environmental requirements applied to land-spreading of DW.
Table 6.1 Summary of example comparative outputs from the DW-STOP mathematical models
204 of 217
System properties
Current herd
“What if” the milking
herd were 10% larger? *
Case study Case study Case study Case study
1
2
1
2
No of cows in milking herd
430
111
473
122
Annual milk production, million litres
2.7
1.0
2.9
1.1
Annual DW applied to land, m3
7600
3890
8000
4000
Typical input BOD5 , mg/L
3300
4200
3400
4500
Typical input TS , mg/L
7000
9300
7200
10000
IAP (CSTR IAP (CSTR
OFP
OFP
Example specifications of systems to reduce
only)
only)
pollutant concentration by 90%
% reduction in BOD5 before spreading
% reduction in TS before spreading
Capital cost, £
Capital cost, less OFP membrane liner, £
Design life, years
Running costs, £/year
93%
31%
£296k
-15
£6,759
OFP+
DRB.
93%
31%
£214k
-15
£5,200
OFP+
DRB.
% reduction in BOD5 before re-use
99%
% reduction in TS before re-use
83%
Capital cost, £
£650k
Capital cost, less OFP membrane liner, £
£155k
Design life
15
Running costs, £/yr
£870
* i.e. properties of systems with capacity for 10% more cows
99%
90%
£467k
£129k
15
£610
Example specifications of systems for re-use of
DW for yard and machine washing
6.2
92%
79%
£206k
£26k
5
£380
IAP
(CSTR) +
DRB.
99%
45%
£379k
-15
£7,530
95%
88%
£135k
£19.5k
5
£260
IAP
(CSTR) +
DRB.
99%
45%
£277k
-15
£5,640
RECOMMENDATIONS FOR NEXT STEPS
Full details of the Exploitation Plan for DW-STOP are described in Chapter 5. In essence, there are
opportunities to use the outputs from the project in connection with three fields of activity:
commercial use; technology transfer and strategic issues. These are summarised in Table 6.2.
Table 6.2 Summary of next steps for exploitation of the outputs from DW-STOP
Area of
Activity
Commercial
use of DWSTOP
Outputs
Specific activity
Construction of
systems
Use of systems
Consultancy
Relevant
Partners
ARM
Pallinghurst
Ex-staff of
SRI
Cross
references
to
Chapter 5
Relevance of DW-STOP outputs to the
main subject areas
Environmental
Water
Other
protection
re-use /
applications
re(non-dairy)
cycling
Refer to
Section
5.1.1
5.1.2
5.1.3
5.3.1.1
M, C, D
M, C, D
M, C
5.3.1.2
C, D
C, D
na
5.3.1.3
M, C, D
M, C, D
M, C
205 of 217
Area of
Activity
Technology
Transfer
Specific activity
Communications
to dairy farmers
Demonstration
project(s)
Other
publications
Strategic
Issues
Further
research
Relevance of
DW-STOP
outputs:
6.3
M=
C=
D=
na =
Cross
references
to
Relevant
Partners
Chapter 5
MDC, Exstaff of SRI,
IGER, ADAS
All
All
MDC, Exstaff of SRI,
IGER, ADAS
ADAS, CUS,
IGER, UoB,
Ex-staff of
SRI
mathematical models
case studies
data set
not applicable
Relevance of DW-STOP outputs to the
main subject areas
Environmental
Water
Other
protection
re-use /
applications
re(non-dairy)
cycling
Refer to
Section
5.1.1
5.1.2
5.1.3
5.3.2.1
C
C
na
5.3.2.2
M, C, D
M, C, D
na
5.3.2.3
M, C, D
M, C, D
na
5.3.3
M, C, D
M, C, D
na
5.3.4
M, C, D
M, C, D
M, C, D
5.2.1
5.2.2
5.2.3
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by irrigation onto vegetated treatment planes, Water Research 36:291-299.
Verdoodt A., Van Ranst E., Ye L. and Verplancke H. (2005) A daily mulit-layered water balance model to
predict water and oxygen availability in tropical cropping systems. Soil Use and Management 21, 312-321.
Weatherhead, E. K.; Knox, J. W. (2000) Predicting and mapping the future demand for irrigation water in
England and Wales. Agricultural Water Management 43: 2, 203-218.
Willers H.C.; Derikx, P. J. L.; Ten Have, P.J.W; Vijn T.K. (1996) Emission of ammonia and nitrous oxide
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Williams J. R. Nicholson, R. J. (1995) Low rate irrigation of dirty water and its effect on drainage water
quality. The Agricultural Engineer, 50 (2), 24 28
Williams, A. G. , Shaw, M. , Selviah, C. M. , Cumby, R. J. (1989) The oxygen requirements for deodorizing
and stabilizing pig slurry by aerobic treatment Journal of Agricultural Engineering Research 43 291-311
Zhao, Y. Q., Sun, G. and Allen, S. J., 2004, Anti sized reed bed system for animal wastewater treatment: a
comparative study. Water Research 38, 2907 – 2917.
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APPENDICES
APPENDIX 1 Glossary
Specific term
5-Day Biochemical
Demand (BOD5):
Definition or meaning
Oxygen
Carbon-Nitrogen Ratio (C:N)
Chemical Oxidation Demand
(COD):
Colloidal Matter:
Fertiliser Value
Notifiable disease
Odour Threshold
Organic Matter.
Organic Nitrogen
Population Equivalent (PE):
r.h or RH
Redox Potential (sometimes
known as Standard Redox
Potential)
The quantity of oxygen used by micro-organisms in the biological oxidation
of organic matter in five days, at a specified temperature (20oc), and under
specified conditions. A standard test used in assessing wastewater strength.
.The weight ratio of carbon to nitrogen in a waste material.
A measure of the oxygen-consuming capacity of inorganic and organic
matter present in water or wastewater. It is expressed as the amount of
oxygen consumed from a chemical oxidant in a specified test. It does not
differentiate between stable and unstable organic matter and thus does not
necessarily correlate with biochemical oxygen demand. This is also
sometimes known as oxygen consumed (OC) or dichromate oxygen
consumed (DOC)
Finely divided solids that will not settle but may be removed by coagulation
or biochemical action or membrane filtration.
The potential worth of the plant nutrients (especially nitrogen and
phosphorus and potassium) contained in the wastes and that could become
available to plants when applied onto the soil. A monetary value assigned
to a quantity of organic wastes represents the cost of obtaining the same
quantity of plant nutrients in their commercial form as that found in the
waste. The worth of the waste as a fertiliser can be estimated only for given
soil conditions and other pertinent factors such as land availability, time,
and handling
A notifiable disease is a disease named in section 88 of the Animal Health
Act 1981 or an Order made under that Act. Section 15(1) of the Act says
that: "any person having in their possession or under their charge an animal
affected or suspected of having one of these diseases must, with all
practicable speed, notify that fact to a police constable."
The point at which, after successive dilutions with an odourless medium,
(e.g. Air) the odour of the sample can just be detected. The threshold odour
is expressed quantitatively by the number of times the sample is diluted.
Chemical substances of animal or vegetable origin, comprising cell matter
and the products of cell decay
This collective term represents the mass of nitrogen in various compounds
in a waste, except ammoniacal nitrogen, nitrate and nitrite. It is measured
as the Total (Kjeldahl) nitrogen – ammoniacal nitrogen.
A means of expressing the strength of organic material in wastewater.
Equivalence can be estimated based on a number of parameters, most
commonly flow, BOD5 or suspended solids. For example, domestic
wastewater consumes, on average 0.08 kg of oxygen per capita per day as
measured by the standard BOD5 test. This figure has been used to measure
the strength of organic industrial waste in terms of an equivalent number of
persons. For example, if an industry discharges 480 kg of BOD5 per day, its
waste is equivalent to the domestic wastewater from 6 000 persons
(480/0.08 = 6 000). Caution must be exercised when using population
equivalents because of the difficulty in comparing processing plant wastes
directly with municipal wastes.
Relative humidity (%)
A measure of the electrical voltage (or potential) at which a substance or
mixture of substances will be reduced by electrons, relative to that at which
hydrogen ions will be reduced. It applies to solutions containing both
reactants and products and is given the symbol E°. Redox potentials are
expressed using a standard hydrogen electrode as a refer ence (EoH 0v.) For
convenience, it is usually measured using a calomel (mercury I chloride)
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Specific term
Definition or meaning
RNA
Settleable Solids
Supernatant
Suspended Solids
Total (Kjeldahl) Nitrogen
Total Ammoniacal
(TAN)
Nitrogen
Total Solids:
Volatile Acids:
Volatile Solids (VS):.
Volatile Suspended Solids (VSS)
electrode, consisting of an Hg/Hg2Cl2 electrode in a saturated solution of
KCl At 25o C, the potential of a calomel electrode (E°cal), is related to the
potential of the standard hydrogen electrode, E°H, according to E°H = E°cal
+ 0.242 V. In the context of waste treatment, redox potential indicates the
tendency towards aerobic or anaerobic activity,
Ribonucleic acid
(1) The matter in wastewater that will not stay in suspension during a
prescribed settling period, such as 1 hr, but either settles to the bottom or
floats to the top. {2) In the Imhoff cone test, the volume of matter that
settles to the bottom of the cone in 1 hr.
The clarified liquid left after the removal of a sediment or precipitate.
(1) Solids that either float on the surface of, or are in suspension in, water,
wastewater, or other liquids, and that are largely removable by laboratory
filtering. (2) The quantity of material removed from wastewater in a
laboratory test, as prescribed in Standard Methods for the Examination of
Water and Wastewater and referred to as non-filterable residue.
This chemical test measures the combined amount of organic nitrogen and
ammoniacal nitrogen in a waste. It also detects other nitrogen compounds
including: azides, amines, hydrozones, oximes, semicarbazones and
azonitrile, nitro and nitroso compounds. However, it does not detect
nitrogen in the form of nitrite or nitrate. The test involves acidic digestion
with a catalyst (normally mercuric sulphate), followed by back titration of
the absorbing acid.
This is a chemical test to measure the amount of nitrogen present in the
form of dissolved ammonia (NH3) and ammonium ions (NH4+). The test
involves alkaline distillation, absorption of the distillate in acid, followed by
back titration of the absorbing acid.
The sum of filterable and non-filterable solids in water or wastewater,
usually stated in milligrams per litre. It is measured as the weight fraction
of residue remaining when a sample of waste is dried at a specified
temperature, usually 105oc for 24 hours.
Fatty acids, containing six or less carbon atoms that are soluble in water and
that can be stream-distilled at atmospheric pressure. Volatile acids are
commonly reported as acetic acid equivalent.
The quantity of solids in water, wastewater, or other liquids lost in ignition
of the dry solids at 550oc. VS are an indication of organic matter present
That portion of the suspended solids residue driven off as volatile
(combustible) gases at 550oc
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APPENDIX 2 Response of behalf of DW-STOP to the Defra Consultation on the Draft Waste
Management (England and Wales) Regulations 2005 (Agricultural Waste Regulations
Consultation) March 2005
--------------------------------------------------------------------------------------------------------------------------Submitted to: Agricultural/Mines and Quarries Waste Consultation on 17 March 2005
Location: Waste Management Division, Defra, Zone 7/H11, Ashdown House, 123 Victoria Street
LONDON SW1E 6DE
Defra Consultation on the Draft Waste Management (England and Wales)
Regulations 2005 (Agricultural Waste Regulations Consultation)
A Response on behalf of the DW-STOP LINK Research Project
1.
Introduction and summary of key points
This response is offered in connection with Consultation Questions Q3(e)(i); (e)(ii); (f) and (g), and is
made on behalf of a current Defra-supported LINK research project: LK0650 / WA0525. The title of
this project is: Dairy Farm Dirty Water - Seeking the Best Solutions to Avoid Pollution (DW-STOP)
Further details are available on the Defra web site, see:
http://www2.defra.gov.uk/research/project_data/More.asp?I=LK0650&SCOPE=0&M=PSA&V=EP%
3A120A
and
http://defrafarmingandfoodscience.csl.gov.uk/unit/floatingtable.cfm?id=4
In view of the possible implications of the Draft Waste Management (England and Wales) Regulations
2005 for the management of dirty water on UK dairy farms, it essential to highlight and act upon the
aspects of these Regulations that might impair the environmentally-sound management of dairy farm
dirty water. The key issues are summarised in the following six points, and are discussed further in
the subsequent sections of this response.
•
The criteria to determine whether any spreading operation is “beneficial to the land” must
be objective, clear and practical. Subjective judgements, introduced through the processes
of interpretation and implementation of the Draft Statutory Instrument (SI) would be
inappropriate (see Section 2).
•
The statutory guidance to be provided by Defra needs to clarify how the proposed limits
on the annual application rate should be applied on individual farms. Use of the whole
farm area would provide the simplest approach, provided that localised pollution is
avoided. The findings from DW-STOP and related studies would help to support this
approach. For example, this research has shown that dairy farm dirty water typically
contains 0.5 - 1 kg/t total nitrogen, N, (including 0.5 kg/t as ammoniacal nitrogen), 0.5
kg/t of P2O5 and 1-1.5 kg/t of K2O. Thus, the minimum value is approximately £0.6 /t so
an application of 50 t/ha has a potential value of £30 (see Section 3).
•
Defra’s statutory guidance needs to clarify the circumstances when dirty water is regarded
as a sub-set either of “slurry” or of “effluent”, as mentioned in Paragraph 49 of the draft
SI. Further clarification is needed to indicate the maximum proportion of waste milk that
can be included in dirty water for licence exemption subject to Paragraph 49, rather than
Paragraph 47 (see Section 3).
•
Any notion that the “treatment” of “livestock manure, slurry or effluent” automatically
means that the material is a “waste”, (requiring either a waste management licence or a
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licence exemption), is illogical and shows a profound misunderstanding of the role of
relevant treatment technologies. Furthermore, such technologies are poorly defined in the
documents supporting the SI. Consequently, acceptable clarification is essential, and
implementation of the SI should be based on well-informed assessments of the source(s),
nature and application of the materials involved (see Section 4).
2.
•
Treatment technologies, including those used in DW-STOP, can help farmers to meet
nutrient application limits, (e.g. nitrogen and phosphorus). Therefore, implementation of
the SI must accept such measures as part and parcel of manure/slurry management, so that
returning these materials to land, within the prescribed limits, remains outside of the
Waste Regulations (see Section 4).
•
Where hydrologically isolated volumes of soil form part of a treatment process,
application of “livestock manure, slurry or effluent” to this soil, and its collection there
from, must be recognised as being outside the controls that relate to conventional field
applications. Where “used” soil is removed from such systems after a period of use, this
material should be regarded as a mixture of soil plus “livestock manure, slurry or
effluent”. Accordingly, it should be outside of the Waste Regulations (see Section 4).
Exclusions and exemptions from the Regulations
Appropriately, the Consultation Paper recognises that where “manure, slurry and effluent” are used on
the farm of origin in accordance with good agricultural practice, they are not regarded as wastes, and
are therefore not covered by the Regulations. However, this exclusion from the Regulations requires
that the material is not used in quantities that exceed the requirements of normal land use.
Within the consultation paper, this exclusion is described as follows:
Consultation Paper Page 23:
(a) Where a farmer is using manure/slurry on the farm on which it is produced as a fertiliser or
soil conditioner to meet the requirements of agricultural land (i.e. the use is beneficial to the land),
then it is not being discarded as waste and does not fall within the WFD's controls.
(b) Manure/slurry may be waste:(i) where a farmer uses it on the farm on which it is produced in quantities which exceed
the requirements of agricultural land (i.e. it is not beneficial); or
(ii) it is transferred from the farm on which it is produced for use by someone else. In this
case, Defra confirmed its intention to exercise the UK's discretion under Article 11 of the
WFD to provide a licence exemption where the use of the manure/slurry provides "benefit
to agriculture or ecological improvement." This commitment is fulfilled in the 2005
Regulations – see Chapter 5 paragraphs 5.32(g) (iii)-(vi) below.
Thus, it appears that the same test of whether spreading material is “beneficial to the land” applies
under two conditions:
• To determine whether the application of material on the farm of origin is excluded from
the Regulations, and
• To determine whether the application of material imported from another farm qualifies for
a licence exemption.
Clearly, the conditions specified to establish what is meant by “beneficial to the land” are very
important. They must be objective, clear and practical. Interpretation and implementation of the SI
should not lead to subjective judgements. For example, this applies in connection with the following
text to establish whether a licence exemption applies for imported material:
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Consultation Paper Page 49 “Paragraph 49”.
(1) The treatment of land by the spreading of agricultural waste so as to benefit
agriculture or lead to ecological improvement if—
(a) the waste consists of—
(i) animal faeces, urine, spoiled straw or manure;
(ii) effluent from any such waste; or
(iii) slurry from washing buildings or yards used for keeping livestock;
(b) in any period of twelve months the quantity of the waste used to treat the land per
hectare is no more than—
(i) 250 cubic metres (tonnes), in a case where at least half the waste used during
that
period is waste referred to in paragraph (a) (i);
(ii) 850 cubic metres (tonnes), in any other case;
(c) the waste is spread as evenly on the ground as is reasonably practicable;
(d) the amount of total nitrogen added to the soil as a result of the treatment does not
exceed 250 kg per hectare in any period of twelve months;
(e) the land is not—
(i) frozen hard (that is, the soil surface has been frozen for at least 12 hours
during the preceding 24 hours) or snow covered;
(ii) waterlogged or flooded; or
(iii) less than 10 metres from a watercourse or 50 metres from a spring, well or
borehole; and
(2) The storage of the waste intended to be submitted to such treatment if–
(a) storage takes place—
(i) in the case of waste referred to in sub-paragraph (1)(a)(i) above, in a secure
place; and
(ii) in any other case, in a secure container or lagoon;
(b) the total quantity of waste being stored at any time does not exceed 1,250 cubic
metres (tonnes); and
(c) no waste is stored for more than twelve months.”
3.
Possible impacts on the effective use of the results from DW-STOP
Successful exploitation of the results from DW-STOP may be impaired if the associated dirty water
management practices involved are deemed not to be “beneficial to the land”. For example, if dirty
water is to be spread on the farm of origin, but is deemed “non-beneficial”, then neither the exclusion
from the Regulations, nor the “Paragraph 49” exemption would apply (see above). Thus, a costly
waste management licence would be needed.
Why should the spreading of dirty water be deemed “non-beneficial”? One possible interpretation of
the consultation paper is that if the final land spreading of the dirty water complies with the conditions
listed in part 1 of “Paragraph 49”, then it should be “beneficial to the land”. However, there are some
seemingly unresolved points that may impinge on this interpretation, highlighting the need for
acceptable clarification:
•
Will the limits on the annual application rates be applied across the whole farm area, or
will they be related to smaller areas? If the former applies, then heavier applications of
dirty water may be acceptable on some land areas (provided that they do not lead to
pollution).
•
No limit on the application of phosphorus is currently included in the proposed
Regulations, although this issue features prominently in the specific questions included in
the consultation. If a maximum phosphorus loading is also imposed, this may further limit
applications in some circumstances.
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•
As shown by DW-STOP and other studies, dirty water often includes waste milk, and
therefore, it could be argued that land spreading of this material should be subject to the
licence exemption provided through “Paragraph 47”, of the Draft SI, which could lead to
lower limits on the annual application rates of dirty water (see below).
•
Are provisions being made for additional criteria envisaged to establish whether spreading
operations are “beneficial to the land” (e.g. demonstration of agronomic value)?
Consultation Paper Page 49 “Paragraph 47.”
(1) The treatment of land by the spreading of agricultural waste so as to benefit agriculture or lead
to ecological improvement if—
(a) the waste consists of liquid milk;
(b) the waste is diluted with not less than an equal quantity of water or slurry;
(c) the land is not—
(i) frozen hard (that is, the soil surface has been frozen for at least 12 hours
during the preceding 24 hours) or snow covered;
(ii) waterlogged or flooded; or
(iii) less than 10 metres from a watercourse or 50 metres from a spring, well or
borehole;
(d) the activity is carried out in accordance with any requirement imposed implementing
an
action programme under the Action Programme for Nitrate Vulnerable Zones (England
and
Wales) Regulations 1998;
(e) in any period of 24 hours the land is not treated with more than 50 cubic metres
(tonnes)
per hectare of the diluted waste and it is at least one month since the last treatment
of the l and; and
(f) the amount of total nitrogen added to the soil as a result of the treatment does not
exceed 250 kg per hectare in any period of twelve months.
(2) The secure storage or dilution of waste intended to be used to treat land as specified in subparagraph (1) above.
4.
Implementation of the Statutory Instrument
Since various detailed aspects will require specific resolution, the DW-STOP Project Partners have
been advised that in implementing the Regulations, the Environment Agency (EA) will introduce their
own interpretation of the requirements, taking into account the statutory guidance to be provided by
Defra. Furthermore, it has been suggested, informally, that the “interpretation” by the EA might seek
to include the argument that if any manure, slurry and effluent requires “treatment” before landspreading on the farm of origin, then this must imply that it is a “waste”, and that a waste
management licence or exemption would be required. This appears to derive from parts of the Waste
Management Licensing Regulations 1994 (“the 1994 Regulations”), as follows:
Consultation Paper Page 41
(vi) The “Recycling…of organic substances…(including composting and other biological
transformation processes)” is a separately identified waste recovery operation in Annex IIB (R4) to
the WFD – see paragraph 5.37 below. The “processing” of manure and slurry which is discarded
as waste has been the subject of a separate consultation exercise on the revision of the licensing
exemption currently provided in paragraph 12 of Schedule 3 to the 1994 Regulations. A copy of
that consultation paper is available on the Defra website at
http://www.defra.gov.uk/corporate/consult/wastemanlicence/index.htm
In the context of DW-STOP, the argument that “treatment” automatically defines the treated material
as a “waste” is wholly unacceptable. Several areas of uncertainty concerning this point require
effective and realistic clarification, as described below. Such clarification would be appropriate for
inclusion in the statutory guidance to be provided by Defra. Except where indicated, the following
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points are made in connection with: “Report to the Agricultural Waste Stakeholders’ Forum From The
Licensing and Exemptions Sub-group”:
http://www.defra.gov.uk/environment/waste/agforum/meetings/2003/pdf/lexempt-report.pdf
This Report reviews the details of existing exemptions from waste management licences and the
reference to “composting” in its Appendix 1 indicates that the exemption is “not intended to cover
manure and slurry”. It is assumed that this is because “composting” of these materials on the farm
where they are produced is not regarded as waste management, because these materials are not seen as
wastes, as described in Q and A no 45, in The Waste Management (England and Wales) Regulations
2005 “The Agricultural Waste Regulations” Question and Answer Brief :
http://www.defra.gov.uk/corporate/consult/agwaste-regs/qanda.pdf
The Waste Management (England and Wales) Regulations 2005 “The Agricultural Waste
Regulations” Question and Answer Brief Q and A no 45.
What about livestock manure, slurry and effluent?
In most circumstances, when manure, slurry and effluent is applied to your own land to make use
of its fertiliser value in accordance with good agricultural practice, it is not being discarded as
waste. There is no intention to control this use with these Regulations. However, interpretation of
European and national case law suggests that livestock manure, slurry and effluent may sometimes
be discarded as waste. We consider this to be when:• the amount applied to your land is excessive (i.e. beyond good agricultural practice); or
• when it is exported from your farm for use at another farm.
It will be necessary in these circumstances to determine on a case-by-case basis whether or not the
manure or slurry is being discarded by the farmer as “waste”. When it is exported we intend to
control its use as a waste with a licensing exemption (Option 4) that the importing farmer would
need to register with the Environment Agency. This is a very simple form of control and will not
involve too much additional work. For more information on where and why some manure, slurry
and effluent may be considered waste – see:
www.defra.gov.uk/environment/waste/agforum/meetings/2002/pdf/240902-letter.pdf
The joint implications of “Appendix 1” and “Q and A no 45” seem to provide a precedent for
"treatment" to be regarded as part and parcel of manure/slurry management, where returning it to land
is outside of these waste controls. However, although this assumption seems reasonable (i.e. that
composting is not intended to cover manure and slurry because this is not regarded as waste
management), this requires confirmation through the statutory guidance to be provided by Defra. This
is necessary to avoid any possibility of an alternative interpretation that any on-farm “composting” of
manures and slurries does require a waste management licence. Such interpretation would be illogical
and unenforceable!
Since the Report links "composting" and "slurries", a wide/loose definition of “composting” is
implied. Indeed, this is acknowledged as follows:
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Licensing and Exemptions Sub-group Report Page 11 “Paragraph 12”.
(1) Composting biodegradable waste at the place where the waste is produced or where
the compost is to be used, or at any other place occupied by the person producing the waste or
using the compost, if the total quantity of waste being composted at that place at any time does
not exceed (a) in the case of waste composted or to be composted for the purposes of cultivating
mushrooms, 10,000 cubic metres; and
(b) in any other case, 1,000 cubic metres.
(2) The storage of biodegradable waste which is to be composted if that storage is at the
place where the waste is produced or is to be composted.
(3) In this paragraph, “composting” includes any other biological transformation process that
results in materials which may be spread on land for the benefit of agriculture or ecological
improvement.
Further uncertainty arises because dirty water is not mentioned specifically. It is assumed that this is
seen as either a sub-set of “slurry” or “effluent” as mentioned in Paragraph 49 of the draft SI.
Acceptable clarification is essential to avoid misunderstandings.
Dr Trevor Cumby
DW-STOP Project Manager
Environment Group,
Silsoe Research Institute
Wrest Park, Silsoe,
Bedford,
MK45 4HS,
United Kingdom
Wednesday, 16 March 2005
Tel: +44 (0)1525 860000
Tel: +44 (0)1525 864022 (direct line)
Fax: +44 (0)1525 860156
e-mail: [email protected]
APPENDIX 3 MPhil thesis of Gari Fernandez, University of Birmingham
Please refer to (Cumby et al, 2005) for an electronic copy of Appendix 3
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