Sludge production based on organic matter and nitrogen
removal performances
N. Serón1, S. Puig2, S.C.F. Meijer3, M.D. Balaguer1, J. Colprim1
1
Laboratory of Chemical and Environmental Engineering (LEQUIA). Institute of the Environment.
University of Girona. Campus Montilivi s/n, Facultat de Ciències, E-17071 Girona, Catalonia, Spain.
(e-mail: [email protected]; [email protected]; [email protected])
2
Catalan Institute for Water research (ICRA). Parc Científic i Tecnològic de la Universitat de Girona.
C/ Emili Grahit, 101. Edifici H2O. E-17003 Girona, Catalonia, Spain. (e-mail: [email protected])
3
ASM design, Alexander Numankade 205, 3572KX Utrecht, The Netherlands.
(e-mail: [email protected])
Abstract
Excess biomass produced during the biological treatment of wastewater requires costly disposal.
As environmental and legislative constraints increase, there is considerable impetus for reducing
the sludge production. Nowadays, several strategies for minimizing it production are applied but
high costs still limit their application in full-scale wastewater treatment plants (WWTPs). On the
other hand, biological nutrient removal (BNR) process may have an impact on the sludge
production. This paper deals with the effect on the organic matter and nitrogen performances on
the sludge production treating urban wastewater. The results demonstrated that the sewage sludge
production was reduced between 50 to 60% (0.38 to 0.16 kg VSS·kg-1 COD) while improving the
nitrogen removal efficiency from 33% to 79%. Therefore, an efficient way to minimize the sludge
production, it is by operating the WWTP in optimal conditions for nutrient removal.
Keywords Activated sludge; biomass production; nitrogen; observed yield; sludge production;
wastewater treatment plant (WWTP)
INTRODUCTION
The activated sludge process is the most widely used in biological wastewater treatments for
domestic and industrial plants (Wang et al., 2006). One of the drawbacks of conventional activated
sludge (CAS) processes is the relative high sludge production which, in combination with the cost
for aeration, generally make up 60% of the total operational cost of wastewater treatment (Wei et
al.,2003). In this sense it becomes necessary to minimize as much as possible the sludge production.
In literature, different treatments are proposed to minimize sludge management costmanagment
cost. Usually these methods are based on decreasing the dry matter or the water contents of the
sludge. Dehydration, humidity oxidation, drying, incineration, composting, landfill disposal and
agriculture use have been used as methods for sludge decrease or disposal (Moliner, 2007).
Nevertheless, the high cost of these strategies and the environmental legislations are forcing the
community to look for new wastewater treatment technologies with lower sludge production,
instead of developing new technologies for sludge treatment to reduce the cost of disposal (Wei et
al., 2003).
For dealing with sludge production, different engineering approaches are available: (1)
mineralization or disintegration of waste activated sludge using chemical or physical treatment,
such as ozonation, chlorination, heating, or mechanical shear force, (2) modification of a
conventional activated sludge process inserting a sludge holding tank in the sludge return circuit to
form an oxic-settling-anaerobi process (OSA) (Chen et al., 2003), where activated sludge is
exposed to an anoxic zone under no food and low oxidation reduction potential (ORP) conditions
periodically (Saby et al.,2003) and (3) anaerobic digestion where the use of phenomena result in the
conversion of organic materials to methane and carbon dioxide in the absence of molecular oxygen
(Switzenbaum, 1995).
In a biological treatment processes, biomass growth occurs concurrent with the oxidation of organic
or inorganic compounds. The ratio of the amount of biomass produced respect to the amount of
substrate consumed is defined as the biomass yield (Yobs). The coefficient values used to predict the
rate of substrate utilization and biomass growth can vary as a function of the wastewater source,
microbial population and temperature (MetCalf and Eddy, 2003). For municipal wastewater Yobs
values range from 0.10 to 0.30 g/g for the primary treatment and 0.30 to 0.50 without primary
treatment. (MetCalf and Eddy, 2003). On the other hand, nutrients froom wastewater should be
treated before discharging. In this sense, the sludge produced after a BNR performance is more
stable than the sludge froom the primary treatment. Therefore, there is a relationship between
sludge production and the biological removal performance. The goal of this study is to find out the
effect of organic matter and nitrogen performances on the sludge production treating urban
wastewater.
MATERIALS AND METHODS
Pilot plant
The 42 L modified University of Cape Town (MUCT) process pilot plant was designed based on
modelling studies (data not shown) and consisted on a rectangular tank with four compartments:
anaerobic (18% of total volume), anoxic (25%) and aerobic (44%) reactors, followed by a settler
(13%). This configuration is designed to biologically remove organic matter, nitrogen and
phosphorus from wastewater. In the MUCT configuration, the Return Activated Sludge (RAS) is
recycled to the anoxic stage. With an internal recycle from the anoxic stage, the anaerobic stage is
fed with activated sludge low on nitrate thereby, maintaining anaerobic conditions and stimulating
the growth of biological phosphorus removing bacteria (Puig, 2007). The schematic diagram of this
process is shown in Fig1.
RECYCLE
RECYCLE
SECONDARY
CLARIFIER
Influent
ANAEROBIC
ANOXIC
Efluent
AEROBI
RAS
WASTE
Figure 1: Scheme of the pilot plant treating urban wastewater.
The reactor was equipped with a floating-probe system for on-line monitoring of the pH (Crison),
ORP (Crison), temperature (PT-100) and dissolved oxygen (DO)(Crison). An on/off DO control
was applied at 1.5 mgO2·L-1. Filling, recirculation and wastage events were conducted by different
peristaltic pumps (Watson Marlow®). Mixers were equipped in the anoxic, anaerobic and aerobic
compartments. Table 1 shows the operational conditions applied during the experimental period.
Table 1. Operational conditions of the pilot plant during experimental period
Parameter
Value
Units
168
Days
Operational days
20
Hours
Hydraulic retention time (HRT)
44
L·d-1
Daily flow
400
% of the inflow
Anaerobic recirculation percentage
400
% of the inflow
Anoxic recirculation percentage
100
% of the inflow
External recirculation percentage
1.5
mg O2·L-1
DO set point
Domestic wastewater characteristics
The pilot plant treated urban wastewater from the Quart WWTP (Girona, N.E. Spain). Twice a
week 200L of fresh wastewater were transporter to our laboratory and stored at 4ºC in a 250L
stainless steel mixing tank to minimize microbiological activity. The characteristics of the domestic
wastewater fed to the pilot plant are shown in Table 2. The urban wastewater presented a C/N ratio
of 7.6 mg COD/mg N-TKN with a high percentage of particulate COD (calculate as the difference
between total and soluble COD).
Table 2. Characterization of the wastewater during the experimental study.
Parameter
Average Standard deviation
Units
641
214
mgCOD·L-1
Total COD (CODt)
149
75
mgCOD·L-1
Soluble COD (CODs)
85
19
mgN-TKN·L-1
Total Kjeldalh (TKN)
58
12
mg N-NH4+-·L-1
N-NH4+
13
5
mgP-TP·L-1
Total P
35
2
mg P-PO43-·L-1
P-PO4
367
317
mgTSS·L-1
TSS
317
137
mgVSS·L-1
VSS
Simulations of different configurations to minimize treatment sludge production
Different scenario simulations were performed with Biowin simulation software (Envirosim
Associates Ltd.), treating standard wastewater (in the range of the experimental wastewater) with a
fixed sludge age of 11 days. The mathematical model included a hydraulic model (reproducing all
the flows), secondary settler model (ideal type), an activated sludge model for the water line and a
model for the sludge line. Two different simulations were carried out: i) BNR performance with low
nitrification efficiencies (Sim1) and ii) high organic matter and nitrogen efficiencies (Sim2).
Sludge production assays
The solids production in a WWTP was balanced by the mass (solids) removed (per day). The
wastage usually is expressed as total suspended solids (TSS) and volatile suspended solids (VSS).
The sludge production efficiency was calculated from experimental data based on the organic
matter daily converted per amount of sludge daily produced in g VSS produced/g COD converted.
This calculation resulted in the experimentally observed yield, Yobs expressed as grams of VSS
produced per grams of substrate COD removed. The measured solids production included the solids
wasted via the effluent.
Analytical methods
Standard wastewater measurements for organic matter (chemical oxygen demand (COD) and total
organic carbon), nitrogen (total Kjeldalh (TKN) nitrogen, ammonium (NH4+), nitrates (NO3-) and
nitrites (NO2-)), phosphorus (total phosphorus and phosphates), total suspended solids (TSS) and
volatile suspended solids (VSS) were taken regularly and analyzed according to Standard Methods
for the Examination of Water and Wastewater by APHA (2005).
RESULTS AND DISCUSSION
Biological nutrient removal and sludge production performances in the pilot plant
The pilot plant was operated for 168 days. Figure 2 shows the evolution profiles of solids waste(a),
carbon(b) and nitrogen(c) compounds during the experiment.
Time (days)
0
-1
TSS waste (g·d )
30
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150 160 170
a
25
20
15
10
5
COD (mg COD·L-1)
0
b
1000
CODs efluent
800
600
400
200
0
160
-1
Nitrogen (mg N·L )
COD influent
c
140
N-TKN influent
N-TKN efluent
3N-NO efluent
120
100
80
60
40
20
0
0
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150 160 170
time (days)
Figure 2. Evolution profiles of TSS wasted (a), COD (b) and nitrogen (c) during the experiment.
Figure 2a shows the total suspended solids (TSS) wasted (effluent plus the wastage), which ranged
from 20 to 2 g·d-1. This high variability was caused by settlings problems in the settler.
Figure 2b presents the organic matter evolution during the experimental study. Almost immediately
after start up, the pilot plant achieved complete COD removal. COD removal efficiencies of around
90% were achieved with effluent concentrations in average 54 mg COD·L-1, in spite of the influent
COD variability (from 327 to 922 mg COD·L-1) due to daily WWTP dynamics.
Regarding the conversion of nitrogen compounds (Figure 2c), the variability in the solids wasted
also affected the nitrogen removal performance. During the experiment periods, complete
nitrification/denitrification (i.e 14-21, 38-49, 60-61, 89-101 and 157-164) were seen, as well as
periods of incomplete nitrogen removal (i.e 103-112 and 138-151). During 80% of the operational
days, the effluent TKN concentration was close to the European Directive 91/217/CEE standards
(15 mg N-TKN·L-1). However, during days 55-59, 103-112 and 138-151, the effluent nitrate
concentration increased up to 20 mg·L-1. Once the solids wasted wereere stabilized, also the
nitrogen performance improved.
Modelling studies
Two simulations were done based on the pilot plant treating an influent of similar characteristics of
the experimental one: i) with incomplete nitrification (Sim 1) and ii) with complete organic matter
and nitrogen performances (Sim 2). The model predicted the measured effluent quality and sludge
production under both performances. Table 3 shows the results of the two performances simulated.
Table 3. Comparison of the two simulations based on the effluent quality and sludge production
Inflow
Sim1
Sim2
Units
Parameter
6
4
5.87
m3·d-1
Flow
575
36
48
mg COD·L-1
Total COD (CODt)
291
7
5
mg BOD·L-1
Total BOD (BODt)
53
35.3
4.3
mgN-TN·L-1
N-TKN
+
35.9
32.4
0.71
mg N-NH4+-·L-1
N-NH4
0
0
6.77
mg N-NO3-·L-1
N-NO3
264
4
24
mgSST·L-1
TSS
-1
172
3
12
mgSSV·L
VSS
92
92
%
COD removal efficiency
33
79
%
Nitrogen removal efficiency
2
0.13
m3·d-1
Flow wasted
936
7785
mgSST·L-1
TSS wasted
635
3919
mgSSV·L-1
VSS wasted
68
50
---VSS/TSS
1.27
0.53
kg VSS·d-1
Sludge production, Yobs
0.73
0.30
kgVSS·kg-1DBOinfluent
Sludge Production, Yobs
0.38
0.16
kgVSS·kg-1 COD remove
Sludge Production, Yobs
The simulations showed that the sludge production was affected by the nitrogen removal
performance. The simulation with reduced nitrification capacity (Sim1), the sludge production was
considerably higher than in the simulation 2 wherehere full nitrification improved the nitrogen
removal. Here the sludge production was reduced between 50 to 60% (0.73 to 0.30 kg VSS · kg-1
BOD or 0.38 to 0.16 kg VSS· kg-1 COD).
In Sim1 less organic matter was denitrified and more organic matter was available and oxidized
aerobically, resulting in a higher sludge production. On the other hand, in simulation 2 (with full
nitrification), a large fraction of organic matter was used for denitrification, resulting in a lower
sludge production. In simulation 1, 0.38 kg VSS was produced per kg COD removed together with
a nitrogen removal efficiency of 33%. In simulation 2, only 0.16 kg VSS was produced per kg COD
removed with an improved nitrogen removal efficiency of 79%.
Acumulated produced VSS (g·d-1 )
Experimental determination of the sludge production
Three experimental assays of the sludge production were carried out under different periods: i) first
assay during days 24-27, ii) second assay during days 53-61 and iii) third assay during days 159168. Figure 3 shows the results of the experimental determination of the sludge production.
90
80
y = 0,3554x
R² = 0,9798
70
60
50
y = 0,3184x
R² = 0,9913
40
30
20
y = 0,1419x
R² = 0,9881
10
0
0
20
40
60
80
100
120
140
160
180
200
220
240
Acumulated removed COD (g·d-1 )
Figure 3. Sludge production in different periods during the experimental study.
The values of observed yield Yobs were obtained from the slope of the regression line and relate to
the amounts of VSS produced versus COD removed. Table 4 shows the removal efficiencies, Y obs
and STR for the three experimental assays. In the first assay, the lowest value of Yobs was measured
(0.142 gVSS produced·g-1 COD removed).
Table 4. Comparison of the observed yield, Yobs and the nutrient removal efficiency for the different assays
Parameter
Assay 1 Assay 2 Assay 3 Theorical value
Units
0.142
0.355
0.318
0.40
g VSS· g-1 COD remove
Yobs
87
68
66
-%
COD total removal efficiency
91
91
90
-%
COD soluble removal efficiency
76
65
82
-%
N removal efficiency
12
11
11
-days
Sludge retention time
As long as the nutrient removal efficiencies increased, the Yobs decreased. Comparing the three
assays, when the COD removal efficiencies (from 68 to 87%), Yobs decreased from 0.335 to 0.142
grVSS·g-1COD removed. On the other hand, when improving the efficiencies of nitrogen removal
(from 65 to 82%) also reduced the Yobs from 0.335 to 0.318 grVSS·g-1COD removed.
Sludge production assessment based on WWTP mass balance
The experimental historical data from the pilot plant was evaluated based on the statistical mass
balance method developed in Meijer et al., (2002) and Puig et al., (2008). Balanced data was used
to calculate the sludge production by making an overall WWTP mass balance. Table 5 presents the
results obtained at different periods studied as a function of the Yobs and the removal efficiency.
Table 5. Comparison of the observed yield, Yobs and the nutrient removal efficiency for different periods using
the mass balance method
Parameter
Test 1
Test 2
Test 3
Test4
Units
12-32
60-82
95-101
103-129
days
Period
g VSS· g-1 COD remove
0.112
0.227
0.09
0.325
Yobs
%
88
83
94
66
Total COD removal efficiency
%
91
90
95
91
Soluble COD removal efficiency
%
80
63
81
59
N removal efficiency
12
11
13
9
days
Sludge retention time
As can be seen in Table 5, tests 1 and 3 resulted in similar values for Yobs. Both tests showed high
COD removal achieving 90% as well as high nitrogen removal efficiencies (88 to94%). Regarding
tests 2 and 4, the Yobs were 0.227 and 0.325 gVSS·g-1COD removed achieving 83% and 66% of
COD removal efficiency. For test 4, the observed yield can be related to the incomplete removal of
(mainly particulate) COD. If the particulate COD fraction is not degraded, this directly will result in
an increased sludge production. This experimental data shows that increasing nitrogen removal
efficiency reduces the sludge production, under condition of complete COD removal.
CONCLUSIONS
In this paper, the effect of biological organic matter and nitrogen removal on the sludge production
has been studied successfully using different approaches: simulation studies, experimental test and
mass balance.
Simulation shows that the sludge production is affected by the BNR performance; where the
efficiency of nitrogen removal is improved, sludge production can be reduced up to 60 to 70% (0.38
to 0.14 kg VSS·kg-1 COD).
Both the measured sludge production and the sludge production calculated from the WWTP mass
balances evaluated; demonstrate that complete COD removal is the main condition for a reduced
sludge production. Particulate COD which is not degraded directly increases the amount of sludge
in the process.
Further reduction of Yobs can be obtained by increasing the nitrogen removal efficiency, and
improving conditions where more organic material is removed under anoxic conditions resulting in
lower sludge production. At low nitrogen and phosphorus removal efficiencies, the organic matter
is oxidised aerobically and more sludge is produced as a result.
In conclusion, this study demonstrates how sludge production is a result of BNR performance in
general and nitrogen removal efficiency in particular. If the nutrient removal efficiencies increase,
the observed yield for sludge production Yobs decreases. Therefore, an efficient way to minimize the
sludge production, it is by operating the system in optimal conditions for nutrient removal.
ACKNOWLEDGEMENTS
The authors would like to thank the Ministerio de Medio Ambiente and also the members of
INIMA. This research was financially supported by the Spanish Government (CONSOLIDERCSD2007-00055) and the AGAUR- Catalan Government for the Post-Doctoral fellowship BP-B100193-2007. The authors also thank Gemma Rustullet, Ariadna Cabezas, Anna Rossell and Hèctor
Monclús (Lequia-UdG) for their cooperation during the experimental study.
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