Report
Alexandra Water Supply – Preliminary Design and Cost
Estimates
Prepared for Central Otago District Council (Client)
By CH2M Beca Limited
10 December 2013
© CH2M Beca 2013 (unless CH2M Beca has expressly agreed otherwise with the Client in writing). This report has been prepared by CH2M Beca on the specific instructions of our Client. It is solely for our
Client’s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which CH2M Beca has not given its prior written
consent, is at that person's own risk.
Alexandra Water Supply – Preliminary Design and Cost Estimates
Revision History
Revision Nº
Prepared By
Description
Date
A
Robert Crosbie / Andrew
Watson / Simon Drew
5th June 2013
B
Robert Crosbie / Andrew
Watson / Simon Drew
10th July 2013
C
Robert Crosbie / Andrew
Watson / Simon Drew
10 December
2013
th
Document Acceptance
Action
Name
Prepared by
Robert Crosbie/Andrew
Watson
Reviewed by
Simon Drew
Approved by
Robert Crosbie
on behalf of
CH2M Beca Limited
Signed
Date
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Table of Contents
1
Introduction .......................................................................................................... 4
2
Objectives ............................................................................................................ 4
2.1
Capacity .................................................................................................................. 4
2.2
Treated Water Quality Requirements ....................................................................... 5
2.3
Resilience................................................................................................................ 6
3
Treatment Plant Location .................................................................................... 6
4
Raw Water Quality ............................................................................................... 6
5
6
7
8
9
4.1
Dairy Creek Lakeside Wells ..................................................................................... 6
4.2
Clutha River........................................................................................................... 10
4.3
Existing Bores........................................................................................................ 12
Raw Water Intakes ............................................................................................. 13
5.1
Intake Options ....................................................................................................... 13
5.2
Clyde Dam............................................................................................................. 13
5.3
Bores (Dairy Creek Area) ....................................................................................... 13
Treatment ........................................................................................................... 19
6.1
Process Selection .................................................................................................. 19
6.2
Dairy Creek Lakeside Bore Source ........................................................................ 20
6.3
Clutha River Source............................................................................................... 20
6.4
Reliability, Redundancy and Automation ................................................................ 22
6.5
Process Details for Conventional Treatment........................................................... 22
6.6
Process Details for Membrane Treatment .............................................................. 25
6.7
Wastewater ........................................................................................................... 26
6.8
Existing Borefield ................................................................................................... 28
6.9
Summary of Treatment Option Costs ..................................................................... 30
Treated / Raw Water Pipeline ............................................................................ 31
7.1
Pipeline Alignments ............................................................................................... 31
7.2
Pipe Material and Pressure Class .......................................................................... 32
7.3
Booster Pump Station ............................................................................................ 33
Consenting ......................................................................................................... 33
8.1
Land Use Designation............................................................................................ 33
8.2
Water Take Consent .............................................................................................. 33
8.3
Discharge Consent ................................................................................................ 33
8.4
Building Consent ................................................................................................... 33
Geotechnical Assessment ................................................................................ 34
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Executive Summary
CH2M Beca Ltd has been commissioned to investigate options for a new water source and Water
Treatment Plant (WTP) to supply Alexandra.
Options to be investigated were:
1. Supply direct from Clutha River
2. Supply from Clyde Dam
Discussion with CODC regarding siting of a new WTP indicated that there is a number of potential
sites for a WTP therefore this report assumes a location close to the water source and includes an
estimated cost to connect to the nearest suitable point in the existing reticulation system.
Subsequent to the initial commission, CODC requested that options to use the existing water
source be considered and those options included:

Softening water drawn from the existing source to an acceptable hardness level

Simply improving existing water to achieve DWSNZ compliance without softening
Raw Water Source
A detailed assessment of the existing borefill was not carried out.
After an on-foot inspection of potential river intake sites a suitable site was identified on the left bank
of the Clutha River near Alexandra. An inspection of the area near Clyde Dam identified that an
intake just upstream of the Clyde Dam on the left bank would be feasible to construct either as a
piled structure in the lake or a floating pontoon system although access to the edge of the lake is
difficult given the slope and size of the slope protection rip-rap. In any event, water directly from
Lake Dunstan will be of very similar quality to that in the Clutha River near Alexandra. Therefore,
given the need to construct a pipeline to Alexandra from Lake Dunstan to convey essentially the
same water as that which can be extracted closer to Alexandra at lower capital cost, this option was
not considered further.
Results from testing on the Clyde bore when it was installed in 2002 indicate minimal draw down at
full flow suggesting that local subsurface conditions are suitable for additional bores.
Given that a bore supplying Clyde is located in this area the possibility of adding further bores in the
same area has been investigated.
There is also an existing water intake through Clyde dam which was investigated.
The outcome of investigation with respect to raw water sources is that:
1. River water can be extracted from the Clutha River near Alexandra with an appropriately
designed low maintenance intake.
2. Assuming sub-surface conditions similar to those at the Clyde bore exist over an area wide
enough to accommodate two further bores, a bore based water source in this area is feasible.
3. The existing water intake in the Clyde Dam is insufficient to meet Alexandra’s demand.
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Treatment Plant Options
The Clutha River experiences significant turbidity events on a relatively frequent basis. This is the
result of sediments entering upstream rivers in heavy rainfall events in the mountains which often
bring hot, dry conditions to Alexandra forcing water demand up. It is therefore necessary to design
a WTP with sufficient capacity to meet full demand during a significant turbidity event.
Options considered for treatment process to supply 15 ML/d are:
1A. Conventional clarification to remove most of the sediment followed by filtration and disinfection.
1B. Conventional clarification followed by membrane filtration and disinfection.
These options are applicable to water extracted from either the Clutha River or Lake Dunstan but
given a Lake Dunstan source also requires a pipeline to Alexandra these treatment plant options
were not considered further in conjunction with a Lake Dunstan source.
In reviewing turbidity data from the Clyde bore it became apparent that there is a significant pretreatment effect arising from filtration through the subsurface gravels. The reduction in turbidity is
such that cartridge filtration and UV disinfection would be sufficient to meet the NZ Drinking Water
Standards. Such a system offers significant savings in capital and operating costs and
consequently was investigated further.
Consequently for water extracted from bores adjacent Clyde Dam the treatment process is:
2A. Cartridge filtration, UV disinfection and chlorination to provide a disinfection residual.
A review of photos taken during and after Clyde Dam construction suggests the Clyde bore may be
installed in a man-made fill and that the same fill extends significant distance along the left bank of
Lake Dunstan just upstream of the dam.
This treatment process is reliable and simple to operate requiring no specialist local expertise.
An option to continue to use the existing borefield was also considered both with and without water
softening but in any case achieving compliance with DWSNZ.
Options considered were:
3A. Softening with lime
3B. Softening with Nanofiltration (NF)
3C. Compliance with DWSNZ without softening
Option 3 does not address the risk associated with the closed landfill up-gradient from the existing
borefield.
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The mid-range capital cost estimates for the options above and the associated 20 Year Net Present
Value (NPV) of Capital (Capex) and Operating Costs (Opex) are as follows:
Capex
NPV (Capex & Opex)
Option 1A
$14.534m
$21.275m
Option 1B
$16.934m
$25.452m
Option 2A
$11.426m
$16.059m
Option 3A
$12.030m
$20.821m
Option 3B
$11.93m
$20.559m
Option 3C
$8.560m
$14.594m
Note: Capex estimates allow for a 15MLD facility, however, Opex estimates have been calculated
using an estimated average water demand of 4.1MLD.
All the above are for 15 Ml/d to supply Alexandra. Other options including supply of Alexandra plus
Clyde or Dunstan Flats and plus Clyde and Dunstan Flats are discussed in the report.
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1
Introduction
Central Otago District Council (CODC) has identified the need to substantially upgrade the water
supply at Alexandra to meet demands for improved potable water quality and to meet the treatment
requirements of the Drinking Water Standards for New Zealand (DWSNZ).
This report addresses two concepts for a new treatment plant and presents updated cost estimates
for the proposed upgrade, in accordance with the general scope set out in CODC’s RFT referenced
CON 05-2013-01.
For those concepts, Process Flow Diagrams (PFDs) are attached as Appendix A and draft site
plans are included in Appendix B. Cost estimates are included in Appendix C.
The project brief required an assessment of intake and treatment options to supply Alexandra from
with two broadly defined intake locations being adjacent Alexandra in the Clutha River and in the
Dairy Creek area above Clyde Dam. More specific details of intake options are covered in Section
5.0.
The report also investigates options to retain the current borefield and to treat that water to comply
with DWSNZ with and without water softening.
2
Objectives
2.1
Capacity
The minimum treatment capacity required irrespective of intake location is that capacity required to
supply Alexandra. Clyde currently has a satisfactory water supply drawing water from a bore on the
true left bank of Lake Dunstan approximately 300 m upstream of Clyde Dam.
There would be no benefit in providing capacity in a treatment plant located at Alexandra in order to
supply Clyde because the Clyde supply is considered satisfactory and is a low cost source to
operate. On the other hand a treatment plant located in the Dairy Creek area supplying Alexandra
would present an opportunity to combine supply of treated water to both Clyde and Alexandra from
a common treatment plant with potential savings in operating costs.
Further, a treatment plant located near Dairy Creek would require a treated water pipeline to
Alexandra which presents an opportunity to provide treated water to the Dunstan Flats area
between Alexandra and Clyde at some time in the future. Consequently there are a number of
options which were considered to justify an evaluation of capital and operating costs at this stage of
the project. Those options are set out in Table 1 below along with corresponding treatment plant
capacities.
A treated water capacity of 15 ML/day is required to meet the forecast peak demand for Alexandra.
There are variations to this capacity as described below which would cover the supply to Clyde and
the Dunstan Flats as discussed in Section 5.0.
It should be noted that capital cost estimates have been developed for a 15 Ml/d plant. These were
then scaled to cover 20 Ml/d and 25 Ml/d plants to cover the additional options set out in Table 1.
This report has not considered the future option of supplying Dunstan Flats by augmenting the
existing Clyde supply. This report has considered the option of supplying Dunstan Flats from a
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Alexandra Water Supply – Preliminary Design and Cost Estimates
WTP located at Alexandra because such a supply could be achieved in the future by extending a
WTP at Alexandra and adding reticulation to service the Dunstan Flats.
The following table summarises the treatment plant capacity required to meet demand under
various scenarios.
Table 1 – Treatment Capacity
Area Supplied
Intake Location
1. Alexandra + Alexandra
2. Dairy Creek + Alexandra
Capacity
Alexandra
A
Alexandra
B
Alexandra +
Flats (or
Clyde)
C
Alexandra +
Flats
D
Alexandra +
Flats +
Clyde
E
Conventional
Membrane
Conventional
Membrane
-
UV
-
UV
-
UV
15 Ml/d
-
20 Ml/d
20 Ml/d
25 Ml/d
The above table does not necessarily imply that a UV treatment plant need be located at Alexandra.
Being a relatively compact plant, it could also be located near (and above) Clyde. If a UV plant was
located above Clyde then Option 2C could be taken as being for supply to Clyde or Dunstan Flats.
Similarly, if the UV plant is located above Clyde option 2E would provide sufficient water to supply
Clyde and Dunstan Flats.
Option 2A requires a raw water line from Dairy Creek to Alexandra.
Option 2C or 2E requires a treated water line from Dairy Creek to Alexandra.
For the purposes of developing cost estimates for this report Option 2 costs are considered not to
be sensitive to location of WTP. There would be minor differences in the pipeline cost between it
being a raw water line or treated water line and minor differences in pipe line diameter. These
differences are sufficiently small that they fall within the estimating margin.
A further option, Option 3 (A, B and C see Section 6.8) assesses the costs of providing treatment to
water extracted from the existing borefield only for a 15 ML/d capacity. The existing borefield is
reported as only having capacity for 17 ML/d.
2.2
Treated Water Quality Requirements
CODC’s RFP required the study to consider two levels of treated water quality criteria for the
upgrading:

Compliance with DWSNZ 2005 (revised 2008) requirements

A satisfactorily reduced scaling tendency.
From the RFP, and as discussed at the CODC/CH2M Beca meeting on 22nd April 2013, there have
been occasional taste & odour issues with the existing Alexandra groundwater supply. Although not
specifically stated in the RFP, we have assumed that the upgrading also needs to address this
matter.
The addition of fluoride for dental health has not been allowed for.
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2.3
Resilience
New structures should be designed for being functional post disaster. Under the Building Code this
will require new structures to be designed for a Level of Importance of 4.
3
Treatment Plant Location
Cost estimates have been prepared on the basis of two alternative treatment plant locations.
CODC has confirmed that at present it has no particular site in mind and that there is no land which
has been specifically designated for such purposes.
After discussion with CODC it was agreed that for the purposes of this report, it would be assumed
that a treatment plant would be located in one of two general locations being:
1) Near the proposed river intake in the general vicinity of the western end of Boundary Road at an
elevation above Clutha River flood level. This location would also apply to a plant treating water
from the existing borefield.
2) Near Dairy Creek. Ideally this would be at an elevation which would allow water to flow into
Alexandra without pumping but not so high as to waste energy pumping to a treatment plant
located at an elevation greater than necessary to supply Alexandra.
There may be modifications required to existing reticulation to accommodate connection from a
plant at either location but these modifications are beyond the scope of this report, except that a raw
or treated water line from Dairy Creek to Alexandra has been included in the capital and operating
cost estimates.
Capital cost estimates allow for the cost of pipelines between the intake, or bores, and treatment
plant.
4
Raw Water Quality
4.1
Dairy Creek Lakeside Wells
4.1.1
Turbidity
Hourly measurements of the turbidity of Clyde treated water was obtained from the Clyde Water
Treatment Plant for 2007 to 2013. The Clyde WTP is supplied from a lakeside bore at Dairy Creek
beside Lake Dunstan. As it is a chlorination-only plant the treated water turbidity can be considered
as representative of the groundwater taken from the bore. The data was analysed from October
2010 onwards because of a large number of inaccuracies caused by instrumental errors in the
earlier data. A comparison between the Clutha River data and the Clyde Treated Water data was
used to distinguish additional outliers (refer Figure 1).
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Figure 1 - Comparison between Clutha River and Clyde Treated Water Turbidity Data
From this graph, it was determined that there were three peaks in the Clyde Treated Water data that
could be explained by the turbidity fluctuation in the Clutha River (02/03/2011, 02/01/2013 and
14/01/2013) and three peaks that could not be explained (13/12/2010, 13/04/2012 and 31/10/2012)
and so were considered outliers. The peak at 31/10/2012 is followed by a peak in the turbidity of
Clutha River five days later. This length of time is considered to be too large to provide an
explanation for the Clyde Treated Water peak. Therefore this outlier, along with all others, were
removed from all further graphs and calculations.
Figure 2 shows the turbidity of the Clyde treated water over time. From this graph the five highest
turbidities were found and are shown in Table 3. An average turbidity was found for this entire
range as well as a percentage of the time that turbidity was higher than both 1 and 2 NTU (Table 2).
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Figure 2 – Turbidity Time Series Plot for Clyde Treated Water (With Outliers Removed)
Table 2 - Clyde Treated Water Turbidity
Average Turbidity:
% of time that turbidity is above 1 NTU:
% of time that turbidity is above 2 NTU:
0.151
0.54%
0.01%
Table 3 - Highest Turbidities in Clyde Treated Water
Highest
turbidities
1
2
3
4
5
2.160
1.990
1.660
1.180
0.930
Time and date
14/01/2013 11:02
3/01/2013 8:02
2/03/2011 18:02
31/10/2012 23:02
23/11/2010 20:02
The longest periods of time that turbidity was over 1 or 2 NTU was found using a series of “If”
statements in Excel. There were only five occurrences where turbidity reached a level higher than
1 NTU and so these are all summarised in Table 4. Of these five occurrences, there was only one
reading where turbidity went above 2 NTU (Table 5).
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Table 4 - Largest Range of Turbidities over 1 NTU for Clyde Treated Water
1
2
3
4
5
Start date of
turbidity over 1
NTU
31/12/2012 15:02
13/01/2013 9:02
14/01/2013 16:02
2/03/2011 18:02
31/10/2012 22:02
End date of turbidity
over 1 NTU
3/01/2013 8:02
14/01/2013 13:02
14/01/2013 23:02
2/03/2011 22:02
31/10/2012 23:02
Number
of days
2.7
1.2*
0.3*
0.2
0.0
* Although shown separately, these two exceedences are effectively one event
Table 5 - Largest Range of Turbidities over 2 NTU for Clyde Treated Water
1
4.1.2
Start date of
turbidity over 20
NTU
14/01/2013
11:02
End date of turbidity over
20 NTU
14/01/2013 11:02
Number
of days
0.00
Chemical and Physical Data
In the laboratory analysis reports included in the URS report 1 (Appendix A) there are three reports
for grab samples from the Clyde supply. Two are recorded as “raw water” and one as “WTP”.
There are no chemical determinands that exceed 50% of the MAV. However the full suite of
DWSNZ determinands has not been tested for and we recommend that this be undertaken.
Parameters of interest for process selection and design are summarised in Table 6.
Table 6 – Summary of Chemical and Physical Parameters of Interest
28th April 2005
Raw Water
15th March 2007
Raw Water
17th December 08
WTP
0.013
0.005
0.008
Alkalinity to pH 4.5 (mg/L as CaCO3)
54
42
43
Colour (CPU)
0.5
<0.5
<0.5
Total hardness (mg/L as CaCO3)
55
43
45
Total organic carbon (mg/L)
0.24
<0.5
2.1
pH
7.42
7.42
7.46
Turbidity (NTU)
2.5
0.15
0.25
Parameter
Absorbance (AU)
The turbidity result for the 2005 sample is high compared with the analysis of the Clyde treated
water online turbidimeter data presented above. We presume therefore that the sample was taken
during pump start up and represents a turbidity spike as the bore stabilises.
1
URS, 2009. Review of Riverbank Filtration as a Water Supply Source for Alexandra, URS Christchurch.
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4.2
Clutha River
4.2.1
Protozoal Data
Cryptosporidium monitoring of the Clutha River source has been undertaken from October 2011
until January 2013, with a total of 33 samples collected over this period. The dataset represents a
rolling 12 month series containing 26 or 27 samples in each 12 month period. It therefore meets the
DWSNS requirement of “… at least 26 samples collected over a 12-month period”. All results have
been normalised as per DWSNZ, and are all reported as “less than” values. Accordingly, following
the procedure set out in DWSNZ, all results treated as zeros, resulting in a mean for the dataset of
zero, and therefore a 3 log credit requirement.
4.2.2
Turbidity Data
NIWA provided mean hourly values for the turbidity of the Clutha River at Alexandra Bridge (Site
th
rd
575228) from 12 September 1995 to 23 January 2013 (refer Figure 3). This site is funded by
Contact Energy. The turbidity monitor is a Campbell Scientific OBS-3 submersible probe, which
uses the backscatter method to measure turbidity in the zero to 4,000 NTU range. Campbell
Scientific quotes an accuracy of the greater of 2% of reading or 0.5 NTU 2.
An average turbidity was found for this entire range as well as a percentage of the time that turbidity
was higher than both 10 and 20 NTU (Table 7). There were a number of gaps in the data provided.
Altogether these gaps accounted for 4.8% of the data and approximately 293 days. These data
gaps were excluded from all calculations.
2
This is actually quoted for the OBS3+ model which has replaced the OBS-3.
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Figure 3 - Turbidity Time Series Plot for Clutha River
Table 7 - Clutha River Turbidity
Average Turbidity (NTU):
% of time higher than 10 NTU:
% of time higher than 20 NTU:
4.007
6.97%
2.36%
From Figure 1, the five highest turbidities were found and they were recorded along with their date
of occurrence (Table 8). These peak turbidities most likely relate to flood events in the Clutha River.
Table 8 - Highest Turbidity Events in Clutha River
1
2
3
4
5
Highest Turbidities
189.580
157.120
136.89
109.3
105.26
Time and Date
21/09/2002 3:00
28/06/2000 5:00
9/10/1996 20:00
6/01/2002 0:00
19/12/1995 18:00
A series of “If Statements” were used to find the five largest ranges of turbidities higher than 10 NTU
and 20 NTU. The results are presented in Table 9 and Table 10. The results show that turbidity
remained above 10 NTU for a maximum of 25.2 days and above 20 NTU for a maximum of 11.9
days.
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Table 9 - Largest Range of Turbidities over 10 NTU at Clutha River
1
2
3
4
5
Start date of turbidity
over 10 NTU
25/11/1999 15:00
5/10/2000 14:00
4/10/1996 12:00
19/12/1995 18:00
26/06/2000 0:00
End date of turbidity
over 10 NTU
20/12/1999 19:00
24/10/2000 17:00
17/10/1996 5:00
30/12/1995 23:00
7/07/2000 1:00
Number of
days
25.2
19.1
12.7
11.2
11.0
Table 10 - Largest Range of Turbidities over 20 NTU at Clutha River
1
2
3
4
5
4.2.3
Start date of turbidity
over 20 NTU
25/11/1999 15:00
19/12/1995 18:00
20/09/2002 10:00
26/06/2000 5:00
7/10/1996 19:00
End date of turbidity
over 20 NTU
7/12/1999 13:00
27/12/1995 15:00
27/09/2002 22:00
3/07/2000 16:00
14/10/1996 16:00
Number of
days
11.9
7.9
7.5
7.5
6.9
Chemical and Physical Data
There are no analytical reports available for samples from the Clutha River at Alexandra. For the
purposes of this study, we have assumed that the analyses available for the lakeside bore at Clyde
are representative of the water quality at Alexandra.
4.3
Existing Bores
Grab sampling data presented in the URS report shows total hardness results of between about 90
and 190 mg/L (as CaCO3) for the groundwater quality from the existing Alexandra wellfield. URS
also developed a correlation between electrical conductivity and hardness, and showed that when
the river is in high flow hardness in the groundwater reduces.
Based on the grab sampling data, calcium is between 35 and 62 mg/L, while magnesium is between
1.2 and 3.8 mg/L.
The hardness of the groundwater is actually less that the Drinking-water Standards for New Zealand
Guideline Value of 200 mg/L (as CaCO3).
Internationally, softening of municipal water is normally only regarded as economic when the
hardness of the water being softened is greater than 150 mg/L (as CaCO3) – in other words, if
consumers will tolerate a hardness of up to this value, then the costs of adding softening are
typically not considered worthwhile. While the existing Alexandra wellfield hardness of between
about 90 and 190 mg/L suggests softening would be marginal, the community has given a clear
signal that it regards the water as too hard.
Most New Zealanders would find levels of hardness above about 150 mg/L unacceptable. For
example, the Kapiti Coast when considering options for its water supply, adopted a target treated
water hardness of ≤ 80 mg/L (as CaCO3) when considering options that involved its existing
wellfield, but stated a clear preference for a hardness that was similar to its river source (30-50 mg/L
as CaCO3).
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5
Raw Water Intakes
5.1
Intake Options
CODC’s RFT document indicated that preference was for a raw water source either near Alexandra
or in the Dairy Creek area.
Investigations carried out by Opus (October 2007), URS (7th September 2011), URS (18th May
2012) indicate that bores or galleries adjacent the Clutha River between Alexandra and Clyde suffer
from high hardness depending on proximity to the river. Consequently locating bores in the Dairy
Creek area is the only bore option considered in this report.
Three intake options have been considered being;
1. Existing intake in Clyde dam.
2. New lakeside bores upstream of Clyde dam.
3. A new river intake on the true left bank of the Clutha River approximately 200 m upstream from
the end of Boundary Road and about 300 m downstream of the confluence with the Fraser
River.
Continued use of the existing borefield would not require any additional intake works.
5.2
Clyde Dam
An intake located in the Dairy Creek area implies, as an option, reuse of the existing intake built into
the Clyde Dam. This intake consists of two screened, 200 mm diameter (approximately) pipes
passing through the dam to a dry mount pump chamber located within the dam itself. Preliminary
investigation suggests that it will be very difficult to find a pump which fulfils the required duty even
for 15 Ml/d capacity with acceptable operating characteristics.
At 15 Ml/d (i.e. excluding Clyde and Dunstan Flats demand) and assuming 20 hours pumping per
day and that both existing intake pipes are used together at peak times, velocities are 3.3 m³/s in
the pump suction lines which are well above the recommended maximum of 1.8 m/s. One
reference suggests that approximately 0.3 m of submergence per 0.3 m/s suction pipe velocity is
necessary in order to preclude the possibility of vortexing. This suggests approximately 3.3 m of
submergence is required. At low lake level (bottom of normal operating range) submergence is only
1.2 m and at top of normal operating range submergence is 2.2 m. This is well below the minimum
suggested resulting in a high likelihood of air being sucked into the pump system. Modifications to
the inlet could be completed to mitigate the submergence/high intake velocities.
It should also be noted that the Clyde Dam intake would experience the same turbidity issues that
an intake on the Clutha River closer to Alexandra would experience so there is no advantage in
considering this intake as an alternative to a river intake for that reason. The overall conclusion, for
the purposes of this report, is that the likelihood of using the Clyde Dam intake as a source of raw
water is very low and it is therefore not considered further in this report.
5.3
Bores (Dairy Creek Area)
The water supplied to Clyde township from the existing Clyde bore (upstream of Clyde Dam) is
known to be of reasonable quality and to fluctuate within a narrow range of turbidity. A raw water
source from this location does have the benefit of filtration through gravels adjacent the dam which
results in the removal of turbidity. This source does not experience the elevated hardness of water
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sourced from bore fields along the true left bank of the Clutha River nearer Alexandra although it is
harder than river and lake water.
A site inspection of the area upstream of the dam was made on 28th March 2013. It is concluded
that there are two possible approaches to an intake located in this area. The first is installation of
more bores in the same geological structures as the existing Clyde bore, while the second is a new
intake directly in the lake.
Following the above mentioned site visit the conclusion is that further bores in the vicinity of the
existing Clyde bore should be considered in the first instance as this looks as though it will be a
more cost effective solution than a direct intake from the lake and the filtering effect of the gravels
as pre-treatment is of significant value. This approach assumes that the new bores will yield the
required flow.
If it is shown that bores would not provide adequate capacity then mounting intake pumps on a
floating barge or pontoons positioned behind the log boom would be an alternative solution but in
terms of raw water quality this does not offer any significant advantage over a river intake near
Alexandra.
Alternatively a shore based intake could be built. A lake shore intake would be difficult to construct
on the existing steep bank which is covered with heavy bank erosion protection. If this approach
was adopted it would consist of a jetty supporting pumps and screens but the banks immediately
upstream of the dam are steep and this makes for difficult access. An intake located further up the
lake where access is easier would be outside the existing log boom and would thus require its own
debris protection.
Neither a jetty or pontoon arrangement offers significant benefits in terms of raw water quality
compared with a river intake located near Alexandra. Exactly the same treatment processes would
be required at Dairy Creek as would be required for an intake near Alexandra but with the added
cost of a pipeline to Alexandra. For that reason intakes of this type located near Dairy Creek are
not considered further.
Based on the test results for the present Clyde bore provided by CODC and assuming further bores
in the vicinity of the existing bore would achieve the same capacity, the following has been assumed
for the purposes of this report.
1. Two additional bores of the same capacity as the Clyde bore dedicated to Alexandra supply
only would be sufficient to provide for a peak of 15 Ml/d. This would require an output of 95 l/s
per bore which is at the bottom end of the range reported during testing of the Clyde bore.
2. Coupling the existing Clyde bore with the two additional bores for Alexandra would provide
adequate capacity to supply Alexandra, Clyde and Dunstan Flats.
This report is based on the assumption that two additional bores are installed to supply Alexandra
and that those bores are coupled to the Clyde bore if a WTP is located in the Dairy Creek area and
it provides treated water to Clyde, Dunstan Flats or both.
The above assessment of capacity is based on 20 hours pumping per day and no installed standby
capacity at peak demand.
The likelihood of encountering similar sub-surface conditions for new bores as exist for the present
Clyde bore have been researched. Photo 2 shows Clyde Dam partially fill with the approximate
location of the existing Clyde bore shown. Photo 1 shows the more approximate location on a
photograph of Clyde Dam during construction. The existing bore appears to be in a man-made
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bench constructed near the concrete batching plant. This bench appears to extend from just
upstream of the dam to beyond the existing bore. These photos suggest there is a high likelihood of
encountering similar subsurface conditions for at least 200 m downstream of the Clyde bore which
would provide sufficient separation between the existing and new bores. Photo 3 shows a full aerial
view of potential bore sites in relation to the existing Clyde bore.
The expected cost of carrying out further investigations in the Dairy Creek area are as follows:
Two test bores capable of being converted to production bores (for two
bores)
$70,000 - $80,000
Hydrological assessment of test results and long term capacity
$20,000 - $30,000
Total Cost
$90,000 - $110,000
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Photo 1
Approximate
location of existing
bores
Approximate
location for
two new
bores
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Photo 2
Approximate location –
existing bore
Approximate location for two new bores
Photo 3
Existing Bore
Potential Bore Sites
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Direct Intake from Clutha River
This option involves an intake located on the bank of the Clutha River to the north west of Alexandra
which feeds to a WTP located nearby from where treated water is pumped into the reticulation
which feeds both Alexandra reservoirs.
The site chosen for the purposes of this study meets the general requirements for such an intake
which are:

Deep water adjacent the river bank;

A steady flow past the intake;

Clear of areas where large debris such as logs could impact the intake;

Reasonably accessible from the shore.
th
The chosen location is based on a site inspection conducted on foot (28 March 2013) over a
section of river on the true left bank downstream of the confluence with the Fraser River from
opposite the end of Eclipse Street to about 200 m upstream of Boundary Road. The site inspection
identified the location as meeting the above requirements with the bank rising about 6 to 8 m above
normal river level at about 2H:1V.
Consideration was given to several alternatives for an intake at this location including:
Excavating a side channel in the shape of a loop and in which screens would be located. This would
allow the screens and pump station to be constructed in relatively dry conditions before the side
channel is opened up to the river. However the depth of the channel and flood protection necessary
at the inlet and outlet connection to the river would be significant and would also involve significant
temporary works to construct. Also, the risk of debris and bed load being deposited in the side
channel would be high and then become a maintenance issue. This option also runs the risk of
intercepting poor quality groundwater flowing towards the river.
Consideration has also been given to an intake similar to the Waikato River intake supplying water
to Auckland and which has operated successfully for many years. This concept consists of several
cylindrical wedge wire screens in the river stream connected to a pipeline which feeds into a
submersible pump station inland from the river bank. The screens are protected from damage by
flood debris by several piles positioned upstream. The Waikato intake had to be positioned 20 m
from the bank to achieve adequate depth and stay clear of whitebait habitat. This resulted in the
screens being very exposed to river debris and the need for substantial pile protection. ORC have
indicated that there are no particular concerns related to fish life in this section of the Clutha River
and therefore it is not expected any conditions will be imposed regarding the distance of the screens
from the riverbank so a simpler arrangement has been adopted. The screens required at this site
are relatively small at 400 mm diameter and only require a water depth of about 1.0 m to 1.5 m
maximum which is found close to the bank at the proposed intake site.
The concept intake arrangement shown on the attached Drawings 6518206-C-K002 and 6518206C-K003.
After considering several arrangements using in line borehole type pumps inside a casing or
centrifugal pumps, the most appropriate solution is considered to be the arrangement shown which
uses submersibles pumps. Suction heads are likely to be too high for centrifugal pumps and they
have been eliminated on that basis.
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Sheet piling used to construct the intake would remain as a permanent structure except for a short
length parallel with the river bank which would be installed to allow screen installation in the “dry”
and which would then be removed after construction was completed.
The concept shown on the attached drawing is based on 15 Ml/d (175 l/s).
The pumps (2 duty and one standby) would each have a duty of 87.5 l/s at about 25 m to 30 m head
depending on the WTP site level. The WTP is assumed to be about RL 150 which is on the terrace
above the river giving a 20 m static and 5 m to 10 m friction losses.
An intake system based on three pump wells (circular precast concrete) has been assumed on the
basis that this would be lower cost than one rectangular, cast insitu pump well.
The screen gap has been based on a 3 mm gap and a slot velocity of 0.15 m/s which is considered
suitably conservative. The screens would be approximately 400 mm diameter and about 1.25 m
long. The screens will be air burst backwashed which is an effective means of automatically
clearing screens of accumulated debris. This is particularly important given the potential for didymo
and other material accumulating on the screens.
Hybrid Intake Option
One further option which has not been considered in any detail is a hybrid raw water intake system.
th
It has been observed (URS report 24 September 2009) that during periods of high river flow, which
often coincides with high river turbidity, the hardness of water extracted from the existing bores is
substantially lowered because the ground water gradient towards the river is reduced sufficiently
under these conditions that a greater proportion of river water reaches the existing bores. At the
same time turbidity does not rise significantly in bore water. This therefore presents an opportunity
to simplify the treatment process by eliminating the need to provide specifically for removal of
turbidity under high, but relatively infrequent, turbidity events. This option would still require a full
capacity river intake as described above and would also require continued operation and
maintenance of the existing bore field. On the basis that the location of the existing bore field is
considered to be a risk to water quality, this option has not been assessed further in this report.
6
Treatment
6.1
Process Selection
On the basis of our analysis of the raw water turbidity results for the Clutha River and the Clyde
WTP (from the lakeside bore at Dairy Creek) turbidity results, we have selected suitable treatment
processes for the two source options. These are detailed in the sections following.
As noted in Section 4.0 the available chemical analyses do not indicate any chemicals of concern
with respect to DWSNZ. A check on the Drinking Water for New Zealand website (maintained by
ESR for the MoH) for a number of water supplies drawing from the Clutha River or its lake sources 3
shows no Priority 2 determinands that are related to source water, providing further confirmation
that chemicals should not be of concern. We note however that the full DWSNZ suite of analyses
have not been undertaken from the Clutha River at the Alexandra site.
3
Stirling, Balclutha, Clyde (effectively river water), Wanaka (Lake Wanaka) and Queenstown (Lake Wakatipu)
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In addition, on the basis of the available chemical analyses, the two source options under
consideration do not have hardness values that will cause scaling issues.
6.2
Dairy Creek Lakeside Bore Source
The analysis of the hourly turbidity data from October 2010 to January 2013 (2.3 years) presented
in section 4.1.1 shows that the 1 NTU was exceeded 0.54% of the time, and the longest continuous
period that it did so was 2.7 days (December 2012/January 2013). The fact that during this period
there were three floods of 40 NTU and greater, and another three of about 20 NTU, shows that the
bankside geology of the lake is very effective at providing filtration before the water reaches the
lakeside bore. The fact that the bore has been in place since 2002, and the turbidity is still very low,
suggests that clogging is not an issue.
For a water that has such low levels of turbidity the most cost effective means of achieving
protozoal compliance in terms of DWSNZ is typically by UV disinfection, as long as the UV
transmittance is high. The three UV absorbance values presented in Table 6 show that the UV
transmittance is in excess of 97%, indicating that if further data gathering confirmed this value, then
UV disinfection is potentially a cost-effective means of achieving compliance.
The compliance requirements for UV are for the turbidity of the water to not exceed 1.0 NTU for
more than 5% of the compliance monitoring period (one month), and not to exceed 2.0 NTU for the
duration of any three-minute period.
In terms of the 2.0 NTU limit there is one result that exceeds that figure (i.e. about 1 hour). As long
as such periods are relatively short, the borewater could be diverted to waste until it comes back
within the compliance criterion.
Over the one month compliance period, 5% of the time equates to 1.5 days. This means that the
event in December 2012/January 2013 of 2.7 days would breach the UV compliance criterion of
1.0 NTU, and that the 1.5 day event in January 2013 would come very close.
These turbidity spikes, although infrequent, would require a filtration step to assure compliance. If
further data gathering can show that these spikes are actually more limited, and/or diverting to
waste could be successfully used to overcome them, then it is possible that the filtration step can be
eliminated. However, at this stage we have made allowance for cartridge filtration upstream of the
UV unit.
We note that this is a large scale application of cartridge filters. A significant risk with cartridge
filters is unacceptably short life to replacement, resulting in high operational costs. Both bench
scale testing and a trial would be recommended to confirm the cartridge life if this option is pursued.
Although it is likely that bacterial compliance can be achieved with UV, in order to provide for
disinfection residual we have allowed for chlorination of the treated water. At this stage we have
also allowed for pH correction (using caustic soda) of the final treated water, although we note that
given the raw water pH and alkalinity, pH correction may not be warranted.
6.3
Clutha River Source
6.3.1
Source Water Quality
The Clutha River is a good water source, with a very low average turbidity of 4 NTU. Although the
lakes in the catchment’s headwaters provide capture and attenuation of sediment loads, the
tributaries without lakes (primarily the Kawerau and Shotover Rivers) do provide significant
sediment inputs during floods. The long and narrow shape of Lake Dunstan means that it provides
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only limited capture and attenuation of sediment. The turbidity analysis in section 4.2.2 shows that
over the 18.5 years of record that there have been 23 events of 40 NTU or greater and 5 events of
over 100 NTU. Some of the events are quite prolonged with the longest being 25.2 days over
10 NTU.
6.3.2
Conventional Treatment
For conventional water treatment technologies, the level of solids loading in flood events is such
that clarification would be required. The proposed process train includes:

Coagulation/flocculation

Clarification (up flow)

Granular media filtration

Chlorination

Final pH correction
The process flow diagram shown in Appendix A and Sections 6.5 following provide further details of
the process.
No allowance has been made at this stage for treatment for taste & odour, as this has been
associated with the current wellfield. Whether very low river flows (and/or Lake Roxburgh being
drawn down) could cause this to appear in the river water is a possibility but unlikely. We
recommend that provision should be made in the design for the retrofitting of powdered activated
carbon (PAC) dosing if this risk is realised.
6.3.3
Membrane Filtration Treatment
Although membrane filtration is generally regarded as more expensive than conventional treatment,
it can offer advantages that in certain situations water suppliers consider offsets the cost premium.
These potential advantages are:

Where a high degree of rigour is necessary to demonstrate compliance, membrane technology
gives increased confidence that compliance can be consistently achieved. This is particularly
true where there are source water microbiological risks of concern.

Membrane systems can avoid the need for an upstream clarification process if there are shortduration flood peaks (thereby reducing the cost premium of membranes).

Membrane technology is well suited to small remote sites.

Membrane plants are reasonably simple to operate in terms of achieving DWSNZ compliance,
and are therefore well suited to water suppliers/operators who don’t have extensive experience
in conventional water treatment. The membrane process does have a higher degree of electrical
and mechanical complexity, and there needs to be good technical support available locally.
The potential advantages of relevance to the Alexandra situation are the second and last of the
bullet points above. Although membrane systems can avoid the need for an upstream clarification
process, the raw water turbidity analysis undertaken shows that the flood peaks, while not reaching
the very high peak turbidities often seen in New Zealand rivers (i.e. in excess of 1000 NTU), they
can be of very long duration.
We have not specifically approached membrane vendors with the Clutha River data. We are
currently undertaking a similar-sized membrane plant that draws from the Waikato River, which is
not dissimilar to the Clutha. The four membrane vendors that we have approached on that project
have all been comfortable offering membrane filtration with no clarifier. However, we consider that
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the flux rates they are proposing, although reduced compared to the with-clarifier case, are too high.
We therefore recommend that including a clarifier on the Clutha River source is wise, although it
could be considered as conservative by some vendors.
Including a clarifier provides advantages for reduced membrane fouling 4, the ability to cope with
widely ranging raw water quality, and gives increased membrane flux rates (i.e. smaller membrane
plant and lower cost of periodic membrane replacement). It also gives options for PAC dosing if
needed in the future.
The fact that membrane plants are reasonably simple to operate and are suited to water operators
who don’t have extensive experience in conventional water treatment may be an attractive feature
to CODC. However, this feature has to weighed up in the context of the cost premium.
6.4
Reliability, Redundancy and Automation
The following assumptions have been made in relation to the level of reliability, redundancy and
automation for treatment:

The treatment process is to be automated so that routine daily operations do not require operator
intervention. Operator attendance would be expected to be required for two to three days per
week.

Redundancy is to be provided for key mechanical equipment which has a moderate risk of failure
and is required for continued plant operation.

No redundancy in water quality and control instrumentation is required. CODC will hold spares of
key instruments to enable prompt replacement when required.

A minimum capacity of 85% of full capacity is to be achieved with one filter or one membrane
train out of service.

A minimum of two clarifier units has been included to allow for planned maintenance to be
completed during low demand periods.

Full daily capacity shall be maintained while any routine operations are undertaken that have a
frequency greater than monthly.

A standby diesel generator is required, capable of operating the full plant in the event of a power
outage.

Chemical storage is to be sized for the greater of:
– Minimum of 30 days storage at maximum flow and dose
– Minimum of 14 days storage at maximum flow and dose at time of re-order/delivery.

The existing supply is to remain in service continuously until the new plant has been constructed
and commissioned.
6.5
Process Details for Conventional Treatment
6.5.1
Coagulation and Flocculation
4
For Watercare’s Waikato WTP the use of clarifiers has been proven to be a sound decision – there was a
non-clarified membrane evaluation train built in the planning stages, and operated for about 6 weeks before the
operation ceased and concluded that clarifiers were needed because of excessive fouling.
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For both the conventional and membrane plant options, coagulant dosing will be required.
For the conventional treatment option, coagulation is key to achieving turbidity removal and hence
compliance. Hence the dose control is of high importance. At this stage we have assumed that
polyaluminium chloride (PACl) or aluminium chlorohydrate (ACH) will be used to avoid the need for
pre-pH correction to maintain pH control during coagulation. This needs to be confirmed by benchscale trials (particularly of flood samples), followed by an evaluation of chemical costs in the context
of CODC’s approach to operational complexity.
Polyelectrolyte dosing will be used to strengthen the flocs and thereby improve the effectiveness of
the clarification process.
The s::can scanning UV spectrophotometer coupled with an algorithm to predict the dose rate has
been proven to achieve reliable dose control, and we recommend this be implemented at Alexandra
WTP.
Coagulant mixing will be provided by an in-line static mixer and flocculation will be in a dedicated
tank with hinged baffles that provide a relatively constant mixing intensity irrespective of the flow.
6.5.2
Clarification
The preferred conventional clarification process for this source is conventional up flow clarification.
Other configurations and proprietary processes could also be considered, but this option is
considered to be sound and well proven.
Table 11 - Proposed Clarification Design
Parameter
Value
Maximum Hydraulic Loading Rate
2.5 m/h
Surface Area
265 m2
Number of Units
2
Depth
4.5m
Sludge Removal
Sludge Cones
6.5.3
Filtration
Granular media filtration will include the following features which will provide a higher filtered water
quality, longer filter run times and ability to cope with higher levels of solids.

1.45 m deep media depth – high solids capacity and reduced risk of turbidity breakthrough

Dual media anthracite and sand – high solids capacity and reduced risk of turbidity breakthrough

Collapse pulsing backwash using combined air/water scour – recognised best practice for
maintaining the media in a clean condition, and able to cope with polyelectrolyte residuals with
reduced risk of media fouling. Followed by high rate backwash.

Filter-to-waste – improves certainty of DWSNZ filtered water turbidity criteria, best industry
practice.
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Table 12 - Filtration Design
Parameter
Value
Media (dual)
Anthracite over sand
Sand Depth
450 mm
Sand Grading
0.5 - 1.0 mm
Anthracite Depth
1000 mm
Anthracite Grading
1.2 - 2.0 mm
Backwash Rate (low rate)
9 m/h
Optimal combined air/water
backwash
Backwash Rate (high rate)
40 m/h
20% expansion
Filtration Rate N-1 (i.e. one in
backwash)
12 m/h
Plant inflow restricted to No. of
filters in service x 5,700 m3/d to
meet criteria of 85% capacity
with one filter out of service
and allowing for backwashing.
No of Filters
4
Filtration area (per filter)
18 m2
6.5.4
Basis
3.6 m x 5.0 m
Disinfection
The combination of clarification and filtration to give a filtered water of less than 0.1 NTU will meet
the DWSNZ for protozoa removal by physical removal. Disinfection is required for bacterial
compliance and the most cost-effective option is chlorination with the added advantage of giving a
residual in the distribution network for improved protection of public health.
We have assumed that the form of chlorine used will be liquefied chlorine gas in 920 kg drums. The
building will include a chlorine drum room to meet the AS/NZS 2927space requirements. The drum
room will be equipped with 2 x 920 kg drums and a vacuum eductor chlorination dosing system.
6.5.5
Final pH Correction
Treated water pH correction is expected to be required to reduce the corrosivity of the treated water
to both the public system and private plumbing. Aggressive or corrosive water both decreases the
life of pipework and elevates the level of metals in the water at the point of use. Normal practice is
to increase the treated water pH to the range of 7.75 to 8.0. The three raw water grab sample
results for alkalinity are 42, 43 and 54 mg/L (as CaCO3) and pH of 7.4 - 7.5 (refer Table ), indicating
a reasonable alkalinity. Collection of additional pH and alkalinity data would be recommended to
confirm the need for pH correction prior to design. By using ACH as the coagulant and producing a
high filtered water with minimal chlorine demand, the consumption of alkalinity through the
treatment process should be minor. However, it is likely that during flood events the alkalinity will
reduce.
Water suppliers that have attempted to optimise their treated water quality to minimise corrosivity
and taking consideration of other water quality impacts have concluded (although this could not be
3
considered a consensus) treated water alkalinities in the range of 30 to 50 g/m as CaCO3 are
optimal, and this is the operating range that we recommend CODC should consider.
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We recommend that caustic soda dosing be used for final pH correction, as for most of the time the
dose will be relatively small, and although lime is the lowest cost pH correction chemical, its
handling difficulties are unlikely to warrant the small chemical cost saving.
6.5.6
Clearwater Tank
The chlorine contact time will be provided in the clearwater tank, and in the trunk main and
distribution system. Industry practice is to provide a minimum 30 minutes contact time prior to the
first consumer, and this is also a requirement to achieve an A grading under the current Ministry of
Health Grading requirements. For the purposes of this study we have assumed that the 30 minutes
contact time will be provided in the clearwater tank prior to leaving the plant. The following table
outlines how this contact time will be achieved as well as proving flow balancing storage for the
treated water pumps.
Table 13 - Clearwater Tank and Chlorine Contact Time
Parameter
Value
Basis
Short-circuiting factor (T10/T)
65%
Typical short circuiting factor
for well baffled tank.
Volume required for 30 minute
chlorine contact
500 m3
15 ML/day peak production,
over 23 hours
Balancing storage
200 m3
12 minutes at full plant flow
Total tank volume
3
700 m
6.6
Process Details for Membrane Treatment
6.6.1
Coagulation
For the membrane plant option, coagulant dosing is used primarily for organics removal (the
membrane pore size is such that micron-sized particles are removed without the need for particle
agglomeration). There are some other benefits from coagulation such as increased virus removal
and in some applications reduced membrane fouling. During normal, good river water quality
conditions a low dose can be used. The dose needs to be substantially increased during floods.
(Order of 6-fold increase between normal conditions and peak flood conditions).
We expect continuous coagulation will be required for this source to control the formation of
disinfection by products and limit chlorine demand.
With coagulant dosing being one of the highest plant operating costs, there is a need for good
control of coagulant dosing to achieve consistent organics removal and minimise coagulant use. As
for the conventional treatment option, the s::can scanning UV spectrophotometer coupled with an
algorithm to predict the dose rate is recommended to optimise coagulant dosing.
6.6.2
Membrane Filtration
The membrane plant alone is capable of providing 4 log protozoa credits, which is 1 log higher than
the source categorisation requirement presented in section 4.2.1.
Verification of the effectiveness of the removal achieved (and hence DWSNZ compliance) is based
primarily on the Membrane Integrity Test (MIT). The MIT involves applying air pressure to the
internal or permeate side of the membrane, and monitoring pressure loss. Based on the air/water
surface tension, at a certain pressure, air will not pass through a hole of a given size (3 micron
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under DWSNZ). If the pressure drop is less than a calculated level, the minimum log removal of
particles greater than 3 micron can be demonstrated. When the pressure decay reaches or comes
close to 4 log, membrane repairs will be required.
In addition to the above direct integrity testing, DWSNZ requires indirect integrity testing to provide
assurance the membrane is performing in the period between direct integrity testing (daily MIT).
DWSNZ allows turbidity monitoring or “other monitoring test specified by the manufacturer”, which
would typically be in the form of particle counting. Particle counting is a more sensitive measure of
solids compared with turbidity, and will detect low level membrane defects before a turbidity
instrument will. However the sensitivity of the particle counter is such that it makes it difficult to use
as a compliance tool. Hence we propose to use turbidity on each of the membrane trains to meet
the indirect integrity testing requirements.
Membrane replacement is a substantial operating cost in membrane plants. Suppliers are offering
up to 10 year pro rata guarantees. If this option is pursued, this will be key criteria in the membrane
tender assessment, as longer membrane life has a substantial impact on the cost of operation.
6.6.3
CIP
The membranes will require periodic chemical cleaning by Clean in Place (CIP) to maintain the
permeability and hence hydraulic performance of the membrane. Chemical cleaning can be
required between every one to eight weeks, with around every four weeks being typical.
A variety of chemicals are used for CIP, with the type being dependant on the membrane type and
material, and type of foulant being removed. Oxidants such as sodium hypochlorite and hydrogen
peroxide are commonly used to remove biological fouling and organic matter. Acid washes (citric
acid, hydrochloric or sulphuric acid typically) are used to remove mineral fouling such as aluminium,
iron and hardness. Caustic washing (typically caustic soda) may be used to remove organic or
biological fouling. CIP chemicals of some membrane systems are heated to improve effectiveness
and reduce chemical consumption.
The selection of CIP chemicals will be determined by the membrane supplier, and volumes and
types can vary significantly between suppliers. We have allowed for neutralising (pH and chlorine)
prior to discharging to the wastewater pond for mixing and dilution prior to final discharge to the
river. This operation will need to be included in the discharge resource consent.
6.6.4
Disinfection, Treated Water pH Correction, and Clearwater Tank
As for the conventional treatment option.
6.7
Wastewater
Wastewater disposal requirements for the two options (conventional or membrane plant) are similar.
A membrane plant may operate under clean water conditions with a lower coagulant dose rate, and
will not use polyelectrolyte, but the impact on the concept design for sludge disposal is expected to
be small.
In terms of the possible acceptance of WTP wastes at the Alexandra WWTP CODC has indicated
that:

The direct discharge of the untreated water treatment waste flows to the wastewater system
would not be acceptable to CODC. This is a matter of the impact of such additional flows on the
relatively small wastewater treatment plant as significant modifications would be required to the
wastewater system to accommodate this discharge.
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
CODC’s preference is that all water treatment discharge should be kept out of the wastewater
system, but a supernatant discharging to the wastewater system on a controlled low-flow basis
could be considered (peak discharges being unacceptable).
With respect to the direct discharge of waste flows to the wastewater system there is some benefit
to the wastewater process with the residual coagulant reducing phosphorus levels in the wastewater
discharge. Alternatives to reduce the volume of the waste discharge from the WTP could be
considered if this feature was of interest to CODC.
The two options for sludge dewatering on the WTP site are ponds or mechanical dewatering.
Using sludge ponds, wastewater is discharged to a pond, settled supernatant flows back to the river
and the solids settle out. Periodically to remove the sludge, the inflow is stopped, the water level in
the pond is drawn down and solids are allowed to dry by evaporation and drainage. Solids can then
be removed by excavator and carted to a suitable disposal site – either landfill or a clean fill with
suitable controls and approvals. Given Alexandra’s very low rainfall (mean annual rainfall of
340 mm) this is an effective option for Alexandra.
Mechanical dewatering, typically using centrifuges is a higher capital and operating cost option, but
compact, requiring minimal land area. Sludge lagoons are the lowest capital and operating cost
option, and as it is assumed that sufficient land is available in the area of the proposed location of
the plant, the use of ponds is recommended.
An unlined pond has been assumed, with geotechnical investigations required on the final site
location to confirm the design parameters.
Table 14 - Estimated Sludge Volume
Parameter
Value
3
Basis
Assumed normal coagulant
dose rate
5 g/m ACH
Estimate based on raw water
quality data.
Assumed Flood average ACH
dose rate
35 g/m3 ACH
Estimate based on raw water
quality data.
Average ACH Dose Rate
11 g/m3
Assume “normal” conditions
80% of time and “flood”
conditions 20% of time
Solids contribution from ACH
4.4 g/m3
Average raw water turbidity
4 NTU
Turbidity to suspended solids
conversion
2
Solids contribution from
suspended solids
8 g/m3
Estimated Average Sludge
Production per m3 Treated
Water
16.1 g/m3
Estimated annual sludge
production (dry solids)
35.2 tonne/annum
Assumed Pond Depth
1.5 m
Assumed sludge density
4%
Based on assumed 6 ML/day
average annual production
Conservative estimate –
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Parameter
Value
Basis
expected to be in the range 4
to 10%
Estimated Annual Sludge
volume
880 m3
Assumed Maximum Sludge
Depth
1m
Pond 1 Area (base)
65 m x 25 m = 1,625 m2
Pond 2 Area (base)
65 m x 25 m = 1,625 m
Based on density with pond in
service. As pond is drained
down solids will reduce to
around 16-20% dry solids or to
3
180 m .
2
Hence each pond would have capacity for a little less than two years of sludge production. This is
an appropriate capacity as a minimum of one years’ capacity needs to be provided in each pond to
allow one pond to be dewatered each year. Pushing this out to a pond being dewatered every
second year will reduce the operational costs of pond dewatering. There could be scope to optimise
the sizing of the ponds with further design effort.
Discussions with Otago Regional Council on the consenting of the supernatant discharge would be
required to clarify this.
6.8
Existing Borefield
6.8.1
Commonly Accepted Softening Options
The commonly accepted options for softening of hard water in a municipal context are:

lime softening – either single-stage or excess lime

nanofiltration.
Based on the grab sampling data, single stage softening could achieve a hardness of 45 to 55 mg/L
(as CaCO3). This hardness value is comparable to the grab sample results from the Dairy Creek
lakeside bore. The excess lime process would not be appropriate because it targets magnesium
hardness as well and magnesium makes only a small contribution to the total hardness (5 to
16 mg/L as CaCO3).
While ion exchange is not normally used at a municipal scale, we are aware that Wanganui has
installed a 30 L/s (2.5 ML/day) ion exchange plant to soften water from a new bore. According to
Wanganui District Council the plant is working well, but the supply cost alone of the salt used for
3
regeneration is costing $0.20/m of water produced, and it has recently been turned off to save
money. The additional sodium that this process adds to the treated water also needs to be
considered as a negative impact on the treated water quality.
6.8.2
Lime Softening (Option 3A)
The process train for a lime softening plant would consist of:

lime and coagulant addition

rapid mixing and flocculation
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
softening basin (similar to a conventional clarifier), with a slightly higher loading rate than the
clarifier proposed for the Clutha River water source option because of the low solids content of
the groundwater

recarbonation basin with CO2 dosing

granular media filtration

chlorination and chlorine contact.
Given the history of taste and odour with the current source the process would also need to include
provision for taste and odour treatment. We consider that it is likely that PAC would not be effective
when dosed with lime and therefore this treatment option is not favoured. The preferred option
would be to substitute the granular media filtration step referred to above with biological activated
carbon (BAC) filtration. The water following recarbonation should be non-scale forming and
therefore should not compromise the activated carbon media. This assumption needs to be
confirmed if this option is pursued.
Although the clarifier would be of a similar size to the Clutha River intake option, the capital cost of
the treatment plant for the lime softening option would be more expensive than the Clutha River
source option although the overall cost is lower because the intake is eliminated. This is because
the following system components are also required over and above those needed to treat Clutha
River water.

lime system – silo, feeder, slurry tank, dosing pumps

recarbonation basin and CO2 system.
6.8.3
Nanofiltration (Option 3B)
Nanofiltration (NF) is a membrane technology process that is able to remove divalent and larger
ions, and thus able reduce the calcium and magnesium concentrations that cause total hardness.
Although the chemical constituents that caused the major taste and odour event about 5 years ago
have not been identified, we have assumed that the pore size of the NF membrane would reject
those constituents and have not allowed for taste and odour-specific treatment along with the NF
train. This assumption will require further investigation if this option proceeds.
A NF plant of 9 ML/day capacity would be required to meet the target water quality for hardness by
reducing the maximum of 190 mg/L to 80 mg/L as CaCO3. This would be treat a side stream of the
groundwater and, followed by mixing with a non-softened stream, will meet the target hardness. We
have allowed for 5 µm cartridge filtration as a pre-filter for the NF feed.
3
The reject stream (provisional figure of 2,300 m /day of brackish water) produced from the NF plant
will require disposal. For the purposes of developing the rough order costs estimates we have
assumed that this could be discharged into the river, but the consentability of this approach needs
to be investigated further and if this approach could not be consented then additional treatment and
disposal costs would be added to this option.
The non-softened stream will also require treatment in order to achieve DWSNZ compliance. A
protozoal credit requirement of 3 log has been assumed. The results from the eight grab samples
presented in the URS report shows turbidity varies from 0.10 to 1.3 NTU. This suggests that UV
disinfection is unlikely to be feasible without some level of pre-filtration. Pre-filtration could be
achieved by cartridge filtration technology but without better long term turbidity data this is a risky
assumption to make at this stage.
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Hardness is a known foulant in UV systems. Modern quartz sleeve wiper systems can cope with
reasonably high hardness levels although the fouling factor is typically down-rated below the 0.95
value used for soft waters.
Given the current state of knowledge of the raw water quality we have assumed for the purposes of
developing cost estimates that coagulation and BAC filtration is the most cost-effective option for
the non-softened stream. If the option of treating the existing bores further is pursued then it would
be worth gathering on-line turbidity data for the raw groundwater. This may show that cartridge
filtration and UV disinfection are feasible.
6.8.4
DWSNZ Compliance without Softening (Option 3C)
The do-minimum option for CODC would be to continue to use the current bore field supply but
bring it up to compliance with DWSNZ without the provision of softening.
As has been assumed for the non-softened stream of the lime softening option above, coagulation
and BAC filtration is considered the most cost-effective option at the current state of knowledge of
the raw water quality. The capital and operating costs for this option have been based on this
assumption.
The options for softening the existing groundwater source are no more financially attractive than
other options considered.
The option of bringing the existing groundwater source up to DWSNZ compliance without softening
is potentially attractive but does not address the long-standing consumer issues around the
hardness of the supply. This option may also be able to be implemented at a lower cost if cartridge
filtration and UV was able to proven to feasible. If CODC is interested in pursuing this option we
recommend that each well is:

fitted with an on-line turbidimeter

fitted with an on-line UVT analyser

able to record pump starts and stops and flow rates.
This data will enable the feasibility of cartridge filtration and UV to be investigated.
The other issue associated with this option is the age and condition of the existing borefield,
including bores and the associated pumps and pipework. If these assets are due for upgrading
within the next 10 to 15 years then this would need to be incorporated into the NPV analysis as a
capital cost at that time.
Option 3 does not address the potential risk to Alexandra water supply arising from the presence of
a closed landfill up-gradient of a closed landfill up-gradient from the borefield.
6.9
Summary of Treatment Option Costs
Description
Option 1A
Alexandra Intake + Conventional
WTP (15 Ml/d)
Likely
20 Year NPV
$14,533,860.00
$21,274,946
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Option 1B
Alexandra Intake + Membrane
WTP (15 Ml/d)
$16,933,860.00
$25,452,467.00
Option 1C
Alexandra Intake + Conventional
WTP (20 Ml/d)
$17,312,060.00
$25,062,488.00
Option 1D
Alexandra Intake + Membrane
WTP (20 Ml/d)
$20,997,360.00
$30,541,261.00
Option 2A
Dairy Creek Intake + UV Plant
(15 Ml/d)
$11,426,000.00
$16,058,857.00
Option 2C
Dairy Creek Intake + UV Plant
(20 Ml/d)
$12,945,400.00
$17,952,802.00
Option 2E
Dairy Creek Intake + UV Plant
(25 Ml/d)
$15,024,500.00
$20,395,514.00
Option 3A
Existing Bores + Lime Softening
(15 ML/d)
$12,030,000.00
$20,821,047.00
Option 3B
Existing Bores + Nanofiltration
(15ML/d)
$11,930,000.00
$20,559,472.00
Option 3C
Existing Bores + Direct Filtration
(15 ML/d)
$8,560,000.00
$14,594,031.00
Likely cost is mid-range of a high and low estimate for each of the options considered. 20 Year
NPV is based on the likely capital cost.
Note: Capex estimates allow for a 15MLD facility, however, Opex estimates have been calculated
using an estimated average water demand of 4.1MLD.
The average water demand has been calculated by reviewing Alexandra annual water consumption
for 2010 – 2013 and determining the existing peak demand to average demand ratio. This showed
an average ratio of 3.65, which was used to convert a 15MLD peak demand to a 4.1MLD average
demand.
7
Treated / Raw Water Pipeline
7.1
Pipeline Alignments
The assumed pipeline alignments to convey raw or treated water from the proposed water source to
the existing reticulation are as follows:
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Option 1 - From the proposed water treatment plant ,(near the new river intake on the true left bank
of the Clutha River) approximately 800m of new DN450 pipe has been allowed for, which would
connect to existing reticulation in the public reserve near Larch Crescent. A similar length of
pipeline would be required for Option 3.
Options 2 & 3 – From near the existing Dairy Creek intake, approximately 9600m of DN450 (or
DN525 for the 25MLD option) pipe has been allowed for following SH8 until in meets existing
reticulation near Henderson Drive in Alexandra.
Capital costs for pipeline installation assume an open trench construction methodology with
approximately 1 meter of cover to the soffit of the pipe.
7.2
Pipe Material and Pressure Class
The pipe material options considered for conveying raw or treated water to the existing reticulation
are as follows:
•
PVC-U PN9 & PN12
•
PVC-M PN9 & PN12
Other material types such as Polyethylene (PE), Concrete Lined Steel, and cement mortar lined
ductile iron have been investigated briefly but have not been considered further because it is
believed these materials will have a prohibitively high cost.
Operating Pressure
The proposed operating pressure has been assumed at 400 kPa (40 metres head) at Alexandra to
meet fire flow pressure requirements.
A booster pump station fitted with soft starters and a single variable frequency drive (VFD) has been
allowed for in cost estimates. This system would maintain the set operating pressure and fill the
Alexandra reservoirs as required. The control of this system is expected to operate smoothly and
maintain the set pressure closely with minimal pressure variations.
For option 1 (Intake at Alexandra), the estimated maximum operating pressure is approximately 95
metres head. This would occur if the WTP is near the RL of the intake and booster pumps are filling
the Bridge Hill reservoir through a DN450 pipeline. This is over the safe long term pressure rating
of a PN 9 pressure pipe (90m) and therefore a PN12 pressure pipe (120m) would be required.
For options 2 & 3 (Intake at Dairy Creek), the estimated maximum working pressure is
approximately 60 metres head. This would occur if the WTP is near the RL of the Dairy Creek
intake and a booster pump system is filling the Bridge Hill reservoir. This is within the safe long
term pressure rating of a PN 9 pressure pipe (90m).
Pressure Surge Design
It is conceivable that pressure surges of up to 70 metres head could result from power failure/s with
the booster pump flow rate of 175L/s. The surge pressure is additive to the operating pressure and
could result in peak pressures of up to 160 metres for Option 1 and 130 metres for Options 2 & 3.
The latest design guidelines for surge and fatigue design for PVC M and PVC-U pipes indicates that
short term surge pressures should not exceed the following:
•
•
PN9 PVC-M pipe - 117 metres head.
PN12 PVC-M pipe - 156 metres head.
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•
PN 9 PVC-U pipe - 135 metres head.
•
PN 12 PVC-U pipe - 180 metres head.
Option 1 would require a PN 15 PVC-M or PN 12 PVC-U pipe. PN 15 PVC-M pipe will be more
expensive (~20%) than PN 12 PVC-U and as such PN12 PVC-U has been allowed for in cost
estimates.
Options 2 & 3 would require a PN 12 PVC-M or PN 9 PVC-U pipe. PN 12 PVC-M pipe will be more
expensive (~20%) than PN 9 PVC-U and as such PN 9 PVC-U has been allowed for in cost
estimates.
Fatigue Design
Fatigue is not expected to be a problem, as the operating pressure will be maintained at a fairly
constant level, well within the pressure capability of the pipe. PVC-U pipe has better fatigue
characteristics than PVC-M (due to its greater wall thickness) and PVC-U pipe is considered to be
satisfactory for this application.
7.3
Booster Pump Station
For all options, a booster pump station will be required to convey raw or treated water from the new
water treatment plant into the existing reticulation. For cost estimating purposes, the location and
therefore operating conditions of the booster pump station is assumed to be in the immediate
vicinity of the source water intake. There are a number of options to pump raw or treated water into
the existing reticulation but developing these options in detail is beyond the scope of this report.
The cost estimates allow for staged flow pumps (from approx. 40L/s up to 175L/s – 325L/s) to
provide functionality for maintaining a constant network pressure and fill the Alexandra reservoirs as
required.
8
Consenting
8.1
Land Use Designation
As stated earlier, CODC does not have land specifically designated for water treatment. No attempt
has been made at this stage to identify a specific site either near Dairy Creek or Alexandra but it is
recommended that work on this issue commences as soon as a decision is made on which of the
options presented in this report is to be adopted.
8.2
Water Take Consent
A consent application to ORC can only be prepared after a decision has been made on site and
source.
8.3
Discharge Consent
There will be discharges to water, and potentially land, for which consents will be required from
ORC. The nature of these discharges will be highly dependent on the process finally chosen.
8.4
Building Consent
A building consent will be required for the new building and tanks.
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9
Geotechnical Assessment
This report has been prepared without consideration of geotechnical conditions. Generally, and
based on local knowledge, it is expected that geotechnical conditions will not be a significant
determinant in the design of treatment plant structures. It is expected that adequate soil bearing
pressures will be available at most locations in the Alexandra and Clyde area such that no specific
provision need be made for geotechnical conditions in the estimates.
It is strongly recommended that geotechnical investigations are carried out at the finally chosen site.
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