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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

CH2M Beca // 10 December 2013 // Page i

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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

10th December 2013

Document Acceptance

Action Name Signed Date

Prepared by Robert Crosbie/Andrew Watson

Reviewed by Simon Drew

Approved by Robert Crosbie

on behalf of CH2M Beca Limited


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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 4.1 Dairy Creek Lakeside Wells ..................................................................................... 6 4.2 Clutha River ........................................................................................................... 10 4.3 Existing Bores........................................................................................................ 12

5 Raw Water Intakes ............................................................................................. 13 5.1 Intake Options ....................................................................................................... 13 5.2 Clyde Dam............................................................................................................. 13 5.3 Bores (Dairy Creek Area) ....................................................................................... 13

6 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

7 Treated / Raw Water Pipeline ............................................................................ 31 7.1 Pipeline Alignments ............................................................................................... 31 7.2 Pipe Material and Pressure Class .......................................................................... 32 7.3 Booster Pump Station ............................................................................................ 33

8 Consenting ......................................................................................................... 33 8.1 Land Use Designation............................................................................................ 33 8.2 Water Take Consent .............................................................................................. 33 8.3 Discharge Consent ................................................................................................ 33 8.4 Building Consent ................................................................................................... 33

9 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 pre-treatment 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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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

Alexandra A

Alexandra B

Alexandra + Flats (or Clyde)

C

Alexandra +

Flats D

Alexandra + Flats + Clyde

E

1. Alexandra + Alexandra Conventional Membrane Conventional Membrane -

2. Dairy Creek + Alexandra UV - UV - UV

Capacity 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: 0.151 % of time that turbidity is above 1 NTU: 0.54% % of time that turbidity is above 2 NTU: 0.01%

Table 3 - Highest Turbidities in Clyde Treated Water

Highest turbidities Time and date

1 2.160 14/01/2013 11:02 2 1.990 3/01/2013 8:02 3 1.660 2/03/2011 18:02 4 1.180 31/10/2012 23:02 5 0.930 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

Start date of turbidity over 1 NTU

End date of turbidity over 1 NTU

Number of days

1 31/12/2012 15:02 3/01/2013 8:02 2.7 2 13/01/2013 9:02 14/01/2013 13:02 1.2* 3 14/01/2013 16:02 14/01/2013 23:02 0.3* 4 2/03/2011 18:02 2/03/2011 22:02 0.2 5 31/10/2012 22:02 31/10/2012 23:02 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



Number of days

1 14/01/2013 11:02

14/01/2013 11:02 0.00

4.1.2 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

Parameter 28th April 2005

Raw Water 15th March 2007

Raw Water 17th December 08

WTP

Absorbance (AU) 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

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 575228) from 12th September 1995 to 23rd 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): 4.007

% of time higher than 10 NTU: 6.97% % of time higher than 20 NTU: 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

Highest Turbidities Time and Date

1 189.580 21/09/2002 3:00 2 157.120 28/06/2000 5:00 3 136.89 9/10/1996 20:00 4 109.3 6/01/2002 0:00 5 105.26 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



Number of days

1 25/11/1999 15:00 20/12/1999 19:00 25.2 2 5/10/2000 14:00 24/10/2000 17:00 19.1 3 4/10/1996 12:00 17/10/1996 5:00 12.7 4 19/12/1995 18:00 30/12/1995 23:00 11.2 5 26/06/2000 0:00 7/07/2000 1:00 11.0

Table 10 - Largest Range of Turbidities over 20 NTU at Clutha River



Number of days

1 25/11/1999 15:00 7/12/1999 13:00 11.9 2 19/12/1995 18:00 27/12/1995 15:00 7.9 3 20/09/2002 10:00 27/09/2002 22:00 7.5 4 26/06/2000 5:00 3/07/2000 16:00 7.5 5 7/10/1996 19:00 14/10/1996 16:00 6.9

4.2.3 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 for

two new bores

Approximate location of existing

bores



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Photo 2

Photo 3

Potential Bore Sites

Existing Bore

Approximate location – existing bore

Approximate location for two new bores



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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.

The chosen location is based on a site inspection conducted on foot (28th 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 6518206-C-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. It has been observed (URS report 24th 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 short-duration 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 bench-scale 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 Basis

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 3.6 m x 5.0 m

6.5.4 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 considered a consensus) treated water alkalinities in the range of 30 to 50 g/m3 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


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 700 m3

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


Assumed normal coagulant dose rate

5 g/m3 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 Based on assumed 6 ML/day average annual production

Assumed Pond Depth 1.5 m

Assumed sludge density 4% Conservative estimate –



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Parameter Value Basis expected to be in the range 4 to 10%

Estimated Annual Sludge volume

880 m3 Based on density with pond in service. As pond is drained down solids will reduce to around 16-20% dry solids or to 180 m3.

Assumed Maximum Sludge Depth

1 m

Pond 1 Area (base) 65 m x 25 m = 1,625 m2

Pond 2 Area (base) 65 m x 25 m = 1,625 m2

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 regeneration is costing $0.20/m3 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.

The reject stream (provisional figure of 2,300 m3/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 Likely 20 Year NPV

Option 1A Alexandra Intake + Conventional WTP (15 Ml/d)

$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.

alexandra water supply – preliminary design and cost … · report alexandra water supply ......

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