​​​​​​4   ​Darfield and Kirwee Wastewater Scheme​​

​​​4.1    Overview & History

Previous documents can be reviewed here Selwyn District Council - Darfield Wastewater

The town of Darfield is one of the largest un-sewered communities in New Zealand.  Although there are currently no measurable environmental or public health impacts from the existing septic tanks, portions of the community have shown interest in a reticulated wastewater system.  Reticulated wastewater networks are known to reduce environmental and public health risks.  Kirwee will also be included into any wastewater scheme for Darfield, to allow for future development.  Refer section 6a 'Views of those affected' for further information on the views of the community.

Following the Council decision to progress with the design, costing and funding options for a reticulated wastewater system for Darfield and possibly Kirwee to be prepared for the 2021-2031 Long Term Plan, a Technical Advisory Group was set up to  fast track this work.  The Technical Advisory Group first met on 13 July 2020 and since then prepared concept designs and cost estimates for a reticulated wastewater system for Darfield and Kirwee. Options considered are summarised below:

4.1.1   Pi​peline to Pines WWTP

BECA Limited were commissioned to prepare the concept design of the trunk sewer from the outlet of the future Darfield and Kirwee reticulated sewer to the Pines Wastewater Treatment Plant.

The report proposed a 24,750m long pipe with a total fall of about 130m, typically falling at a grade of about 1 in 180 until the last 1,600m of the pipeline which is flat, the pipe termination would rise above ground to discharge into the Pines WWTP inlet structure. The outlet from the pipeline will be approximately 2m above ground, which will keep the last 1,600m of pipe submerged.​

Figure below shows a schematic of the proposed pressure section of pipe.

Figure 4-1 Concept for pressure trunk sewer

The fall along the pipeline is such that it can be designed as a gravity sewer except for the last 2,000m. This section needs to be designed as a pressure pipe.  The pipeline needs to cross both State Highway 1 and the South Island Main Trunk Railway line. These crossings will require the pipe to have at least 1.5m cover and to be installed by thrusting or drilling beneath the highway/railway. The pipe is pressurised at these crossings.

Sewerage in Kirwee is proposed to connect to the new trunk sewer via a pumped pressure main.  At this stage no screen has been allowed for. The solids load is expected to be low and will be conveyed along the trunk main by the screened Darfield flow. A 1800m long DN180 PE pumping main has been proposed from​ Kirwee along Courtenay Road with an 8m high pipe loop to keep the pumping main charged. The pipe will need to be drilled beneath the highway and the railway.

The proposed route is shown in green within the Figure 4-2.

Figure 4-2 Proposed pressure sewer route

Within the proposed 2021 Long Term Plan, subject to consultation, staff have budgeted for odour management at the commencement of the gravity line, along the line and at the termination point at Pines.  Odour management is a key consideration of this project.

Flushing of the line during early years will be required until connection numbers provide adequate sustainable flow. ​

4.1.2   ​​​​​Assessment of Treatment options at Darfield

Stantec were commissioned to prepare a report to consider two scenarios: 

1. ​Connection of Darfield to the Pines WWTP located in Rolleston.
2. Construction of a new greenfield Darfield WWTP.
   

In addition to the Darfield flows, those from the neighbouring township of Kirwee were also considered.

Three wastewater treatment processes were considered for the new Darfield WWTP. These are:

1. Oxidation lagoon (natural process; no nutrient removal, high-area option)
2. Trickling filter [TF] (organic treatment with low nutrient removal option)
3. Biological Nutrient Removal (BNR) Activated Sludge Plant (ASP) (organic and nutrient removal option)
   

These technologies provide very different treatment levels with increased costs associated with higher levels of treatment.  The report concluded that the Trickling filter process is the lowest capital cost option, and it produces a treated wastewater quality suitable for application in the available irrigation area.

Conveyance of wastewater to Pines was also considered as either:

1. Untreated raw wastewater
2. Effluent from a wastewater process.
   

The report concluded that the capital costs for raw wastewater conveyance and treatment at Pines WWTP appear significantly lower than the other options and it is unlikely that there is significant benefit to treating locally and discharging to Pines for further treatment and/or irrigation.

Comparison between the raw wastewater conveyed to Pines WWTP and the TF irrigated at Darfield needs to consider the additional costs associated with conveyance (pipeline, pumping, earlier upgrade of the Pines WWTP and increased loading, irrigation requirement and solids generation) with the additional ~$10M capital cost associated with the TF option and associated operational costs.​

4.1.3   ​​​Reticulation Design

Davie Lovell-Smith were commissioned to survey and prepare a concept reticulation layout and staging maps for both Darfield and Kirwee.

Darfield and Kirwee would be served primarily by gravity sewers, with low-pressure sewer catchments used where required.  The concept designs allow for the Darfield & Kirwee reticulation to be built in stages.  Stage 1 (proposed 2021/22 & 22/23) will focus on new development areas. Stage 2 (2024/25) and 3 (2025/26) will extend reticulation to existing properties.

Phasing maps for Darfield and Kirwee reticulation are provided in Figure 4-3.

Figure 4-3 Reticulation staging concept

4.1.4   Costs and Funding​​​​​​​​

Stantec prepared an initial net present value (NPV) costs for treatment of wastewater from the Darfield and Kirwee catchments. The two options considered were:​

1. Treatment of wastewater at a new wastewater treatment plant (WWTP) located in Darfield. Wastewater from Kirwee would be pumped to Darfield and treated by biological trickling filter and irrigated locally.
2. Raw wastewater from Darfield would be pumped, via Kirwee, to the Pines WWTP in Rolleston.
   

This comparison showed that there is no material difference between local treatment or conveyance of raw wastewater to Pines WWTP for treatment. With similar NPV costs, the selection of a preferred options needs to be based on other Rationale.  As such, treatment of the Darfield/Kirwee flow at the Pines WWTP (Option 2) is preferred and should be progressed.​

Key rationale behind this recommendation is:

• NPV profiles are similar over the 30-year period.
• Council will only have to manage one WWTP.
• The Pines WWTP will produce a higher standard of treated wastewater than that of Darfield WWTP.
• Council will not have to apply for and maintain a Resource consent for discharging at Darfield.
• The rating costs to Darfield and Kirwee residents will be lower for pumping to Pines.
• Waste solids can be dried using the solar drying halls and in the future anaerobic digestion will further reduce the quantity of solids and provide return on power instead of disposal of trickling filter solids to landfill.
  

Risks that need to be considered:

• Pumping raw wastewater from Darfield to Pines will result in a highly septic wastewater, allowances have been made to reduce and treat the septicity but the residence time within the conveyance network will need to be considered.
• The longevity of the pipe material and pump stations will be dependent on the ability to mitigate against corrosivity over time and identify issues to avoid reducing asset life and/or loss of service.
• Increase of disposal costs for biosolids over time.
  

PricewaterhouseCoopers (PWC) was engaged to develop a financial model that apportions the capital cost of the enhancements (and other associated works and upgrades), over time amongst the beneficiaries of those enhancements.

They modelled the incremental financial implications of the enhancements to calculate and compare the cost of each beneficiary group (both 'existing' and 'new') under the options considered.

Modelled projects include the following:

1. Pines WWTP upgrades
2. Darfield and Kirwee connection to Pines WWTP
3. Darfield and Kirwee reticulation
4. Ellesmere connection to Pines WWTP
5. ESSS growth works programme
6. Ellesmere WWTP upgrade
7. Ellesmere growth works program
8. New Darfield WWTP
   

Beneficiary groups included both existing and new developments as outlined below:

1. ESSS Growth
   
2. Darfield and Kirwee Existing
3. Darfield and Kirwee Growth
4. Ellesmere, USH and Fords Existing
5. Ellesmere Growth
   

The map below illustrates these indicative communities and proposed projects. In simple terms, Darfield and Kirwee pay for the Capex cost (less stimulus funding) and the District pays for ongoing operation and maintenance.

Figure 4-4 ​Communities and proposed ESSS projects

Selwyn District Council adopted the PWC model and further refined the input costs, timing and inflation assumptions.

The share of the Council's cost of the capital works can be charged to ratepayers as an annual targeted rate. This will not cover the cost to physically connect the property to the scheme, which will need to be paid by the property owner.

The targeted rate is based on an interest rate of 3%, with rate payments being made for 40 years. The amount will change as interest rates change.

In addition to the annual targeted rate, ratepayers will also pay the annual district wide rate, which is paid by all ratepayers that are connected to a Council wastewater system. The estimated rate increase for a typical residential ratepayer in Darfield and Kirwee, assuming connection being available in 2025/26 is outlined in Table 4-1.

Properties that constitute more than a single household unit equivalent (HUE) will pay an additional $6,085 per HUE and this will increase the loan rate proportionately.

Table 4-1 Estimated rate increase

​​​​​4.1.5   Government Stimulus Funding (Three Water)

Council decided to pursue the option of conveyance of wastewater to the Pines WWTP in Rolleston, and sought $10.66 million of stimulus funding from the central government to subsidise this scheme.  Council has since received the first instalment of the stimulus funding ($5.33 million) on 20 November 2020. 

The decision on the extent of the scheme will be the subject of the 2021-31 Long Term Plan consultation.

4.1.6   ​Pines 120 Upgrades

The Pines WWTP has been designed to be modular, allowing the plant to be upgraded in stages to match population growth. The current Pines WWTP will have capacity to treat wastewater from 60,000 Population Equivalents (PE). A masterplan has been developed for the treatment plant to see what it would take to expand the ultimate treatment capacity to 120,000 PE.

Refer the ESSS section of the AcMP for further information [add link]