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5-Waters-Volume-3-Wastewater
17 Upper Selwyn Huts Wastewater Scheme
17.1 Scheme Summary
Estimated Population Served
| 80
(Only 12 dwellings permanently occupied, the balance are occupied periodically)
|
Scheme Coverage (1 Jan 2021)
| Full Charges | 1 charge
97 connections
|
| Half Charges | 0
|
| >1 Charges | 0
|
System Components
| Piped (m) | 1671
|
| Manholes (No.) | 23
|
| Pump Stations (No.) | 1
|
Treatment
| Septic tank followed by oxidation pond |
| Disposal | Border Dyke |
| History | Original scheme installation date | 1920's |
Value ($)
| Replacement Cost | $942,462
|
| Depreciated Replacement Cost | $435,542
|
Financial
| Operator cost (scheduled and reactive maintenance) per connection
| $243.37/connection
|
Demand
(1 Jan - 31 Dec 2020)
| Annually (m3) | 8,220
|
| Average daily (m3) | 22.5
|
| Peak daily (m3) | 101.4
|
| Minimum daily (m3) | -
|
| Infiltration | Yes
|
| Sustainability | Ultimate discharge point | Border dyke Irrigation |
17.2 Key Issues
The following key issues are associated with the Upper Selwyn Huts Wastewater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 17‑1 Upper Selwyn Huts Scheme Issues
| High flows may at times breach resource consent conditions | Monitor recently Installed SCADA system |
| Consent expires 2020 | Progress short term consent application
Consider alternative of piping to Pines WWTP
|
17.3 Overview & History
Selwyn Huts is a settlement of 97 dwellings.
The settlement is located on Council reserve land and the day to day operation of the settlement is managed by the council. This change occurred in June 2011, as a result of local reserve committee governance issues.
The Selwyn Huts reticulated sewerage scheme was initially installed in the 1920s with the overflow from the septic tanks discharged into the Selwyn River. A new sewage treatment and land disposal scheme was constructed in 1988. In the upgraded system, sewage discharges from the reticulation into a septic tank. The sewage from the septic tank then discharges to an adjacent pumping chamber via a 65mm diameter main to an oxidation pond located east of the township. Treated effluent is thereafter discharged into a pipeline linked to six irrigation control boxes and is released onto 0.88 hectares of border dyked irrigation area. In emergency situations or when the ground is too wet for border dyke irrigation, the treated effluent could be discharged into the Selwyn River.
Based on records received, there is a strong correlation between winter groundwater levels/rainfall and increased sewage flows into the ponds. Given the sustained periods of elevated winter sewage flows, the major component is infiltration i.e. leaky pipes, rather than Wastewater via rainfall.
The peak wet weather flow was recorded at 914 m3/day in August 2010. This is 42 times of the consented daily volume and breaches consent conditions.
Figure 17‑2 Scheme Schematic
17.4 System Capacity
High wet weather flows (groundwater and Wastewater) dominated flows.
There is not expected to be significant growth in the area serviced by this scheme and scheme expansion is not expected to be required over the planning horizon of this AcMP.
17.5 Resource Consents
The Upper Selwyn Huts wastewater scheme has one resource consent. Table 17‑2 shows the discharge permitted by the resource consents for this scheme.
Table 17‑2 Resource Consents
CRC991634
| To discharge oxidation pond effluent to land | Days Road, Selwyn huts | 22/06/2000 | 20/06/2020
(currently operating under s124 continuance)
| 650 m3/month |
Consent, CRC991634, expired during 2020. New consent was lodged prior to the previous consent expiring, the activity has been granted a continuance from ECan.
17.6 Scheme Assets
A summary of the assets within this scheme is outlined in this section.
17.6.1 Reticulation Overview
A summary of material and diameter for pipes is shown in Figure 17‑3 and Figure 17‑4.
Figure 17‑3 Pipe Material – Upper Selwyn Huts
Figure 17‑4 Pipe Diameter – Upper Selwyn Huts
17.6.2 Treatment and Disposal
The Upper Selwyn Huts treatment and disposal facilities are detailed below in Table 17‑3.
Table 17‑3 Upper Selwyn Huts Treatment and Disposal Facilities
| Septic tank | Sewage from reticulation discharges into septic tank then discharges to adjacent pumping chamber and is pumped via 65mm dia main to oxidation pond | 1925 |
| Oxidation pond | The two celled oxidation pond is located east of township | 1988 |
| Border dyke irrigation | Treated sewage is discharge into a pipeline linked to 6 irrigation control boxes and released into 0.88ha of border dyke irrigation area | 1988 |
| Emergency discharge | In emergency situations or when ground water too wet for border dyke irrigation, the treated sewage is discharge via 100mm dia main into the Selwyn River | 1988 |
17.6.3 Pump Stations
There is one pump station in Upper Selwyn Huts, this is described in Table 17‑4 below.
Table 17‑4 Pump Stations
| Selwyn Huts | Two submersible pumps (duty/standby) | 1988 & 2000 | 2.4 |
17.7 Future Options
17.7.1 Options considered & recommendation
Council have considered three separate options for wastewater services at Upper Selwyn Huts. Table 17-5 compares these options. Further detail of options is provided below.
Council staff recommend connecting Upper Selwyn Huts and Coes Ford to the Ellesmere – Pines pipeline, if it goes ahead. This option has been included in the LTP budget.
Regardless of which option is chosen for wastewater disposal, the Upper Selwyn Huts reticulation network will need to be upgraded.
Table 17-5 Comparison of wastewater options for Upper Selwyn Huts
1
| Connect Upper Selwyn Huts to the Ellesmere/Pines pipeline | $1.5m
| Subject to Ellesmere to Pines pipeline going ahead; will need to pay the equivalent of the ESSS DCs on top (approx. $400k) |
| 2 | Pipeline from Upper Selwyn Huts to Pines WWTP | $4.2m | Standalone pipeline to Pines WWTP, including 3 pump stations. Joint funding with property team (due to campground connection) |
| 3 | New package plant and disposal field | $2.0m | Alternative option, many environmental & cultural constraints, consenting difficulties |
| - | Reticulation upgrade | $1.0m | Required for all options
|
17.7.2 Option 1 (preferred): connect Upper Selwyn Huts to the Ellesmere/Pines pipeline
If a pipeline is installed from Ellesmere WWTP to Pines WWTP, this presents an opportunity to connect Upper Selwyn Huts to the scheme. Figure 17-5 shows the potential pipe routes and two alternative connection points (on Leeston Road or at Chamberlains Ford).
The two pipe routes are estimated to have similar costs ($1.4m for the route to Chamberlains Ford in red; $1.5m for the direct route along Selwyn Lake Road). These costs include an intermediate pump station at Coes Ford, allowing connection of the camping ground at this location and maintaining flushing velocities in the rising main.
Figure 17-5 Potential pipe routes for connection of Upper Selwyn Huts and Coes Ford to Ellesmere to Pines pipeline.
17.7.3 Option 2: standalone pipe from Upper Selwyn Huts to Pines WWTP
Without the Ellesmere WWTP to Pines WWTP pipeline (i.e. if Upper Selwyn Huts was piped to Pines WWTP as a standalone project), the cost is estimated to be $4.16 million. Figure 17-6 shows the pipe route. The cost estimate includes 3 pump stations (located at Upper Selwyn Huts, Coes Ford and Chamberlains Ford).
Figure 17-6 Standalone pipe route from Upper Selwyn Huts to Pines WWTP
17.7.4 Option 3: Package wastewater treatment plant and disposal field at Upper Selwyn Huts
A package wastewater treatment plant and disposal field at Upper Selwyn Huts is estimated to cost $2.0M, including consenting costs.
Treatment would be provided by a modular package treatment plant.
Three land application systems have been considered:
- Two 60m ecoTrenches and 4,800m2 of raised drip irrigation field
- One 320m ecoTrench
- 20,000m2 of raised drip irrigation field
Table 17-6 compares the land application options. Figure 17-7 shows a typical detail for a raised drip irrigation system.
Table 17-6 comparison of land application options for Upper Selwyn Huts
1
| This option provides some flexibility with managing the effluent land application in accordance with site constraints as they may vary throughout the year |
2
| This is the most expensive LAS option, with the lowest footprint |
3
| This is the cheapest option, with the largest footprint. The adoption of cut and carry provides greater uptake of nutrients such as nitrogen should this be a consent requirement. |
Figure 17-7 Typical detail for the proposed ecoTrench raised drip irrigation system
17.8 Operational Management
The wastewater schemes are operated and maintained under the maintenance contract as follows:
- Contract 1241: Water Services Contract. Contract is with SICON who undertakes investigations, conditions inspections, proactive and reactive maintenance and minor asset renewals.
Wastewater sampling is completed under an agreement with Food and Health Ltd as required.
17.9 Photos of Main Assets
Photo 1 – Treatment Pond
| 
Photo 2 – Border Dykes
|
17.10 Risk Assessment
A risk assessment has been undertaken for the Upper Selwyn Huts scheme. The key output from the risk assessment is the identification of any extreme and high risks which need to be mitigated. In order to mitigate these risks they have been included and budgeted for in the projects within this LTP. Table 17‑7 details the risk priority rating and Table 17‑8 outlines the risks for this scheme.
Table 17‑7 Risk Priority Rating
| > 50 | Extreme | Awareness of the event to be reported to Council. Urgent action to eliminate / mitigate / manage the risk. Document risk and action in the AMP. |
| 35-50 | Very High | Risk to be eliminated / mitigated / managed through normal business planning processes with responsibility assigned. |
| 14-35 | High | Manage risk using routine procedures. |
| 3.5-14 | Moderate | Monitor the risk. |
| < 3.5 | Low | Awareness of the event to be reported to Council. Immediate action required to eliminate / mitigate / manage the risk. Document risk and action in the AMP. |
Table 17‑8 Risks – Upper Selwyn Huts
| Breaching of consent requirements | Review consent conditions and apply for variation (Short Term) | 2014 | 27 | 27 | 45 |
| Breaching of consent requirements | Install monitoring and SCADA equipment | 2014 | 27 | 27 | 27 |
| Consent due to expire in 2020? | Document alternative options | 2014 | 12 | 12 | 12 |
| Non-consented activities | Renewal of consents | 2014 | 12 | 12 | 6 |
The list of district wide risks can be found in 5Waters Activity Management Plan: Volume 1.
17.11 Asset Valuation Details
The total replacement value of assets within the Upper Selwyn Huts Scheme is $942,462 as detailed in Table 17‑9 below.
Table 17‑9 Replacement Value, Upper Selwyn Huts
Plant and Equipment
| $421,617
|
Wastewater Reticulation
| Chamber | $2,898
|
| Lateral | $23,088
|
| Manhole | $40,978
|
| Pipe | $354,173
|
| Valve | $9,652
|
17.12 Renewals
The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 17‑8 below.
Figure 17‑8 Upper Selwyn Huts Wastewater Renewal Profile
17.13 Critical Assets
The criticality model for Upper Selwyn Huts has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 17‑10 and Figure 17‑9 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 17‑10 Length of Assets per Criticality Level
5
| Low | 741
|
4
| Medium-Low | 858
|
3
| Medium | 0 |
2
| Medium-High | 0
|
1
| High | 72
|
17.14 Asset Condition
The asset condition model was run for Upper Selwyn Huts in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 17‑10 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 17‑11 provides a description of the condition rating used within the condition model.
Table 17‑11 Asset Condition Grading
| 1.0 | Excellent |
| 2.0 | Good |
| 3.0 | Moderate |
| 4.0 | Poor |
| 5.0+ | Fail |
17.15 Funding Program
The 10 year budgets for Upper Selwyn Huts are shown by Table 17-12. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1. These can be found by following the links:
Renewal Strategy
Expenditure
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 17-12 Upper Selwyn Huts Budget Summary
| 2021/2022 | -
| -
|
2022/2023
| -
| $100,000
|
2023/2024
| -
| $1,400,000
|
2024/2025
| -
| -
|
2025/2026
| -
| -
|
2026/2027
| -
| -
|
2027/2028
| -
| - |
2028/2029
| -
| -
|
2029/2030
| -
| -
|
2030/2031
| -
| -
|
| Total | -
| $1,500,000
|
An explanation of the categories within the budgets are as follows below:
- Expenditure consists of operation and maintenance costs;
- Renewals are replacement of assets which are nearing or exceeded their useful life;
- Projects are investigations, decisions and planning activities which exclude capital works; and
- Capital projects are activities involving physical works.
Table 17‑13 Key Projects
Capital Projects
| 466990008
| Trunk sewer
| -
| $100,000
| $1,400,000
| -
| TBC
|
* LoS refers to Level of Service; G refers to Growth
The list of district wide projects can be found in 5Waters Activity Management Plan: Volume 1.
Discussion on Projects
Projects have been determined based on their:
- Relevance to the scheme
- Requirement to be completed under legislation
- Ability to bring the scheme up to or maintain the Level of Service required under council's Asset Management Policy.
Many projects are jointly funded by more than one scheme and activity. Each scheme pays a pro-rata share only, equivalent to the number of connections.
Discussion on Capital and Projects
Where relevant, Capital (Levels of Service) and Capital (Growth) projects have been included in the scheme financial details.
Levels of Service Projects and growth splits have been provided to ensure the costs of population driven works are clear.
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5-Waters-Volume-3-Wastewater
