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5-Waters-Volume-3-Wastewater
16 Tai Tapu Wastewater Scheme
16.1 Scheme Summary
| Estimated Population Served | 743
|
Scheme Coverage (1 Jan 2021)
| Full Charges
| 210
|
| Half Charges | 41
|
| >1 Charges | 5
|
System Components
| Piped (m) | 18,250
|
| Manholes (No.) | 57
|
| Pump Stations (No.) | 1
|
| Treatment | To Christchurch
|
| Disposal | To Christchurch |
| History | Original scheme installation date | 1998 |
Value ($)
| Replacement Cost | $6,006,183
|
| Depreciated Replacement Cost | $4,800,789
|
Financial
| Operator cost (scheduled and reactive maintenance) per connection
| $136.32/connection
|
Demand
(1 Jan - 31 Dec 2020)
| Annually (m3) | 41,487
|
| Average daily (m3) | 114
|
| Peak daily (m3) | 1,076
|
| Minimum daily (m3) | -
|
| Infiltration | Yes
|
| Sustainability | Ultimate discharge point | To Christchurch city network |
16.2 Key Issues
The following key issues are associated with the Tai Tapu Wastewater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 16‑1 Tai Tapu Scheme Issues
| Significant infiltration and inflow is experienced during wet weather | Target renewals to address infiltration and investigate sources of infiltration |
| Discharge capacity (via CCC agreement) is limited. | Discuss options with Christchurch City to increase discharge allocation. And investigate options to pump to the ESSS network. |
16.3 Overview & History
An agreement with Christchurch City Council allows the pumping of raw sewage to Christchurch City reticulation, up to a maximum flow of 7.5 Litres/sec with a maximum discharge volume of 90,000m3 in each financial year – (24 hours/day pumping is raw sewage).
Following the September 2010 and February 2011 earthquakes, liquefaction was observed throughout the Tai Tapu township and over to Halswell. Inspection of the PVC sewer pipes did not reveal any acute damage though change in pipe grades if any have not been confirmed. In November 2011 the Department of Building and Housing stated the land was classified as TP2 – Foundation Technical Category 2 for the purpose of vertical infrastructure works. This recognises that minor-moderate and damage from liquefaction occurs during large earthquakes and strengthening of floors and foundations are required. Any future renewals, repairs and capital works on the horizontal infrastructure will be required at a higher construction standard.
Figure 16‑2 Scheme Schematic
16.4 System Capacity
The report presented to Council on 13 July 2005 summaries the situation best. Originally the Tai Tapu scheme was designed for 232 Lots. A public excluded report to Council 6 October 2004 recommended that the limit of serviced properties be extended to 279 domestic connections – the information from this report has since been made public.
The scheme is at capacity. New connections onto the pressure main to Christchurch City are controlled by Council policy S102 and S103 which can be found at the following link
Selwyn-District-Council-Policy-Manual-APPROVED-BY-COUNCIL-on-2-September-2020-Website.pdf
16.5 Resource Consents
No consents are required for the Tai Tapu discharge as this is covered by the Christchurch City Council consents.
16.6 Scheme Assets
A summary of the assets within this scheme is outlined in this section.
16.6.1 Reticulation Overview
A summary of material and diameter for pipes is shown in Figure 16‑3 and Figure 16‑4.
Figure 16‑3 Pipe Material – Tai Tapu
Figure 16‑4 Pipe Diameter – Tai Tapu
16.6.2 Treatment and Disposal
No treatment is carried out with all wastewater pumped to Christchurch City.
16.6.3 Pump Stations
There is only one pump station within the Tai Tapu system. This pump station pumps wastewater to the Christchurch City network, described in Table 16‑2 below.
Table 16‑2 Pump Stations
| Two surface mounted mono pumps with munchers (mechanical screens before pumping) | 1998 | 7.5 |
16.7 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.
16.8 Photos of Main Assets
Photo 1 – Pump Station
16.9 Risk Assessment
A risk assessment has been undertaken for the Tai Tapu 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 16‑3 details the risk priority rating, Table 16‑4 outlines the risks and the list of key projects is found in Table 16‑9.
Table 16‑3 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 16‑4 Risks – Tai Tapu
| High risk of infiltration | Investigate the source of infiltration | 2014 | 45 | 45 | 20 |
| inadequate capacity | Pump to Lincoln | 2017 | | 20 | 10 |
The list of district wide risks can be found in 5Waters Activity Management Plan: Volume 1.
16.10 Asset Valuation Details
The total replacement value of assets within the Tai Tapu Scheme is $6,006,183 as detailed in Table 16‑5 below.
Table 16‑5 Replacement Value, Tai Tapu
|
Plant and Equipment | $218,942
|
Wastewater Reticulation
| Chamber | $213,614
|
| Lateral | $883,257
|
| Manhole | $434,935
|
| Pipe | $4,219,106
|
| Valve | $36,329
|
16.11 Renewals
The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 16‑5 below.
Figure 16‑5 Tai Tapu Wastewater Renewal Profile
16.12 Critical Assets
The criticality model for Tai Tapu has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 16‑6 and Figure 16‑6 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 16‑6 Length of Assets per Criticality Level
5
| Low | 8,792
|
4
| Medium-Low | 8,046
|
3
| Medium | 922
|
2
| Medium-High | 472
|
1
| High | 0
|
16.13 Asset Condition
The asset condition model was run for Tai Tapu in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 16‑7 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 16‑7 provides a description of the condition rating used within the condition model.
Table 16-7 Asset Condition Grading
| 1.0 | Excellent |
| 2.0 | Good |
| 3.0 | Moderate |
| 4.0 | Poor |
| 5.0+ | Fail |
16.14 Funding Program
The 10 year budgets for Tai Tapu are shown by Table 16‑8. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1.
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 16‑8 Tai Tapu Budget Summary
| 2021/2022 | -
| - |
| 2022/2023 | -
| -
|
| 2023/2024 | -
| - |
| 2024/2025 | -
| -
|
| 2025/2026 | -
| -
|
| 2026/2027 | -
| -
|
2027/2028
| -
| -
|
2028/2029
| -
| $2,000,000
|
2029/2030
| -
| -
|
2030/2031
| -
| -
|
Total
| -
| $2,000,000
|
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 16‑9 Key Projects
| Capital Projects | -
| New pipeline to Lincoln + upgrade pump station on School Road
| - | -
| -
| $2,000,000
| 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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