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5-Waters-Volume-3-Wastewater
8 Lake Coleridge Wastewater Scheme
8.1 Scheme Summary
Estimated Population Served
| 195
|
Scheme Coverage
(1 Jan 2021)
| Full Charges | 51
|
| Half Charges | 14
|
| >1 Charges | 2
|
System Components
| Piped (m) | 3,140
|
| Manholes (No.) | 44
|
| Pump Stations (No.) | 1 (at treatment plant)
|
| Treatment | Imhoff tank, wetland and UV treatment |
| Disposal | Overland treatment and denitrification trench |
| History | Original scheme installation date | 1965
|
Value ($)
| Replacement Cost | $1,573,791
|
| Depreciated Replacement Cost | $668,439
|
| Financial | Operator cost (scheduled and reactive maintenance) per connection
| $1,297.40/connection
|
Demand
(1 Jan - 31 Dec 2020)
| Annually (m3) | 6,736
|
| Average daily (m3) | 18.5
|
| Peak daily (m3) | 37.6
|
| Minimum daily (m3) | 1
|
| Infiltration | No
|
| Sustainability | Ultimate discharge point | Overland flow, denitrification trench and discharge to stream
|
8.2 Key Issues
The following key issues are associated with the Lake Coleridge Wastewater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 8‑1 Lake Coleridge Scheme Issues
| Physical distance and remote nature of plant increases operation and maintenance costs. | Review operation costs with the aim of finding optimised maintenance procedures including plant upgrades as required. |
8.3 Overview & History
Currently raw domestic sewage from the village gravitates to the pump station at the treatment plant. From the pump station sewage is pumped to the top of an Imhoff tank where the solids are separated. The partially treated sewage then gravitates to the subsurface wetland, passing through a natural treatment process prior to UV disinfection. The treated and disinfected sewage then flows via a dosing chamber to the overland flow area, a denitrification process occurs. If necessary, (due to saturated ground conditions) treated sewage can then flow into Post Office Creek through the rock diffuser structure.
There is a small residential population but little commercial activity in the village. Recent subdivision developments in the Lake Coleridge basin have connected to the village wastewater scheme.
Historically there has been high infiltration and inflow.
In November 2011 a variation to the Rakaia River Water Conservation Order was made by Trust Power Ltd. Council staff had a number of concerns which they were heard on. An agreement was signed dated 11 July 2012 between the Council and TrustPower which stated:
-
Trustpower will not, through the Coleridge Project, adversely affect the existing assets held and operated by the Council for the purposes of conveying, treating and disposing of wastewater from the village.
-
Trustpower will continue to provide, through existing Coleridge infrastructure, or through the Coleridge Project, flushing flows for the Councils sewerage outflow/overflow as may be reasonably required.
-
Trustpower will ensure that any effects on the Village wastewater scheme as a result of TrustPower's future infrastructure works will not be disadvantage or burden the Village wastewater scheme.
The community is located within the alpine fault line, and is expected to incur significant damage to its water infrastructure resulting from a major event.
Figure 8‑2 Scheme Schematic
8.4 System Capacity
In general the Lake Coleridge scheme is underutilised as the occupancy rate within the houses that the scheme serves is low. There are peak periods but these too are well within the consented limits.
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.
8.5 Resource Consents
The Lake Coleridge wastewater scheme has a number of resource consents. Table 8‑2 shows the discharge permitted by the resource consents for this scheme.
Table 8‑2 Resource Consents
CRC012170.1
| To discharge treated domestic sewage to land. | Hummocks Road, Lake Coleridge
| 3/05/2011 | 9/07/2027 | 41
|
CRC012169
| To discharge treated domestic sewage to Post Office Creek. | Hummocks Road, Lake Coleridge | 12-Jul-02 | 9/07/2027 | 41 |
8.6 Scheme Assets
A summary of the assets within this scheme is outlined in this section.
8.6.1 Reticulation Overview
A summary of material and diameter for pipes is shown below in Figure 8‑3 and Figure 8‑4.
Figure 8‑3 Pipe Material – Lake Coleridge
Figure 8‑4 Pipe Diameter – Lake Coleridge
8.6.2 Treatment and Disposal
Table 8‑3 outlines treatment and disposal systems used within the Lake Coleridge scheme.
Table 8‑3 Treatment Plant Components
| Imhoff tank | Provides primary treatment for the raw sewerage | 1965 & 2004 |
| Subsurface wet land | The submerged vegetated bed wetland with a surface area of 500m2. It has a working depth of 0.55 m at the outlet producing a hydraulic retention time of approximately 2 days | 2004 |
| UV disinfection | The UV unit is sized to accommodate the flow through the wastewater treatment plant with a design flow of 1.8 L/s | 2004 |
| Overland flow area | The overland flow area is approximately 500m2. The overland flow area is divided by a denitrification trench into two cells. The slope of two overland flow cells is approximately 2%. It is planted with a variety of local grasses | 2004 |
8.6.3 Design
The Lake Coleridge scheme was designed for a resident population of 110 PE, equivalent to approximately 44 occupied dwellings. This is shown below in Table 8‑4.
Table 8‑4 Design Data
| Design Population | 110
(366 30% Occupancy | 370 | 41
| 14.2
| 73 (PF 1.8) |
8.6.4 Pump Stations
There are no pump stations within the Lake Coleridge reticulation. There is a pump chamber and two pumps at the treatment plant pumping sewage to the Imhoff tank. These are detailed below in Table 8‑5.
Table 8‑5 Pump Stations
At Treatment Plant
| Two pumps (1 submersible & 1 surface mounted)
| 1996 | 3.8 |
| | standby |
8.7 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.
8.8 Photos of Main Assets
Photo 1 – Pump house and Imhoff tank
| 
Photo 2 - Wetland
|
8.9 Risk Assessment
A risk assessment has been undertaken for the Lake Coleridge 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 8‑6 details the risk priority rating, Table 8‑7 outlines the risks and the list of key projects is found in Table 8‑12.
Table 8‑6 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 8‑7 Risks – Lake Coleridge
| Impact of power failure on receiving environment | Install generator plug | 2014 | 12 | 6 | 6 |
| Plant failure impact on receiving environment | Review failure consequence of individual items of plant | 2014 | 12 | 12 | 6 |
| Poor functioning UV treatment may lead to consent breaches | Install wipers on UV unit or renew UV unit with unit which has self cleansing wipers. | 2014 | 12 | 2.1 | 2.1 |
| Non-consented activities | Renewal of consents | 2014 | 27 | 27 | 6 |
| Poor functioning treatment may lead to consent breaches | New TP Design | 2017 | | 2.1 | 6 |
| Poor functioning treatment may lead to consent breaches | New TP Construction | 2017 | | 2.1 | 6 |
| Plant renewal | New TP Construction | 2017 | | 2.1 | 6 |
The list of district wide risks can be found in 5Waters Activity Management Plan: Volume 1.
8.10 Asset Valuation Details
The total replacement value of assets within the Lake Coleridge Scheme is $1,573,791 as detailed in Table 8‑8 below.
Table 8‑8 Replacement Value, Lake Coleridge
Plant and Equipment
| $480,442
|
Wastewater Reticulation
| Chamber | $12,234
|
| Lateral | $237,339
|
| Manhole | $310,599
|
| Pipe | $533,176
|
8.11 Renewals
The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 8‑5 below.
Figure 8‑5 Lake Coleridge Wastewater Renewal Profile
8.12 Critical Assets
The criticality model for Lake Coleridge has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 8‑9 and Figure 8‑6 below shows the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 8‑9 Length of Assets per Criticality Level
5
| Low | 1,637
|
4
| Medium-Low | 1,503
|
3
| Medium | 0 |
2
| Medium-High | 0 |
1
| High | 0 |
8.13 Asset Condition
The asset condition model was run for Lake Coleridge in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 8‑7 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 8‑10 provides a description of the condition rating used within the condition model.
Table 8‑10 Asset Condition Grading
| 1.0 | Excellent |
| 2.0 | Good |
| 3.0 | Moderate |
| 4.0 | Poor |
| 5.0+ | Fail |
8.14 Funding Program
The 10 year budgets for Lake Coleridge are shown by Table 8‑11. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1.
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 8‑11 Lake Coleridge Budget Summary
| 2021/2022 | -
| $101,000
|
| 2022/2023 | -
| -
|
| 2023/2024 | -
| -
|
| 2024/2025 | -
| -
|
| 2025/2026 | -
| -
|
| 2026/2027 | -
| -
|
2027/2028
| -
| -
|
2028/2029
| -
| -
|
2029/2030
| -
| -
|
2030/2031
| -
| -
|
| Total | -
| $101,000
|
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 8‑12 Key Projects
| Capital Projects | -
| Upgrade UV to automated wiping system
| $101,000
| -
| -
| -
| 100% LoS |
* LoS refers to Level of Service
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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