Parts of Shirley, Aranui and all of Prestons are serviced by vacuum sewer systems which either have limited or no spare capacity.
We use different types of wastewater systems(external link) across the city. Shirley, Aranui and Prestons had vacuum sewer systems installed post-earthquake because they cope better with liquefaction and land settlement.
The vacuum sewer systems were designed for the housing density that was foreseen at the time. Since then, the Christchurch District Plan has enabled intensification in parts of the Shirley vacuum sewer area but the capacity of the vacuum sewer system has now been reached.
Both the Shirley and Aranui vacuum sewer systems are experiencing severe operational issues when it rains because of excessive stormwater and groundwater that enters the system from private property. The public vacuum pipes are completely sealed to avoid groundwater seepage.
Similarly, the Prestons vacuum sewer system is designed to service the number of properties established and not the intensification that would be enabled by the Resource Management (Enabling Housing Supply and Other Matters) Amendment Act 2021.
We are proposing, as part of our Draft Housing and Business Choice Plan Change (external link)(PC14), that Shirley, Aranui and Prestons vacuum catchments be excluded from intensification. Areas that should be exempt, will be listed as Qualifying Matters in the District Plan.
The wastewater gravity networks in Shirley and Aranui were significantly damaged in the 2010/11 Canterbury earthquakes. The Stronger Christchurch Infrastructure Rebuild Team (SCIRT) was tasked to return the infrastructure networks to a condition to meet the levels of service prior to the 4 September 2010 earthquake, within the time constraints of the rebuild.
SCIRT considered different options for Shirley and Aranui – including gravity system replacement, enhanced gravity system, vacuum sewer system and pressure sewer system. These options were evaluated in terms of constructability, resilience, planning/communication and estimated lifecycle costs such as capital and operational costs, inflow and infiltration savings as well as further seismic damage costs.
For both the Shirley and Aranui areas, the vacuum sewer system option was considered the best option and was approved by the SCIRT Scope and Standards Committee.
At the same time, Ngāi Tahu concluded that the Prestons greenfield subdivision should also be developed as a vacuum sewer catchment.
During the SCIRT rebuild, only like-for-like restoration was funded by the Crown. Improvements, if economically favourable and required for the rebuild, had to be funded by the Council.
In the Shirley and Aranui vacuum systems, SCIRT designed the sizes of the vacuum sewer mains and the vacuum pump stations to accommodate flow from existing dwellings and from future development using the land zoning and density requirements of the Christchurch City Plan, operative at the time, which set a minimum requirement of one dwelling per 450 m2 land parcel.
The Christchurch City Plan was replaced by the Christchurch District Plan (District Plan) in 2017 and approximately 30 percent of the Shirley vacuum catchment was rezoned from ‘Living 1’ to ‘Residential Medium Density’. This changed land use requirements from one dwelling per 450 m2 land parcel to one dwelling per 333 m2 land parcel.
Infill development has been occurring under the updated density rules, although the capacity of the vacuum sewer systems has not changed. This is because they are not as easily upgraded as a gravity sewer system. All components – including the vacuum pump station – would need to be improved at the same time to increase capacity or an alternative option, such as splitting an existing vacuum sewer system or the creation of satellite wastewater storage and pump stations, would need to be implemented.
The current District Plan allows for significantly denser infill development than the previous City Plan. For example in areas zoned ‘Residential Medium Density’, the District Plan specifies a minimum density of 30 dwellings per hectare (one dwelling per 333 m2 land parcel). Based on consents processed in the previous 12 months, consented densities have ranged between 40 and 135 dwellings per hectare, with the average being 71 dwellings per hectare (average of one dwelling per 141 m2 land parcel).
For comparison, the design densities for the vacuum sewer systems, utilising the SCIRT design parameters, range from 11 to 29 dwellings per hectare in Aranui and 10 to 16 dwellings per hectare in Shirley.
The changes to the intensification rules between the City Plan and the District Plan have resulted in the number of current households connected to the vacuum sewer system exceeding the SCIRT design. In Shirley, all vacuum arms exceed the design number of households, with households ranging between 107 percent and 115 percent of the SCIRT design. In Aranui, one arm has reached 78 percent of the design and the remaining five arms are between 97 percent and 104 percent of the SCIRT design.
Vacuum sewer system performance is dependent on maintaining the balance between air and liquid in the pipes, also known as the air-to-liquid ratio. This requires regular checking and setting of individual valve controls and ensuring that the vacuum mains do not become waterlogged.
Both the Shirley and Aranui vacuum sewer systems experience significant operational issues during wet weather, which is an indicator that inflow and infiltration (I&I) from private property laterals exceed the design allowance.
Where I&I flows exceed the design allowance into the collection chamber and through the vacuum valves, the system responds as follows:
The amount of development that can be accommodated in Shirley, Aranui and Prestons is restricted due to the capacity issues on the vacuum sewer systems. Allowing additional development would further exacerbate existing capacity issues in these areas.
The vacuum sewer system has reached capacity and only like-for-like development can be accommodated in these areas.
Enquiries about what constitutes like for like should be sent to WastewaterCapacity@ccc.govt.nz
The vacuum sewer system has limited capacity and development must align with the Prestons Sewer Master Plan.
Enquiries about development in the Prestons area should be sent to WastewaterCapacity@ccc.govt.nz
Vacuum sewer monitoring devices were installed in 2021 and can be used to identify the properties contributing to the high inflow and infiltration.
It is proposed that the Water Supply and Wastewater Bylaw 2022(external link) will be used to require individual property owners to inspect and repair their sewer drains. If they don’t, we can do the repair and recover the costs from the property owner. We will also continue to monitor the inflow and infiltration during wet weather to determine the degree of reduction achieved.
We are continuing to identify and assess whether wastewater from areas within the vacuum system can be discharged elsewhere in order to open up more capacity for development. The focus area is the Residential Medium Density zone in Shirley.
We plan to investigate infrastructure options and costs for increasing the vacuum sewer system capacity for different levels of development within the Shirley and Aranui vacuum sewer areas.
The options and costs will be presented to Council for a decision on whether funding should be included in the Long Term Plan. The target for completing an options and cost analysis is the end of the 2022/23 financial year (July 2022 to June 2023).