Extra investment in wastewater network to be considered
Published on 03 August 2023
A report on the performance of Whanganui’s wastewater network during wet weather was received by councillors at a full Whanganui District Council meeting today, Thursday 3 August.
Councillors heard that while Whanganui has completed close to 98 percent of a water separation project that began in the 1980s – with stormwater diverted away from the wastewater system on public and private land – a substantial amount of stormwater has been observed entering the wastewater system in wet weather.
The council’s Three Waters manager, Kritzo Venter, says stormwater getting into the system puts pressure on the wastewater network, resulting in diluted wastewater spilling into the environment – particularly in low-lying areas – which ultimately makes its way to the Whanganui River. This overflow could come from drains, sumps and wastewater access chambers.
The overflows are in breach of Horizons Regional Council’s Regional Plan and Whanganui District Council must mitigate any health issues under public health legislation. Any areas on public or private property where overflows occur are decontaminated by council contractors.
Part of the solution will require a programme of new work to be considered in the council’s Long-Term Plan 2024-2034, including a capital investment of an additional $50M over the next 30 years to strengthen the wastewater network. This would include pipe upgrades, pump station upgrades and new wet weather storage facilities.
Some interim reconfigurations within the network have also been made at strategic locations, to minimise and isolate any health risk to high-use areas or private properties, Kritzo Venter says.
Horizons Regional Council and Ngā Tangata Tiaki o Whanganui are aware of these issues and the intent to address them through the Long-Term Plan 2024-2034 process.
Whanganui Mayor Andrew Tripe says the council is acutely aware of the economic reality for many Whanganui households and news that an extra $50M is likely to be needed over the next 30 years to improve the wastewater network may be hard to accept.
“Even though there is uncertainty around how our water services may be delivered in future, with three waters reform legislation currently going through Parliament, it is important we do the planning we can, while we can. We need to do the right thing by making sure wastewater isn’t getting into the environment,” Mayor Andrew says.
“We are also working with the proposed new Manawatū-Whanganui water services entity to make sure our asset management plans are aligned.”
The council’s chief executive, David Langford, says it’s important the community is aware of the issue, what is required to fix it and how the work will be funded.
“We want to be open and transparent with our community and be clear about what they can expect to see going into the future. Currently the network is not performing as it should in wet weather and the community should rightly expect us to come up with a proper and permanent fix.”
David Langford says the council also needs to consider a more robust compliance framework to eliminate any non-compliant inflows into the wastewater system (ie, via stormwater downpipes and yard sumps connected to gully traps).
“The council’s wastewater system has been built properly and to industry standards and hydraulic modelling shows that during dry weather it performs really well. However, during wet weather the volume of water in the wastewater network increases by significantly more than five times the dry weather volumes, which is the industry standard. Such high volumes of stormwater in the wastewater network overwhelms the system and causes sewage overflows," David Langford says.
“While the source of the overflows can’t always be identified it is critical that the council runs an enforcement programme to find those private properties that have non-compliant stormwater connections into the wastewater network and get the property owners to remove them.”
Currently about $200,000 is budgeted annually in the long-term plan to investigate where the infiltration is happening, along with identifying underground faults and scheduling repairs or renewals. As part of the council’s Infrastructure Strategy 2021-2051, roughly $50M over the next 30 years has also been budgeted for upgrading the stormwater system’s public network.
The council’s wastewater discharge consent with Horizons expires in 2026. The process for preparing the new consent has begun, with an application for a consent renewal to be made in approximately November 2025.
More information will be presented to the council over the next six months in the lead-up to the adoption of the Long-Term Plan 2024-2034 in May next year.