Green light for wastewater upgrade

Council-appointed independent commissioners this week granted Watercare resource consent for the proposed Central Interceptor project, a $950 million initiative the company says will allow it to effectively cater for the region’s anticipated population growth in an environmentally and economically sustainable fashion. Construction is scheduled to begin in 2017.

Watercare Resource Consents Manager Belinda Petersen says while the company is still reviewing the fine detail of the decision, the overall message is positive for the region as a whole.

“The Commissioners have agreed that the Central Interceptor is the best option to manage expected future demand while minimising environmental impact both during the construction period and beyond,” Ms Petersen says.

Central to the project is a 13-kilometre long tunnel between Western Springs and Watercare’s Mangere wastewater treatment plant. The tunnel, which will run 30 metres below the Manukau Harbour seabed, will replace pipework Ms Petersen says is “reaching the end of its useful life”.

“The lower section of the Hillsborough Tunnel and the Manukau Siphon have an estimated residual life of between 15 and 25 years. Any failure of that part of the network could result in significant discharges of untreated wastewater into the Manukau Harbour for an unknown period of time, including the majority of industrial flows presently treated at the Mangere plant.

“That consideration alone makes this a very important piece of work for us,” Ms Petersen says.

The Central Interceptor’s extra capacity will reduce overflows throughout the network by 80 percent. At the same time, other work planned will see wastewater from West Auckland that is currently treated at Mangere being diverted to Watercare’s Rosedale wastewater treatment plant.

The proposal also entails the addition of an emergency pressure relief (EPR) structure at Mangere, which would allow untreated storm and wastewater to be discharged into the harbour should the Mangere pump station fail while the tunnel was full to capacity. Experts have assessed the chance of plant failure coinciding with the volume of rain needed to fill the tunnel as a one in 50 year event.

Ms Petersen said the EPR was necessary to give some certainty over where the discharge went, in order to minimise the effects on public health and safety and enable effective mitigation and clean-up procedures. Other options had been thoroughly evaluated, but none provided an adequate solution.

The Commissioners described the EPR as essential, and noted it represented best practice as a last-ditch fail-safe option.