Filters delay Gisborne wastewater treatment plant upgrade one year

Gisborne District Council is a year behind schedule for stage two of its wastewater treatment plant upgrade due to two tertiary filters.

Gisborne’s wastewater treatment plant upgrade has been delayed because two tertiary filters are not meeting expected performance.

A council spokesperson said the upgrades were a year behind schedule, but they were leading the charge with the designs, so they were unable to learn from others.

The $77 million improvements are in the second stage of the two-stage upgrade.

Stage 1 involved ground improvement and building a pump station, while stage 2 involved adding a clarifier, tertiary filtration, ultra-violet disinfection and solids removal.

The site had passed its first year of operation in discharging liquid waste. However, the tertiary filters were not consistently meeting compliance.

Gisborne District Council director of community lifelines Tim Barry said the tertiary filters “polish” (a stage of filtration) the recycled water to help ensure compliance.

“Achieving design performance for the tertiary filters remains the last step in the process.”

Council staff were working with the supplier of the tertiary filters to find a solution for issues as soon as possible, he said.

The upgrades at the plant meant all domestic wastewater was recycled to a high level, which benefited the receiving environment when the water was released, Barry said.

“The Wastewater Treatment Plant (WWTP) currently uses the recycled water within the treatment processes, reducing waste.”

Barry said the council was exploring options to use the recycled water for beneficial reasons.

“There is scope for this to significantly reduce water use, but this would require the development of a business case. Presently, all water leaving the WWTP is returned to the environment.”

During a Waste Water Management Committee meeting this month, council’s water manager Leo Kelso described it as “a journey”.

“We are a year from where we ultimately wanted to be.

“We are leading the pack, we’re not fast followers ... so we are unable to learn from others effectively from a tertiary filter following,” he said.

Councillor Larry Foster said this is a whole new system – there has never been one like it before.

The council allocated $500k for Stage 2 further treatment in the 2024/2025 financial year budget.

Kelso said in December the filters were working at their upper limit of performance, for less than half the flow they were designed to filter.

“The recommencement of the tertiary filter operation will be crucial to support the polishing of the effluent to consistently achieve compliance,” said the report.

“Beyond unforeseen significant rain events, plant performance has been steady and consistent.”

The suppliers of the filters, Veolia, were in the region in February, and it was a productive session, said Kelso during March’s meeting.

He said the council had continued an active engagement weekly with Veolia as the suppliers process the information to support their understanding of the issues.

“From what has been observed to date, there is potential for the filter pore size to [achieve] solids removal well above consent limits, which is compromising their ongoing function.

“This was highlighted during the trial, where low flows were seen to trouble the filters,” said the report.

Correction: The headline of an earlier version of this story described the wastewater treatment plant filters as faulty. Gisborne District Council says the filters are not faulty; they are currently not achieving the expected design performance for their situation. The headline and first sentence of the story have been amended to reflect this.