Whanganui council spending $435,700 on wastewater plant concrete corrosion

Whanganui's wastewater treatment plant was built in 2017 and cost $39 million. Photo / NZME

Concrete corrosion at Whanganui’s wastewater treatment plant has left the council with a $435,700 refurbishment bill, and a similar amount is likely to be spent next year.

Whanganui District Council senior wastewater engineer Tony Hooper told the council’s operations and performance committee the damage was in a clear storage tank.

The 7m deep tank was designed to be covered to control odour so staff could not see the state inside, he said.

It was filled with about 6m of effluent all year round, but 1m was exposed.

“Last year, we took the covers off to do some maintenance on the diffusers at the bottom of the tank and were alarmed to see the deterioration in that top metre.

“Obviously, this tank is designed to have quite a long lifetime and we’re only eight years in.”

A $435,700 contract was awarded to Presco Hire for phase one of the refurbishment - half of the tank.

Work began in February.

A council report said it was a two-part life extension project to mitigate further deterioration and extend the tank’s 50-year design life.

“Basically, they waterblast all the loose material off, spray a new layer of concrete on and paint an epoxy on,” Hooper said.

“It’s a quite new asset and we’re doing maintenance work on it already.”

He said some of the concrete remained “as it was when it was poured” but there was significant damage in other parts of the tank.

Councillor Charlotte Melser asked what the cause was.

“Is it something in the water or was it a manufacturing issue?” she said.

Tony Hooper says additives may not have been added to the concrete during construction of the plant. Photo / NZME

Hooper said a common byproduct of wastewater was hydrogen sulfide, which did “not mix well” with concrete.

“Where the tank sits full of water, the concrete is perfect.

“Above the tide line, there are issues.”

Additives could be added to the concrete to prevent that kind of corrosion, he said.

“It looks like that potentially wasn’t done, which may have been a shortfall given it’s a wastewater treatment plant.”

In a statement to the Chronicle, Hooper said the company that built the plant, Hawkins/Downer, had been informed of the corrosion issue and “discussions are ongoing”.

The plant, off Airport Rd, was finished in 2017 for $39 million.

According to a council report from April that year, Hawkins/Downer was the main contractor, with six direct sub-contractors.

When asked if there was a chance the council could recoup any of the money spent on the refurbishment, he said officers were verifying whether it was a construction or design issue to “better determine our next steps”.

Hooper said phase two would begin next year and was likely to cost about the same as phase one.

Funding for the work came from the plant’s renewal budget.

“That is separate from our operating expenses at the plant so it won’t affect any operations,” he said.

Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.