Whanganui's wastewater treatment plant under construction in 2017. Photo / NZME
A repeat of the environmental disaster at Wellington’s Moa Point sewage plant is unlikely in Whanganui, but there are “still risks” with the city’s wastewater network.
The Moa Point plant flooded and became inoperable earlier this month, resulting in 70 million cubic metres of sewage flowing into the sea off Wellington’s South Coast every day.
At a council operations and performance committee, Whanganui District Council senior wastewater engineer Tony Hooper said a similar event happened in Whanganui in 2014 at its former wastewater plant.
“For a long time, we were discharging partially treated wastewater to the ocean,” he said.
“We went through a process with Horizons (Regional Council) to gain a short-term consent to allow us to legally do that, with an eye on building a new plant.”
The new wastewater treatment plant, off Airport Rd, was completed in 2017 for $39 million.
But Hooper said there were still risks with several critical assets related to the wastewater network, specifically the Beach Rd pump station.
“For example, the rising main that goes from Beach Rd pumps all the wastewater from town up to the treatment plant,” he said.
“It runs parallel within 20m of the airport runway, and we don’t have a backup for that pipe.”
Hooper said the plant, wastewater network, operations and alarming systems were very well set up, but “that’s not to say something catastrophic can’t happen”.
If the pipe failed, the council would be forced to divert wastewater to the sea, he said.
“It’s the same with the falling main from the wastewater treatment plant. If something happens to that, we have no discharge from the plant.”
Council senior stormwater engineer Kritzo Venter said in the worst-case scenario, untreated wastewater could be spilt into the Whanganui River.
“If the diversion valve were to fail at a point downstream of where we can divert successfully to sea, we would have a problem,” he said.
But nothing was imminent, and staff “really keep an eye on the assets”.
Council staff were looking at how the valve could be augmented, and if an additional valve could be built, Venter said.
“We had our Moa Point moment 12 years ago, with the first treatment plant dramatically failing, but the second plant was probably the most scrutinised design and plant in the country, at least at that time."
In 2014, then-Mayor Annette Main said wastewater was bypassing the plant and going into the sea through a 1.7km-long ocean outfall, a necessary step in “the desludging process” of ponds at the facility.
It was “the next major step in our project to build a new, fully-functioning wastewater treatment plant”, she said.
Venter said one aspect of the new plant that made him uneasy at times was its primary, or anaerobic, pond.
The pond holds 57 million litres.
Sludge settles on the bottom of the pond, and the rest of the wastewater gets separated.
It was retrofitted on existing infrastructure, creating operational risk, Venter said.
“We can’t actually access properly into the anaerobic pond.
“It’s really dangerous. We tried to have a diver go in there in the past, but there’s zero visibility.
“At a point, we had to get the diver back with a lanyard or a safety rope, because the person got disoriented.”
Venter said the status of key infrastructure in the pond was unknown, but the plant remained within consenting limits.
“There may be a day of reckoning on what we need to do. It would be expensive, and it would be disruptive.”
He said the new council-controlled organisation for water, with Ruapehu District Council, would have close governance over the issues into the future.
“I’d imagine that board would very much ask questions around these risks, want to understand what it is, and make sure our forward contingency planning is as robust as it can possibly be.”
Wellington’s South Coast beaches remain off limits until the Moa Point plant is running again, a process which could take months.
On February 16, the Government announced an independent review into the failure, with a Crown review team to be appointed to both Wellington City Council and Wellington Water.
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 Whanganui District Council.