The likely cause of Wellington’s sewage disaster has been revealed, with a report finding trapped air in the system may have caused millions of litres of raw, untreated waste to be pumped into the city’s coast.
A hydraulic report commissioned by Wellington Water found air can become trapped in the wastewater bypass pipeline and disrupt the flow of wastewater, pushing it backwards and flooding the plant.
“While this is valuable information, these findings are just one piece of the puzzle. There are other areas of interest that require a more detailed investigation,” Wellington Mayor Andrew Little said.
The failure is currently at the centre of a Government-ordered investigation.
Wellington Water chief operating officer Charles Barker told media today air needs to be allowed into the system to ensure the pipe can “breathe easily”, but rapid water flows can cause turbulence in the pipe.
“The report is indicating turbulence could occur here which is grabbing air and trapping it, and then that acts as a form of block”, Barker said.
“If the flow is more than can get past that point, it’s got to go somewhere,” he said, and in this case likely meant the wastewater flowed back into the plant.
Asked whether it was a design flaw of the pipe, Barker said the plant was designed to take 4000 litres per second through the pipe, and the plant was not experiencing “anywhere near that at the time that the event occurred”.
No one was in the plant the night of the failure, he said, but noted it was not standard practice for wastewater treatments to be manned overnight, and the Moa Point plant is monitored remotely.
Wellington Water had previously said a duty manager was alerted to the incident following an alarm activation and Fire and Emergency New Zealand alert.
Asked whether this suggested the plant was not being adequately monitored, Barker said he was “not going to be drawn out into speculation about what people did or didn’t do or what should or shouldn’t have been”.
Raw sewage continues to be pumped out to the Cook Strait from the Moa Point plant’s long outfall pipe near the mouth of Lyall Bay following the total plant failure in early February.
The “catastrophic failure” happened while remedial work was being carried out, Wellington Water said at the time.
The report was not commissioned to identify the cause but had revealed the “valuable” information.
It is one of four reports commissioned by the agency into the failure, including a wider investigation, a damage assessment, and repair options, as well as the Crown Review,
“It’s natural to want a quick answer, but it’s important that we take the time to uncover all the facts and not draw conclusions too early,” Little said.
“It’s crucial we keep an open mind while the other reports and findings of the Crown Review Team are prepared. We’ll continue to be fully transparent with the public, and all reports will be released at the appropriate time.”
Ethan Manera is a Wellington-based journalist covering Wellington issues, local politics and business in the capital. He can be emailed at ethan.manera@nzme.co.nz.