The first report into the failure of the Moa Point wastewater plant has highlighted a possible design flaw in a bypass pipe that could have trapped air that then blocked an outflow pipe.
Untreated wastewater began discharging from a short pipe near Moa Point on February 4, after the long outfall pipe blocked up and flooded the plant itself.
The failure came after heavy rain and forced the discharge of up to 70 million litres of untreated wastewater in the sea along the South Coast.
The plant, run by Wellington Water, is still shut but screened untreated wastewater is now discharging from the long outfall, about 1.8 km into Cook Strait.
Wellington City Council today released the first in what it says will be a series of reports by Wellington Water into the incident.
“What happened at Moa Point is unacceptable. The bottom line is transparency because Wellingtonians deserve to know why the plant failed, and to have assurance about the response and recovery,” Mayor Andrew Little said.
The report from external consultants focussed on hydraulic modelling and the design of the outfall pipe that normally carries treated wastewater out into Cook Strait.
The council said the report was not done to identify the cause, but it has helped understand what appears to be a key factor.
It found air can get trapped in the wastewater bypass pipeline when there are heavy flows during rainy weather. This would stop the flow of wastewater, pushing it back up the pipe 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, and Wellington Water has commissioned an external specialist to conduct a wider investigation into these other factors." Little said.
“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. Wellingtonians deserve assurance that our recovery addresses the root cause and any related factors, so we can be confident we’re preventing this from happening again."
The mayor described the water as “refreshing and bracing” as he urged others to go for a dip. (Source: 1News)
All beaches in the area were closed for weeks after the plant failure. The public were told in late February that it was safe to returnto the beach and swim, subject to updates on the LAWA website.
Tarakena Bay remains shut, due to its proximity to Moa Point.
An independent Crown review into the failure began earlier this month and is due to release a final report in August.
Wellington Water's board chair Nick Leggett quit over the disaster.
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