Wellington City Council has released the first of a series of reports commissioned by Wellington Water related to the failure of the Moa Point wastewater treatment plant on 4 February.
As part of its emergency response, Wellington Water commissioned the report on 5 February to help increase the flow of wastewater to the 1.8km long outfall pipe.
The report found that air can become trapped in the wastewater bypass pipeline, particularly when there is a high flow of wastewater passing through the plant. Trapped air could possibly disrupt the flow of wastewater, pushing it backwards and flooding the plant.
“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,” said Wellington Mayor, Andrew Little.
“This is a complex situation and it’s appropriate Wellington Water reached out for external expertise. Today’s report is one of the key reports Wellington Water have requested to date, and more may be required. The knowledge we get from these reports will be supplemented by the work of an independent Crown Review Team.”
The report developed a hydraulic model of:
• the effluent pipelines, which carry wastewater out of the plant
• the bypass system, which allows screened wastewater to bypass further treatment (e.g. disinfection) and be discharged out the long outfall pipe when the plant is at maximum capacity, and
• the long outfall pipe.
It was commissioned to help Wellington Water understand:
• how the bypass system could be operated during the recovery phase, and
• the hydraulic constraints on the system once the plant’s treatment process is brought back online, while the UV upgrades are underway.
While the hydraulic report was not commissioned to identify the cause, the Council says it has revealed some valuable information, which will help authorities to understand what appears to be a significant factor at the plant during the flooding event.
“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,” said Mayor Little.
“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.”
He said the hydraulic report will be a key input into the wider investigations.
“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.”
Read the full report here.
