* This story has been udpated to clarify the total cubic metres of sewage that flowed into the Mahurangi River
A critical overflow alarm that was not working and a faulty surge protection component in a North Auckland wastewater pump station were factors that led to what is believed to be the biggest wastewater spill into the Mahurangi River that happened in October last year, according to independent reviews.
Wastewater poured into the Mahurangi River from about 2.30pm on 29 October, after a power surge at Watercare’s Warkworth Street wastewater pump station, and subsequent pump failures.
Watercare estimated that there was 1200 cubic metres of overflow, which was not discovered and stopped until 8am the next day.
It said 600 cubic metres of the overflow went into the Mahurangi River, and the rest was contained to storage tanks on the wastewater treatment plant.
Mahurangi oyster farmers were furious with the spill and contamination, with one reporting that much of their 80,000 oysters had to be dumped, at the peak of the oyster season.
On Thursday, Watercare confirmed that independent reviews into the cause of the overflow had been completed.
“The reviews found the incident resulted from a unique chain of events involving technical and operational factors at the newly commissioned Warkworth Street Pump Station, triggered by a power outage,” said Watercare in a press release.
Watercare’s chief operations officer Mark Bourne told RNZ the reviews found that a faulty safety relay – which was part of the plant’s surge protection system – was in conflict with other safety relays, and prevented the pumps from restarting following the power surge.
He said the treatment plant was designed in a way that its protection system would kick in when individual components failed.
Bourne said if the safety relay was not faulty, the pumps would have restarted following the brief power surge – which lasted about half a second.
The most critical of alarms – the overflow alarm – had also failed to operate, said Bourne.
“That was tested during the pre-commissioning phase before the pump station went into operation but for whatever reason, failed during that operation phase,” said Bourne.
Bourne said the Warkworth Street plant was new, and was going through its commissioning and start up phase when the October event happened.
He said prior to commissioning, Watercare had run tests on the station with clean water, and added that the plant could not be tested across its full ranges until it was in service.
“We try and simulate all activities during the initial testing process, but as I indicated before this was a very unique set of circumstances,” said Bourne.
Bourne said the faulty safety relay had since been fixed and all alarms had been tested and checked.
Another Mahurangi spill raises serious questions for Watercare
When asked if he was confident that station could withstand future possible power surges, Bourne said he had “absolute confidence” in the Warkworth Street pump station.
“This event is just so unfortunate and quite frankly we’re gutted that it occurred,” he said.
Asked where responsibility could be traced to with the multiple failures of the treatment plant, Bourne said Watercare took accountability.
“It’s our pump station, it’s our operation, if there’s further conversations to be had, they’ll be for us to have at a later date, but at this point Watercare takes responsibility for the fault that occurred,” he said.
He added that this was a unique circumstance leading up to the failure of the station, and that there was no systemic issue.
Bourne said Watercare had over 550 pump stations across Auckland.
On Thursday, Watercare announced its final tranche of compensation payment for impacted oyster farmers, bringing the total payout to $2.75 million.
This article was originally published on rnz.co.nz, written by ,
