An investigation has found Waterloo Treatment Plant is compliant after a power cut stopped ultra-violet filters from working and partially untreated drinking water had to be drained from the network.
Water at the Lower Hutt plant is treated with a “double barrier” system where both chlorine and ultra-violet units are used.
But in November last year an electrical issue deactivated the ultra-violet units.
It meant water from supply bores was pumped through the units, which were not working, to the treatment plant storage reservoir.
Wellington Water’s drinking water chief adviser Laurence Edwards said operators immediately responded to alarms indicating there was a problem.
The reservoir was drained before restarting the plant, he said.
However, it’s possible that a small amount of drinking water treated with chlorine but not ultra-violet light was delivered to distribution, Edwards said.
Wellington Water advised Wai Comply, which acts as the drinking water assessor on behalf of the Ministry of Health, of the incident, as well as conducting its own investigation.
Wai Comply found the plant had breached water performance requirements, but was overall assessed as compliant due to the swift response and corrective actions undertaken by Wellington Water.
Edwards said since the incident a new interlock feature between the ultra-violet units and the supply bores has been installed to ensure water cannot be delivered to the treatment plant if the units are off.
Internal improvements have also been made to strengthen resiliency in power outages.
“We are also looking at improvements to the system that will allow us to restart the plant faster should another power loss incident occur”, Edwards said.
Wellington City councillors felt blindsided by the incident when they learnt about it through the council’s quarterly reporting in March this year.
Councillor Fleur Fitzsimons said at the time she was surprised to only be finding out about the incident, which she characterised as serious, months after the fact.
“That’s unusual and alarming that we don’t know.”
Fitzsimons questioned whether Wellington Water was treating the council with the respect it deserved given its fundamental role in the company.
“This doesn’t feel like it,” she said.
Wellington Water manages water assets for the Hutt, Porirua, Upper Hutt and Wellington City councils, South Wairarapa District Council and Greater Wellington Regional Council.
The Herald subsequently asked for all correspondence between Wellington Water and Wellington City Council about the incident under the Local Government Official Information and Meetings Act.
It was mentioned briefly in Wellington Water’s half-yearly report to the end of December 2020.
A final draft of that report was sent to officers from shareholder councils in February this year and put before the Wellington Water Committee in early March.
Apart from that, no other communication was revealed in the official information response leading up to the meeting later in March when councillors discussed the Quarter 2 report.
There was then a flurry of exchanges between the two parties about how to address the concerns of councillors and how to answer a media query from the Herald.
Wellington City Council’s Infrastructure Committee chairman Sean Rush said any unplanned outage was never ideal.
“But it’s good that our contingency plan kicked into action and mitigated what could have been a bigger problem.”
Also a member of the Wellington Water Committee, Rush said the issue was originally described to him as a glitch that had been handled.
But he acknowledged that when the issue came to the attention of city councillors it was in an unexpected way that overstated the gravity of what actually happened.
Rush said he was keen to “get under the hood” of water issues so that he and his councillors could be properly briefed on any issues.
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