Wellington Water regularly monitors water supplied from the Waiwhetu aquifer and the Waterloo plant. Photo: 123rf
Hundreds of Lower Hutt households are being supplied water that doesn't meet national standards.
Taumata Arowai increased the chlorine requirements for drinking water in November last year, but Wellington Water says it needs more time to upgrade its Waterloo Water Treatment Plant.
Wellington Water acting chief executive Tonia Haskell said the plant supplied most of Lower Hutt but only certain households in Epuni and Fairfield - up to 800 at any given time - would be affected, depending on supply and demand.
"It is important to note that the water being supplied from the Waterloo Water Treatment Plant is still safe to drink," she said.
"The plant was compliant under previous rules, and it is only recently when the rules changed, that it stopped meeting the chlorine rules."
Wellington Water was continuing regular monitoring of water from the bores supplied from the Waiwhetu aquifer and the Waterloo plant, Haskell said, with results indicating there were no E coli or other potentially disease-causing organisms.
Chlorine reduces the risk of treated water becoming contaminated as it flows from the treatment plant to households.
The changed rules require water suppliers to increase the Ct value, a calculation which takes into account how much chlorine is added and how long it spends in contact with the water.
To meet the new rules, Wellington Water would have to either significantly increase the concentration of chlorine added at the treatment plant or increase the time it was in contact with the water before it reached the first customers in Lower Hutt, Haskell said.
"Increasing the chlorine concentration to this level, which is around two-and-a-half times the current dose, could result in a significant change to the taste of the water or cause skin irritation. Alternatively, significant network upgrades and investment are needed to increase the contact time between chlorine and water," she said.
The agency was working with its council owners and the regulator, and had completed an independent technical study in the meantime, Haskell said.
"We have submitted this to Taumata Arowai and have asked them for an exemption until we can complete the upgrades needed to meet the new requirement. We are awaiting the regulator's decision."
Taumata Arowai confirmed it had received Wellington Water's exemption application, and said it was one of 14 applications from suppliers currently being processed - two other applications also related to chlorine requirements.
The rules had been changed to align with international best practice, the regulator said, but non-compliance with a single rule did not automatically mean the water was unsafe.
"The Rules set out a comprehensive set of measures that address the different areas of risk that can affect drinking water safety ... while there is a slight increase in the level of risk to the safety of the drinking water supplied through the Waterloo Water Treatment Plant where the Ct value is not currently being met, we consider this risk is manageable by the supplier while the exemption application is being processed."
Meanwhile, Hutt City Council strategic advisor Bruce Hodgins said the council was satisfied with Wellington Water's advice that the water was safe to drink.
"We note that an independent technical study commissioned by Wellington Water confirms that the current treatment process is effective."