Cost of treating drinking water rises to $11m

‘‘The costs associated with the diversion of the 'pump start-up water' escalated quickly.’’

The cost of treating Lower Hutt’s contaminated water supply has escalated to $11 million, with a 1.5-kilometre pipeline needed to divert water away from the Waterloo treatment plant.

E-coli had been found in water drawn from the Waiwhetu Aquifer, which goes to the plant, on three occasions since late last year, and an increase in coliform bacteria was also detected.

With an investigation unable to determine the cause of the problem, Greater Wellington Regional Council agreed to continue permanent chlorination and ultra-violet (UV) treatment of the water, at an up-front cost of $4.6m.

However, further investigation has revealed the amount of ‘‘misty’’ water not suitable for UV treatment is higher than first thought, and that water will have to be shifted elsewhere via a pipeline.

Misty water, which is generated by bore pumps when they first start up, appears the same as clear water, but contains particles which can mask potentially harmful organisms during treatment.

Wellington Water, which carried out the investigations, had hoped the water could be diverted to the nearby Opahu stream or stored at the treatment plant until it ran clear, but it underestimated the volume of water it had to shift.

‘‘As the programme of work has progressed, the costs associated with the diversion of the ‘pump start-up water’ escalated quickly,’’ the report said.

‘‘Multiple approaches to managing the significant volume of water were considered during the design phase.

‘‘However, the only viable option involves the construction of a 1.5km pipeline from the bore field to Te Awa Kairangi/Hutt River.’’

The Waterloo plant supplies water to up to 240,000 people across Wellington and Lower Hutt, Wellington Water group manager for network, strategy and planning Mark Kinvig said.

Four bore pumps in the Waterloo wellfield, along Knights Rd, are still out of action due to the water they pump being contaminated, and it is hoped they will be operating again in time to meet summer demand.

The first of two UV units needed to return to full operation was installed earlier this month, with the second to begin operating once the pipeline is constructed, hopefully by January.

Of the additional $6.4m required for the pipeline, $4.2m will be taken from the council’s planned 2017-18 spending, including $2.7m from a resilience project aimed at ensuring the Wellington region has dual water supplies in an emergency.