Whanganui water supply’s ultraviolet treatment plant will cost $1.5 million

Groundwater sources must have fully operational protozoa barriers by the end of 2025. Photo / 123rf

Designing and constructing an ultraviolet disinfection plant for Whanganui’s water supply will cost the district council $1.5 million over the next two years.

There’s no getting around it, however, with timeframes set by new national water regulator Taumata Arowai fast approaching.

The council must have a funding plan for protozoa barriers in place by next June and all designated groundwater sources must have fully operational protozoa barriers by the end of 2025.

Protozoa refers to a group of micro-organisms that include cryptosporidium and giardia.

Ultraviolet (UV) disinfection kills microorganisms by altering their DNA and impeding reproduction.

Once completed, the plant will provide UV for the Whanganui, Fordell and Pākaraka water supplies, all of which source water from underground aquifers.

Speaking at a council meeting last week, council senior water engineer Dave Rudolph said Whanganui’s supply came from six bores, with one each for the other two supplies.

Whanganui serves 44,000 people, Fordell serves 350 and Pākaraka serves 260.

He said the council was currently meeting bacterial compliance but there was “a fine line” with protozoa.

All the district’s bores except number three at Kai Iwi were currently classed as interim class one and didn’t require UV treatment.

However, some could soon become interim class two and that meant it would be mandatory.

“We are looking at where best we position this UV - whether it’s at the current source supply or whether it’s at the reservoirs,” Rudolph said.

“Scoping and planning have already started and we’re looking at sizing and units.”

Whanganui District Council senior water engineer Dave Rudolph. Photo / Bevan Conley

Queenstown and Frankton residents had to boil water from September 18 until December 7 following confirmed cases of illness caused by cryptosporidium.

As a result, Queenstown Lakes District Council was ordered to upgrade its water treatment plant.

“Even before Queenstown happened, we were looking at UV treatment. We knew we couldn’t stay in [interim class one] because it was so work-intensive,” Rudolph said.

A report from water process engineer Julianne Martin said four out of the six Whanganui bores currently required daily sampling to remain in that class, which represented “a significant workload and expense”.

According to a business case on the project, designing the plant will cost $400,000 in the 2024/25 financial year, with construction costing $1.1m in 2025/26.

From then on, operations will cost $100,000 per year.

In October, Taumata Arowai wrote to 27 councils that had water supplies without protozoa barriers, including the Ruapehu District Council.

Speaking to the Chronicle, Rudolph said UV equipment could be in hot demand as a result.

He said UV treatment was completed using a pressurised capsule.

“The capsule contains multiple lamps and water flows through the light.

“It doesn’t need much contact time. The light gives the water an instant barrier for viruses.”

Martin’s report said the number three bore at Kai Iwi was shallow and there were ongoing issues relating to consent.

As a result, the council’s infrastructure team decided to only use it in an emergency.

If it was used, a boil water notice could be issued to all of Whanganui.

Mike Tweed is an assistant news director and multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.