The debate about affordability might still be ongoing but major works are well under way with the rebuild of Whanganui's wastewater treatment plant next to the city's airport.

The previous council signed off on a $41 million rebuild of the plant in August after the original plant failed and was shut down in 2012.

The contract with Hawkins Infrastructure was signed in September and machinery has been on site for a few weeks now.

Australian Brian Walker is overseeing the rebuild for Hawkins and he told the Chronicle that most of the earthworks will be done by Christmas or soon after, with structural work starting in early in the New Year.


Mr Walker specialises in wastewater treatment projects and has worked on projects like Whanganui's for 40 years.

He said the new plant contained more conventional treatment processes than the original plant which was beset with problems virtually from the time it was commissioned in 2007. Odour problems from the plant saw council shut it down five years later.

The redesign reduces the size of the original main pond by about two-thirds and machinery is currently working inside that pond to create an earth wall to enclose that first pond. Domestic and trade waste will enter that pond through the existing inlet.

"At the moment we're reshaping the main pond which will be covered over once its operating. It will be fully covered and any odours coming off that anaerobic pond will be burned off," Mr Walker said.

The anaerobically-treated waste from that 10m deep pond will then be transferred to a rectangular stabilising pond where oxygen will be pumped into the through large blowers in a building next to the pond.

"The oxygen is pumped to the bottom of that concrete stabilisation tank to allow air to bubble up through it. It's almost what a river does, cleaning itself up going through rapids," he said.

From there the waste is fed into two smaller round settlement tanks.

"The clear and treated liquid from that stage goes through ultra-violet light treatment and then out through the South Beach ocean outfall.

"The solids from the clarifying ponds are extracted and then de-watered. The sludge is then put through a centrifuge because it's all about reducing the moisture content. The final stage is putting it through a dryer. It comes out as a dried product that easily disposed of," Mr Walker said.

He said the new treatment plant was a "robust and simple system" but one that he had overseen the construction of before."

Other earthworks at the site will see the second pond used in the initial plant filled in. He said they were re-using as much of the old infrastructure as they could to keep costs down and while not putting a figure on it said considerable savings had been identified.

Mr Walker said the contract also involved an exacting environmental monitoring process and staff from Horizons Regional Council are visiting the site once every month to carry out tests.

The new plant is scheduled to be operating in early 2019.