From the drain to the sea: Whanganui's wastewater treatment explained

Senior stormwater engineer Tony Hooper spends most of his working life at the wastewater treatment plant. Photo / Bevan Conley.

What happens to wastewater after it leaves your home, goes through Whanganui's wastewater treatment plant and finally out to the Tasman Sea?

Laurel Stowell visited the year old $40 million plan to find out.

It's something most people don't give a second thought to, but Whanganui's senior wastewater engineer Tony Hooper would like more people to know about it.

Whanganui's wastewater treatment plant, costing $40 million and in full operation since early February this year, treats up to 35,000cu m of water a day.

About 50 to 60 per cent of the wastewater from the houses in Whanganui town, and about half of that is from toilets.

But the houses only contribute 20 to 30 per cent of the "organic load" the plant deals with - the substances that actually need treatment.

Most of the rest of comes from the big factories here that process animals into meat, leather, dried milk and petfood. The quantity from each is being constantly monitored.

The plant also treats five truckloads a day of leachate from Bonny Glen Landfill, near Marton. The content of that leachate is analysed every month.

"We get the lab results of what's in it, and there's nothing that's harmful," Hooper said.

Whatever its origin, all of the wastewater is first piped to Beach Road pumping station, where it is screened. A sieve-like screen with 1mm holes pulls out all the large objects, organic and inorganic, and they are trucked to Bonny Glen Landfill.

"When you see it, it just looks like a pile of sloppy mess, although the corn [kernels] do stick out," Hooper said.

Most of what's left is water, with perhaps 20 cubic metres a day of solids, all in very small particles.

After that it moves to an inlet at the plant, where it is circulated and heavy objects drop out. This removes more solids, mainly fine sand.

After drying and rolling the sludge is 10 per cent water in fine crunchy particles. Photo / Bevan Conley.

Then it's into the huge, covered anaerobic pond, where bacteria and other organisms begin their work of feeding on and converting the solids. The pond is big enough to hold extra water in abnormal situations - situations such as heavy rain.

After that it's into the smaller aerobic tank, where oxygen is blasted in to boost up the organisms' activity. They are a varied lot - various kinds of bacteria, protozoa, ciliates and others.

They are too small to be seen with the naked eye, but workers at the plant take water samples and can watch the little beasties moving around among the solid particles.

"The material coming out is food for them to flourish, so they can eat and multiply."

They multiply so much that they make up two thirds of the solids in the wastewater system.

Then our mix of water, waste and micro-organisms goes into a clarifying pond where the solids - sludge - sinks to the bottom and a "sludge train" starts. Some sludge, with its micro-organisms, is sucked out and moved to the anaerobic tank. Some will stay in the aerobic tank.

After drying and rolling the sludge is 10 per cent water in fine crunchy particles. Photo / Bevan Conley.

The rest is put through a dewatering process, which uses a centrifuge with two drums. The liquid spins out one way, and the sludge - with 25 per cent solids - goes another.

About five truckloads of it is trucked straight to Bonny Glen Landfill for disposal. At that stage it's black, from its time in the anaerobic tank, and spongy, with a texture like cow dung or playdough.

"It just looks like a handful of sand," Hooper said.

The rest continues into the plant's drier. There, on a conveyor belt and in temperatures of 95C to 130C it is dried to less than 10 per cent water and crushed by a roller. The result is a fine and crunchy mixture of small particles. Hooper compares it to rice bubbles.

Dried sludge is stored in a lined and unused pond. Photo / Bevan Conley.

It is all trucked to a lined and empty pond, a remnant from Whanganui's failed wastewater treatment plant which stopped operating in 2012 when the pond was already 3m deep in sludge. It will be stored there indefinitely, with each layer covered by a clean fill.

Water in that storage pond is pumped out, and treated at the plant. The pond is huge, but at its present filling rate it will be full in two to four years, Hooper said.

After that it will be capped and left.

It is the water spun off in the dewatering process that makes that final journey out into the sea. Some of it is used to cool the drier and the rest pours out of a race for one last treatment - ultraviolet light to kill any remaining micro-organisms. At that point it still looks slightly murky, with a little bit of scum floating on top.

Ultraviolet light kills organisms in the wastewater. Photo / Bevan Conley.

From there it is pumped to the ocean outfall 1.8km out into the Tasman Sea from South Beach.

If enough chromium from the tanning industry can be removed from the wastewater, the sludge could eventually be applied to land, or even sold as a fertiliser.

"Even if the chromium levels drop tomorrow, we will still fill up that pond. It's saving a transport cost, even if we could manage to dump it for free," Hooper said.

The plant has at least five people present every day, and one person on call, and the water keeps flowing in regardless.

The electricity to run it costs $500,000 to $600,000 a year. The drier uses much of it and runs for about 100 hours a week, usually with someone on hand to monitor it.

Within the plant people sitting at computer screens can see exactly what's happening at any time. It can also be monitored automatically, from a distance.