State of the art plant to solve Wellington’s stinky sludge problem

Wednesday, 26 June 2024

Wellington is spending half-a-billion dollars to solve a stinky problem most residents haven’t even thought about: sewage sludge.

Wellington has a stinky problem that saw poo trucks have to carry untreated sewage across the city – but the city council is building a processing plant to avoid a repeat.

Despite the rain, the construction site out at Moa Point was all go when The Post visited on Tuesday, a hive of ladders and cranes working towards the city’s new sludge minimisation plant.

The main pieces of construction are the plant, which will dehydrate sewage sludge, and the digesters – two giant tanks which will be filled with bacteria to digest the sewage. Together the two parts of the plant will transform poo into an inoffensive, almost-odourless pile of pellets which can be reused in gardens.

The plant, which costs upwards of $400m, would be funded through a new levy added on top of the usual rates bill, using the Infrastructure Funding and Financing Act.

At the moment, the whole wastewater system relied on one pipeline covering the 9km between the Southern Landfill and the Moa Point Treatment Plant.

Eventually the digesters will be about the same height as a four-storey building.

Because of its age and the poor state of the city’s sewerage network, that pipe burst in 2020, meaning ‘turd taxis’ had to truck the sludge between the two locations.

“It was incredibly unpleasant for of us in Wellington at the time. We had to try and truck sludge across in what became known by the community as the ‘poo taxis’. There were lots of them every day. It was expensive. It was messy,” said council project manager Janet Molyneux.

But that pipe burst was hopefully the most residents would ever see of the sewage sludge.

“We want to put an alternative in place before an event like that can happen again,” Molyneux said.

Who could forget the turd taxis of 2020? (File photo)

The weakness in that critical pipe was not the only problem. The current process used by cities across New Zealand was described as “outdated” by Molyneux. In Wellington it required four tonnes of rubbish to be mixed with every tonne of sludge.

The sludge had to be kept in a liquid form so it could be piped to the Southern Landfill, where it was dehydrated and mixed with rubbish.

The problem was that Wellington’s rubbish volumes were slowly reducing as people shifted to reusable products and recycling.

Pretty soon, this would have led to a situation where Wellington had to import rubbish from other cities to mix with the sludge, said Molyneux.

The new plant, which used technology that has been successful across Europe and Asia, was a better solution, said Peter Hodgson from HEB Construction.

Council project director Janet Molyneux says the plant will be a gamechanger for the city.

The thermal hydrolysis process was the first project of its kind in New Zealand. But it was likely other cities would follow suit, with other water organisations including Auckland’s Watercare looking into the technology.

“All treatment plants around the country, around the world have the same issue. What do you do with wastewater sludges?” said Hodgson.

The sludge minimisation plant would take wastewater from the treatment plant, and heat it to dehydrate it before feeding it to the bacteria.

In a few years this digester tank will be filled with poo-eating bacteria.

It makes something like a “baby food” for the bacteria that digest the sludge, allowing them to break it down quickly and easily. A by-product of the anaerobic digestion process was natural gas, which would be transformed back into power to keep the plant running.

At the end of the process was a dry, innocuous product. A sample of the biosolids provided by the council rattled around in its plastic container, with a texture similar to cat biscuits and a “musty” smell similar to a dried fertiliser.

The dehydrated sludge will be turned into pellets like these ones, which have almost no odour and are hard like cat biscuits.

“What we will produce at the plant is something that has no odour, and it's a pellet that you will quite happily holding your hand. It looks like dried dirt and it's quite inoffensive,” said Hodgson.

The pellets were much smaller in size and importantly, less sewage-like than the so-called sludge “cake” which was sent to the landfill under the status quo.

The city council was investigating how these biosolids, produced by the plant, could be reused – most likely in gardening or to be burned as fuel.

The construction of the plant was about one-third complete and “tracking on time”, said construction manager Richard Atkin.

The sludge minimisation facility, named Te Whare Wai Para Nuku, was scheduled to be up and running by the end of 2026.