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Wellington sewage could be the future for powering homes and heating the airport

The Moa Point treatment plant is set to undergo a complete overhaul, with a new system for reducing sludge into fertiliser and fuel planned for the capital.

Kevin Stent/Stuff

The Moa Point treatment plant is set to undergo a complete overhaul, with a new system for reducing sludge into fertiliser and fuel planned for the capital.

As the capital’s sewage system faces a total overhaul, one solution could see homes powered by poo.

The council’s preferred option to replace an increasingly risky pipe system running under the city is a thermal hydrolysis and digestion plant at Moa Point.

It would reduce sludge volumes by 82 per cent – a necessity as the Southern Landfill nears capacity – and lower carbon emissions by 63 per cent.

The process mimics the natural decomposition process of waste, reducing sewage to a product resembling potting soil. Anaerobic digestion produces biogas; it is the council’s intention to use this to generate electricity.

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It would be similar to the process at the Southern Landfill, where methane is run through a generator, meant to make enough electricity for 1000 homes – although the generator has recently been out of action.

The sludge goes into the thermal hydrolysis tank, which is like pressure cooker, breaking down the sludge at a cellular level to allow the next step to get the most biogas (methane) possible.

Supplied

The sludge goes into the thermal hydrolysis tank, which is like pressure cooker, breaking down the sludge at a cellular level to allow the next step to get the most biogas (methane) possible.

Councillor Laurie Foon said the overhaul created the opportunity to implement a “low carbon, circular option” – something for which she had been advocating for some time.

Councillor Sean Rush said with New Zealand’s electricity mix still underpinned by coal, this system reduced the country’s carbon footprint.

The project’s technical director, Chris French, said the aim of the system was to be as self-sufficient as possible. Heat from the digestion tank would be used to heat the thermal hydrolysis chamber, with any excess put into electricity generation, which would likely be returned to the grid.

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“We estimate that up to 500kW of electricity could be generated from the new plant, depending on how we configure the process.”

There was the potential for some of this heat to be transferred to the plant’s neighbour, the airport, to heat the terminal.

The resulting pellets of fertiliser would be used for non-food crops, or fuel. Construction is expected to start in the first or second quarter of 2023, and take three years to complete.

This is what the sewage waste comes out as at the end of the process of thermal hydrolysis and thermal drying. This particular sample came from New Plymouth.

Kate Green/Stuff

This is what the sewage waste comes out as at the end of the process of thermal hydrolysis and thermal drying. This particular sample came from New Plymouth.

There are more than 150 plants of this kind worldwide, but none in New Zealand. This would be the third in Australasia, and Auckland had plans to build one too.

Covid-19 had normalised working remotely, which had opened up opportunities for this project. A local engineering team was in daily contact with experts in Washington DC, allowing the council to “train up the local supply chain”, French said.

This plant would be constrained by the size of the site, with around 6000 square metres set to be purchased from the airport. Its proximity to the airport itself set some unique constraints.

The pipeline under Wellington will be decommissioned if this project goes ahead, with no need to transport sludge across the city to the landfill.

Supplied/WCC

The pipeline under Wellington will be decommissioned if this project goes ahead, with no need to transport sludge across the city to the landfill.

Whereas other plants around the world had large silver tanks outdoors, Wellington’s would be inside a building. This prevented the sun reflecting off the tanks and affecting the vision of inbound pilots.

Heat plumes distorting the air, along with steam, would be minimised. “None of these are insurmountable problems by any means,” French said.

“The wastewater treatment plants produce around 10,000 cubic metres of sludge per week, which we need to process. In 50 years’ time, we expect this to grow to around 17,500 cubic metres per week,” French said.



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