22 Oct 2020

Wellington council considers significantly higher sludge reduction investment

12:51 pm on 22 October 2020

Wellington City Council is considering investing $185 million to reduce the amount of sludge the city produces every day.

Sludge being pumped into trucks.

Sludge being pumped into trucks. Photo: RNZ / Jonathan Mitchell

The money would fund a thermal hydrolysis and digestion plant with a thermal dryer located at Moa Point.

It's reported such a development would reduce sludge volumes by more than 82 percent, and carbon emissions by 63 percent.

But it would come at a cost, and a significantly higher one than had first been expected.

In the Council's 2018 Long-Term Plan, just $30 million had been budgeted to minimise sludge - a sixth of the cost being considered now.

At a meeting this morning, councillors agreed to undertake further work into the viability of the project. A more detailed report is expected later this year.

What is the problem with sludge?

Sludge is a by-product of the wastewater treatment process. The Moa Point and Western Waste Water Treatment Plants process nearly 400,000 tonnes of it each year.

The disposal of the sludge to landfill creates methane emissions which contribute to climate change, and its high moisture content means it's not an easy material to dispose of and can be incredibly bad smelling if not treated.

To treat it properly, it is pumped to the Southern Landfill where it must be mixed with a certain amount of solid waste.

The city is in a bind - the landfill's conditions mean they have to mix the sludge with the waste at a ratio of 4:1 general waste to sludge.

As the city's population increases, so too will the amount of sludge produced (more people equals more waste water equals more sludge). Meanwhile, the council is trying to reduce the amount of household waste being sent to the landfill.

The ratio is feared to be unsustainable in the long term as sludge goes up and general waste goes down.

"Working toward a waste-free Wellington is a small passion of mine," said councillor Laurie Foon, "but since I have been on the council, the way we treat our sludge is the reason that I, and many other waste-free advocates are being told we can't go on that journey right now."

The need to do something quickly was pressed home this year, one of the pipes which transported the sludge from the treatment plant at Moa Point to the landfill failed.

The short-term solution was to have to truck the sludge to the landfill directly.

The fix was to bring in engineers from Germany who repaired the broken pipe, but at considerable cost.

A 21st Century solution to a stinky problem

In total, 16 options were considered by Wellington Water - the regional authority which manages Wellington City's water network.

The options varied from a lower budget of $86 million which would fund a thermal dryer only, up to a possible $363 million wet air oxidation option.

The recommended option is a $185 million project, which would see the creation of a thermal hydrolysis and digestion plant with a thermal dryer.

While it wouldn't remove sludge completely, it would reduce it by more than 80 percent. It would also reduce the carbon footprint currently caused by the sludge being sent to the Landfill.

Councillor Sean Rush described it as a "twenty-first century solution, a sustainable solution, a low-emitting solution, and it does provide the potential to recover the resource".

Concerns were raised over the cost of the new plant, however.

"I am concerned about the project management of it, given the experiences of cost blowouts on Omāroro Reservoir," said Councillor Fleur Fitzsimsons, "and I will as this project develops, ask for a require really detailed commitments to containing costs, and risk management.

"And I do maintain concerns around the contracting out model of core infrastructure which I don't think serves our city well."

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