News | 5 April 2023

What the fudge is sludge?

Ever wondered what happens after you flush the toilet? Much of what ends up at Wellinton city’s wastewater treatment facility at Moa Point is turned into a substance called sludge.

Sludge getting tipped out at the landfill.

We’re changing the way we deal with sludge, creating a solution that’s better for the city’s residents, the environment, and our waste management goals. Read on to learn more about sludge. 

1. Sludge is a by-product that comes from wastewater treatment. Untreated it contains microbiological material that needs to be disposed of in a well-managed way to keep people and the environment safe. Together with its high moisture content it is not an easy material to dispose of and can create a bad smell if not treated properly. 

 2. Currently, more than a million litres of sludge per day is piped 9km from the wastewater treatment plant at Moa Point to Carey’s Gully sludge dewatering plant at the Southern Landfill. The dewatering plant at the landfill removes most of the water, leaving 40-50 tonnes to be buried with landfill waste each day. 

 3. In 2020 the pipes broke – and what became known as ‘poo trucks’ had to shift a million litres of sludge from Moa Point to the landfill – taking 150 return trips every day.   

 4. The current landfill consent dictates that dewatered sludge is mixed with waste in the landfill, 1 part sludge to 4 parts waste – limiting the council’s ability to reduce waste to landfill.   

 5. We’ve got a new solution in the pipeline (that doesn’t need the pipeline) – at the end of 2022 Council agreed to a business case to build a Sludge Minimisation Facility.  

Sludge minimisation facility Moa Point.

6. Before deciding on the Sludge Minimisation Facility 16 other options were considered. After a process of cost-to-benefit analysis, and assessment for community and business needs, and mana whenua values, this was selected as the best fit for Wellington. 

7. The new facility will reduce the amount of treated sludge produced by up to 60 percent, or the equivalent of 11 full Olympic-sized swimming pools, per year.   

8. It will reduce carbon emissions created by the treatment and processing process by up to 60 percent.  

9. Sludge will be treated first by thermal hydrolysis – which acts like a pressure cooker on the sludge, breaking down its molecules so it’s easier to digest in the anaerobic digester.  

10. What’s an anaerobic digester? It’s a sludge treatment process which mimics the natural decomposition process of waste, breaking down biodegradable matter in the sludge. 

11. The sludge will help power all this treatment – as it breaks down it will produce a biogas that is captured and used to create heat and electricity to help run the process.  

12. This process creates what’s known as a Grade A biosolids product, which is pasteurised (so safe from a germ and disease perspective) and odourless. 

13. That new product will have the potential to be used productively, including as a soil conditioner, fertiliser, and fuel for industrial heat. 

14. If we don’t provide a sludge treatment solution for Wellington, we’ll have to truck raw sludge out of the city when the current Southern Landfill consent expires in 2026. 

Site works to make it possible to build our new facility at Moa Point are due to start soon, keep an eye on our channels – we’ll make sure you’re updated.