The sludge issue
What is sludge?
Sludge is the term for the solids produced from the wastewater treatment process. Untreated it contains microbiological contaminants that need to be disposed of in a well-managed way. Due to its high moisture content it is not an easy material to dispose of and if not treated appropriately creates odour.
How do we currently manage sludge?
More than a million litres of sludge per day is usually piped 9km from the wastewater treatment plant at Moa Point to Carey’s Gully sludge dewatering plant at the Southern Landfill. Once most of the water is removed, about 45 tonnes of solids per day are buried in the landfill and the water returned to Moa Point via the wastewater network.
The sludge must be mixed with solid general waste for disposal in the landfill. The mix ratio is no less than 4:1 general waste to sludge; a condition of the current landfill consent.
Existing sludge disposal methods work against Wellington City Council's waste minimisation and carbon emissions reduction strategies. They are also a risk to the sustainability of landfill operations.
- The Council’s Waste Minimisation Strategy is forecast to reduce volumes of solid waste at the same time as volumes of sludge are increasing with population growth.
- The consent to dispose of waste at the Southern Landfill is due for renewal in 2023 and a new sludge management solution is required to support the re-consenting.
- The disposal of sludge to landfill creates methane emissions which contribute to climate change.
- We also need to consider the vulnerability of the sludge pipelines that currently carry sludge from Moa Point to Carey's Gully. The pipes failed in January 2020 resulting in a sludge trucking operation over several months until the pipes were fixed.
What are the options?
We need to create a new solution for minimising and managing sludge so Wellington can get on with reducing our solid waste to landfill, as well as achieving these critical objectives:
- Remove the landfill from future disposal
- Reduce the carbon emissions from sludge processing and disposal
- Minimise odour generation
- Increase the resilience of the overall sludge management process
- Align with mana whenua values.