The Sludge Project

Wastewater composting trial

Carterton District Council is trialling a new, environmentally responsible method for managing wastewater sludge.

This project transforms sludge from one of our sewerage ponds into a safe, nutrient-rich compost that can be used to improve soil health and support land rehabilitation.

What is sludge?

Sludge is the semi-solid material left over after wastewater treatment. Traditionally, it’s disposed of in landfills or requires complex processing. Our approach uses lime pads to naturally compost the sludge, turning it into a usable product without the need for a resource consent.

How this composting process works

Lime stabilisation: The sludge is mixed with lime, which raises its pH and temperature. This kills harmful pathogens and stabilises the material.

Natural composting: The treated sludge is laid out on lime pads and allowed to compost over time. Microorganisms break down the organic matter, transforming it into a soil-like material.

Safe reuse: Once fully composted, the material is suitable for use in land rehabilitation and other non-food applications. It meets environmental safety standards and supports sustainable land management.

Environmental benefits

Reduces landfill waste
Minimises greenhouse gas emissions
Produces a valuable resource for land improvement
Avoids the need for resource consent due to its non-food application

Global inspiration

Similar projects around the world have successfully reused treated sludge for land restoration, agriculture (non-food), and energy production. Our initiative builds on these proven methods while tailoring the process to local needs and environmental conditions.