Moa Point sludge minimisation facility

About the project

• The purpose of this project is to create a new solution for minimising and managing sludge so we can reduce the amount of waste going to landfill.
• The project is already underway. A preferred option has been identified, and we are now starting to engage with local communities. A solution is expected to be in place by 2026.
• This project ties to Te Atakura: First to Zero plan and our Regional Waste Minimisation and Management Plan.

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 can create odour if not treated appropriately.

How sludge is currently managed

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.

Current issues

Existing sludge disposal methods work against our waste minimisation and carbon emissions reduction strategies. They are also a risk to the sustainability of landfill operations.

• In our current Waste Management and Minimisation Plan (WMMP) we have committed to reducing the volume of waste going to landfill by one third, yet volumes of sludge are increasing with population growth.
• The existing consents at the Southern Landfill are due to expire in April 2026, and a new sludge management solution is required to support the consenting of future waste disposal options.
• 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.

Objectives

We need to create a new solution for minimising and managing sludge which will:

• Reduce the amount of solid waste going to landfill.
• Break the link between sludge and the landfill.
• 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.

What’s happened so far

Wellington Water has been through a process of identifying a range of viable options for a new sludge minimisation plant, to be located at either Carey's Gully or at Moa Point. They then worked with representatives from Wellington City Council, mana whenua, and local and international technical experts to identify a preferred option.

Our preferred option 

Our preferred option is a thermal hydrolysis and digestion plant with a thermal dryer to be located at Moa Point. 

This option:

• Reduces sludge volumes by 82%.
• Reduces carbon emissions from the treatment and processing process by 63%.
• Produces a non-odorous, stabilised (inert) product which is acceptable to mana whenua.
• Allows treated sludge to be stored and/or transported to other sites for disposal.
• Offers the potential for sludge to be used productively.

Location

Locating the facility at Moa Point means we would no longer need to pump sludge through pressurised pipelines to the landfill. Ultimately locating the facility at Moa Point reduces the overall complexity and risk of the sludge management process.

The option to locate it here has also been made possible because Wellington International Airport Limited has land available and odour reduction technology has now advanced to the point where it won’t be a nuisance to the local community.

About thermal hydrolysis and digestion

This process breaks down large molecules by heat (thermal hydrolysis) followed by anaerobic digestion, reducing the total mass of sludge into a stable product which is easier to dewater or dry. 

The process mimics the natural decomposition process of waste, resulting in a product that is inoffensive and non-odorous with the look and characteristics of a rich potting soil. It also produces biogas which can be captured for use as fuel for the thermal dryer.

Additionally, because the product is inert, it can be easily stored or transported to other locations for disposal or potentially used productively for other purposes.