Moa Point sludge minimisation facility

In the 2021 Long-term Plan, we agreed to invest in alternate sewage sludge disposal technologies to break the link between the landfill and sewage. We need to do this to achieve our waste and carbon reduction targets. A solution is expected to be in place by 2026.

Moa Point treatment plant in Miramar, viewed from the hillside above.

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 the business case has been approved. 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. Together with 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 some 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.

For appropriate landfill management (including pathogen control) the sludge must be mixed with "clean" solid general waste for disposal in the landfill. That mix ratio is no less than 4:1 general waste to sludge and is a condition of the current landfill consent.

Current issues

  • The need to mix the sludge with solid waste at Southern Landfill makes it difficult to operate the landfill. It also limits the Council’s ability to achieve its waste minimisation and carbon emissions reduction targets.
  • 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 considered 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.

We have been actively engaging with the local community since late 2020. 

In April 2022, we sought ratepayer feedback on the use of the Infrastructure Funding and Finance Act (IFFA) to finance the development of the new facility.

The business case, including the selection of the preferred option, was approved by the Council in June 2022.

From mid-August to late-September 2022, submissions were open on the Notice of Requirement for an alteration to Designation 58 (WCC6 under Proposed District Plan) for the construction, operation and maintenance of a new Sludge Minimisation Facility (SMF) at 127 and 141 Stewart Duff Drive.  Submissions  are currently being reviewed, and if required, oral hearings are likely to take place in November 2022.

Our preferred option 

Our preferred option is a process facility that includes thermal hydrolysis, anaerobic digestion, dewatering and thermal drying. 

Key steps in the proposed Wellington Sludge Minimisation Facility
Key steps in the proposed Wellington Sludge Minimisation Facility

This option:

  • Reduces sludge volumes by 82%.
  • Reduces carbon emissions from the treatment and processing process by 63%.
  • Produces a low odour, stabilised (inert) product.
  • Allows treated sludge to be stored and/or transported to other sites for disposal.
  • Offers the potential for sludge to be used productively, including as a soil conditioner, fertiliser, and fuel for industrial heat.
An example of dried sludge like the product from the proposed Wellington Sludge Minimisation Facility.
An example of dried sludge like the product from the proposed Wellington Sludge Minimisation Facility

Location

Locating the facility at Moa Point means we will no longer need to pump sludge through pressurised pipelines to the landfill. 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.

Early concept of the proposed Wellington Sludge Minimisation Facility.
Early concept of the proposed Wellington Sludge Minimisation Facility. For clarity, the walls of the buildings have not been shown in this illustration.

About thermal hydrolysis and digestion

The heart of the new Wellington Sludge Minimisation Facility is the anaerobic digestion process, which mimics are natural decomposition process of waste, breaking down biodegradable matter in the sludge. As the sludge breaks down, it produces biogas which is captured and used to produce heat and electricity, which helps run the process.

To make the anaerobic digestion process even more efficient and harness more energy from the sludge as it breaks down, the sludge is first put through thermal hydrolysis. This acts like a pressure cooker on the sludge, breaking down the sludge molecules so that the sludge becomes easier to digest in the anaerobic digesters. It also makes the sludge easier to dewater and dry. 

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.

Next steps

The project will return to Council in December 2022 for final funding approval.

Physical site works are expected to be underway early in 2023.

Media releases

News - Moa Point Notice of Requirement for alteration to the Designation open for feedback 23.08.22
News - Business case approved for Wellington’s new sludge minimisation facility 30.06.22
News - Feedback sought on rates levy for Wellington’s new sludge minimisation facility 11.04.22