The solution

A new sludge minimisation plant

We have 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. We then worked with representatives from Council, mana whenua, and local and international technical experts to identify the preferred option.

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


Concept drawing

This option:

  • Reduces the 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

Why at Moa Point?

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 this will not be a nuisance to locals.

Why 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.