Reusing biosolids is one of the ways we are responding to the impacts of climate change in Western Australia. Biosolids are the organic sludge from domestic and industrial wastewater that has undergone treatment to significantly reduce pathogens and volatile organic matter so they can be used as natural fertilisers and soil improvers. In WA, we use biosolids to grow broad-acre crops such as wheat, oat, canola, lupins and for forestry applications.

As we continue to deliver on our commitment to sustainability, waste reduction and a circular economy in which human health and the environment are protected we aim to recover 100% of this valuable resource from Perth metro and 75% from regional areas. 

Biosolid fertlisers being used on Canola fields

How do we produce biosolids?

We treat 80% of WA’s wastewater at our 3 largest wastewater treatment plants – Woodman Point, Beenyup and Subiaco.

We separate the liquid and solids during the wastewater treatment process. After treatment, the liquid is released via ocean outfalls, or recycled for groundwater replenishment, irrigation of public open space or industrial uses.

The sludge receives further treatment before either having lime applied or undergoing anaerobic digestion. The liming increases the pH to reduce pathogens, and anaerobic digestion stabilises the organic matter while reducing the number of solids and level of pathogens.

During the anaerobic digestion, a large amount of organic matter is biologically converted into methane gas that can be used by the treatment facility to generate power or heat for the treatment process.

The solids resulting from the liming and digestion treatment process are called biosolids.

Finally, a polymer is added to assist in the mechanical dewatering of the biosolids. This results in a black, soft-textured substance with an earthy odour and easy handling properties.

Types of biosolids we produce

We produce 2 types of biosolids suitable for direct land application in broad-acre agriculture. They are:

  • Biosolids cake – produced at both the Beenyup and Woodman Point wastewater treatment plants. Raw sludge is stabilised by anaerobic digestion, producing biosolids. The liquid biosolids are dewatered, resulting in biosolids cake. The biosolids cake has an average of 20% solids and 80% water.
  • Lime-amended biosolids – produced at the Subiaco wastewater treatment plant. The raw sludge is first dewatered followed by stabilisation with the addition of lime. The lime amended biosolids have an average of 30% solids.

Another more processed form of biosolids is produced by composting biosolids with green waste and other organic materials. This form of biosolids is suitable for use in domestic gardens, as well as public parks and gardens. Several private companies undertake this activity using biosolids we produce.

What are the benefits of biosolids?

Biosolids are rich in nutrients and organic matter so they are a good natural fertiliser and soil improver.

Among their benefits, biosolids can:

  • improve crop production
  • enrich tree plantations
  • reduce landfill
  • improve economic returns
  • provide topsoil for land used for recreational uses.

How are biosolids used?

Farmers and other users have been beneficially using biosolids for years because they help promote crop and plant growth. Unlike mineral fertilisers that can leach excess nutrients into waterways, biosolids bind quickly to soil particles with nutrients slowly mineralising for plant growth over a longer period.

Bisolid fertiliser being applied directly to land

In WA, biosolids can only be:

  • applied directly to land at the rate at which plants can uptake the nutrients - most of the biosolids produced in Western Australia are used in this way
  • composted at a licensed facility where the end-product is suitable for use on home gardens or municipal parks and gardens - about 15% of the overall sludge production from the metropolitan wastewater treatment plants is used this way
  • disposed at a suitable landfill site.

Only biosolids that meet specific quality criteria are suitable for land application.