Biosolids are a renewable resource that we produce from wastewater sludge.

Biosolids are the wastewater sludge that has undergone further treatment to produce a stabilised product suitable for beneficial use.

How are biosolids produced?

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

The liquid and solids are separated during the wastewater treatment process. After treatment, the liquid is released via ocean outfalls, allowed to infiltrate back into the ground, or recycled for industrial or other uses.

The sludge undergoes extensive stabilisation by liming or anaerobic digestion. This process stabilises the organic matter while reducing the quantity of solids and the level of pathogens.

In this process, 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 process.

The stabilised solids resulting from the digestion 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. 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 cake has an average of 80% water, with the balance being solids.
  • Lime-amended biosolids – produced at the Subiaco Wastewater Treatment Plant. The raw sludge is first dewatered followed by stabilisation with the addition of lime.

Another more processed form of biosolids is produced by blending 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 gardeners have been beneficially using biosolids for years because they help promote crop and plant growth. Unlike mineral fertilisers that have the potential for excess nutrients to leach into waterways, biosolids bind quickly to soil particles with nutrients slowly mineralising for plant growth over a longer period of time.

In WA biosolids can only be:

  • applied directly to land at the rate at which plants can up take 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 20% of the overall biosolids cake production from the metropolitan wastewater treatment plants is used in this way
  • disposed at a suitable landfill site.

Only biosolids that meet specific quality criteria are suitable for land application. In WA, biosolids are used to grow broadacre crops such as wheat, oat, canola and lupins, as well as forestry applications.