Teacher background information
How are biosolids produced?
Biosolids are the wastewater sludge that has undergone further treatment to produce a stabilised product suitable for beneficial use.
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 volatile 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 dewatered sludge. Finally, a polymer is added to assist in the mechanical dewatering of the dewatered sludge to produce biosolids, which is a black, soft-textured substance with an earthy odour and easy handling properties.
Types of biosolids produced in WA
- Biosolids cake: Produced at 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.
Biosolids in compost
Biosolids are also used as a component of compost. Composting further treats the biosolids by temperature and pH changes. The compost produced is suitable for use in domestic gardens, as well as public parks and gardens. Several private companies make compost with biosolids, sourced from our wastewater treatment plants, as a component.
Farmers and gardeners have been beneficially using biosolids as part of composted products 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.
Approved uses for biosolids in WA
- Biosolids can only be applied directly to land at the rate that considers background soil quality to ensure plants can up take any extra nutrients – most of the biosolids produced in WA 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 of at a suitable landfill site.
Only biosolids that meet specific quality criteria are suitable for land application. In WA, biosolids are used to grow broad acre crops such as wheat, oat, canola and lupins, as well as forestry applications.
Benefits of biosolids
Biosolids are rich in nutrients and organic matter so they are a good natural fertiliser and soil improver.
It can also:
- improve crop production
- enrich tree plantations
- reduce landfill
- improve economic returns
- provide topsoil for land used for recreational uses.