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What are biosolids?

About this lesson

Students plan, edit and publish a marketing campaign for a compost containing biosolids.

Year level: 7

Theme: Wastewater

Learning objectives

Students can:
  • explain what biosolids are
  • investigate what is meant by the term ‘renewable’
  • recognise that human waste can be used beneficially for farming
  • plan, draft and publish a marketing campaign for compost made using biosolids.

Curriculum links


  • Some of the Earth's resources are renewable, including water that cycles through the environment, but others are non-renewable ACSSU116
  • People use science understanding and skills in their occupations and these have influenced the development of practices in areas of human activity ACSHE121


  • Plan, draft and publish imaginative, informative and persuasive texts, selecting aspects of subject matter and particular language, visual, and audio features to convey information and ideas ACELY1725

Things you will need

  • Computer

Lesson description


  • Consider the statement ‘Sewage can no longer be seen as waste’. Ask students what they think this means and discuss the implications.
  • Explain what biosolids are.
  • Kojonup farmer Keith Asthton turns all the human waste of Albany into compost. Listen to the audio file of the ABC radio interview ‘Turning poo into profit’ 3/6/2011.

NOTE: the farmer in the article is shown handling compost, which is best done wearing gloves. Biosolids are not safe to handle.

  • Mind map and discuss: what are the considerations/issues around using human waste as a fertiliser?


  • Biosolids are a renewable resource. Have students investigate what this means.
  • Highlight the activity scenario to students: imagine you work for a marketing company. You have been asked to create a marketing campaign for your client who manufactures garden compost, which includes biosolids from Perth wastewater treatment plants.
  • Explain to students they will plan, write and edit a marketing campaign document using word processing. They will need to:
    • detail their campaign strategy and approach
    • detail the expected benefits for the client
    • include the benefits of biosolids, in particular it being a renewable resource
    • outline how to overcome sensitive issues such as health warnings and the ‘yuck’ factor of human waste
    • include detailed advertising ideas for at least two channels of advertising (e.g. press advertising, radio, television, direct mail, outdoor, cinema, point of sale, or online).
  • Produce an interesting advert to present to the class.

Reflect & summarise

  • Students critique each other's ads and vote on the top 3 ads produced.
  • Test students' understanding by asking them what a renewable resource is and provide examples.

Extension activities

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.


Did you know?

The average household creates enough wastewater to produce between 0.5 and 1 kilogram of biosolids (dry weight) per week.

Key vocabulary

  • Biosolids: Stabilised, nutrient-rich, organic residues generated from the wastewater treatment process.
  • Polymer: A chemical compound or mixture of compounds that consists of repeating structural units.
  • Renewable: Capable of being replaced by natural ecological cycles or sound management practices.
  • Wastewater: Water that has been used inside a home, business or industry that travels to a wastewater treatment plant.
  • Wastewater System: The system that collects wastewater from households, businesses and industries then directs it to treatment plants where it is treated to an environmentally acceptable standard before being reused or safely discharged back to the environment.