Biosolids are the solid organic material left over after sewage treatment. Rich in nutrients and similar to soil once dried, they can be reused a number of ways to make the most of this resource.

Producing biosolids

Large, open areas of land drying biosolids
Biosolids drying pans at the Western Treatment Plant in Werribee

Our biosolids are one of the products created as a result of processing sewage at our two treatment plants, along with recycled water and biogas used to generate electricity. This process involves:

  1. Separating solids from wastewater in large settling tanks, where they sink to the bottom to form a layer
  2. Biological treatment, using bacteria to break down organic material and reduce pathogens and odours
  3. Naturally air-drying the solids in drying pans, then storing them on site for at least three years before they can be reused in accordance with Environment Protection Authority (EPA) guidelines

These treated biosolids look and smell like soil, and contain beneficial nutrients like nitrogen, carbon and phosphorus.

Reuse opportunities

Truck spreading dirt-like biosolids on farmland
Melbourne Water biosolids being spread on agricultural land at Balliang, west of Melbourne

Across Australia and around the world, biosolids have been used for many years on farmland to improve soil, as well as in compost and fertiliser. Their uses include:

  • Agriculture – biosolids slowly release nutrients as plants grow, improving the soil’s ability to hold water and produce crops and replacing nutrients lost through the natural harvesting cycle.
  • Composting – biosolids naturally composted with organic material like green waste can improve soil structure and water retention.
  • Resource recovery – the nutrients and energy in biosolids can be recovered with the latest technology. For example, heating them releases gas that can be combusted for electricity, while the remaining ash (biochar) can be used for soil improvement or urban amenity.

Not all biosolids can be used for all purposes. This depends on the properties of the biosolids themselves, and is governed by some of the strictest guidelines in the world.

Classification and regulation

EPA Victoria regulates the production, quality and use of biosolids to maximise their reuse while protecting the environment and public health. Their guidelines classify biosolids based on two factors:

  • concentration of contaminants – C1 (least contaminated), C2
  • level of treatment – T1 (most processed), T2, T3

Our biosolids are of the highest treatment and second-highest contaminant grades (T1C2). Samples are tested at independent laboratories for contaminants like metals, pesticides, pharmaceuticals and pathogens, following the EPA’s strict testing regime. Further information is on the EPA website:

EPA Guidelines for environmental management: Biosolids land application

Land application program
We are supplying biosolids to local farms to spread on existing cropland as a soil conditioner, like manure or compost. Transport and land application, including buffer zones, will comply with the EPA’s conditions – for more information please view our fact sheet.

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