Reusing Biosolids
To ensure a sustainable water future we are committed to exploring reuse options for biosolids stored at our sewage treatment plants.
Biosolids are the dried, stabilised matter that remains after the sewage treatment process has finished. Biosolids are made up of both organic and inorganic compounds, and include household and industrial wastes. Once dry, biosolids look and smell like soil.
There are large stockpiles of biosolids at both the Western Treatment Plant in Werribee and the Eastern Treatment Plant in Bangholme.
What can biosolids be used for?
Biosolids can be recycled for different uses, depending on their characteristics. Biosolids produced and used at the Eastern Treatment Plant, for example, have been used (mainly offsite) for:
- landscaping
- site rehabilitation
- blended organic soil products.
Other beneficial uses include:
Nutrients
Biosolids contain nitrogen and phosphorus which can be used for forestry and farming. In rural areas biosolids are already being sold for use as fertiliser and there is strong demand for this. There are traditionally less contaminants in biosolids from treatment plants where there are comparatively lower amounts of industrial activity.
In future, newly produced biosolids from our Eastern Treatment Plant are likely to be appropriate for this use.
Energy
Biosolids contain methane gas that can captured and combusted to produce electricity. The conversion process involves heating the biosolids to break down the solid materials and capturing the gases (and the ash) which are then converted to energy. Both the energy and the ash from biosolids combustion can be used to generate electricity.
We are currently investigating the energy recovery options for our historic biosolids at the Western Treatment Plant.
Structural purposes
Biosolids can be used for geotechnical applications such as fill for road surfaces. Biosolids used for this purpose must be managed carefully so there are no dangers of contaminants leaching. Historic biosolids from our Eastern Treatment Plant are considered most appropriate for this application and we are currently trialling biosolids from the plant for this purpose.
Regulating biosolids
Not all biosolids can be used for all applications. For example, only the highest grade of biosolids (Grade 1) can be used to grow crops for human consumption. The lowest grade (Grade 3) is not suitable for land use but may be used as a substitute for non-renewable fossil fuels.
EPA Victoria is responsible for regulating biosolids. Regulation strictly controls the application, production and quality of biosolids.
The main purpose of regulation is to ensure the environment and public health are protected, and to maximise the sustainable use of biosolids.
Biosolids are graded according to their chemical composition and the level of pathogens remaining after production. Each contaminant is graded C1, C2 or C3.
Biosolids graded C1 are the highest quality (i.e. have the lowest concentration of contaminants). Grade C3 biosolids are the lowest quality (i.e. have the highest concentration of contaminants).
Biosolids at the Western Treatment Plant
The content of our biosolids varies between batches and between treatment plants.
Biosolids at Western Treatment Plant are considered in two parts – historic stockpiles and current biosolids production.
Due to the presence of heavy industry in Melbourne’s western suburbs, biosolids from the Western Treatment Plant have traditionally contained metals. These historic stockpiles contain higher concentrations of industrial contaminants and are graded C3.
The first harvest of current biosolids production at the plant has been completed and we are currently assessing reuse options for ongoing biosolids production at the Western Treatment Plant. Given reductions in the pollutant levels in sewage over recent years these biosolids are expected to be of better quality than the historic stockpiles.
Biosolids at the Eastern Treatment Plant
Biosolids from the Eastern Treatment Plant have traditionally had higher clay content due to harvesting practices at the plant. Stockpiles of biosolids at the Eastern Treatment Plant are mostly graded C2.
We have improved harvesting practices at the Eastern Treatment Plant, so biosolids no longer have such a high clay content. We expect these improvements will make the biosolids suitable for a wider range of uses in future.