What are biosolids?


Biosolids are the major by-product of the wastewater treatment process. When treated and managed appropriately, they can be beneficially used for a number of purposes.

Biosolids are treated sewage sludges. The ANZBP is careful to distinguish between sewage sludge and biosolids. Biosolids can only be considered as such once they fulfil the requirements of a set of approved biosolids management guidelines.

Sewage sludge is the solids that are collected from the wastewater treatment process but which have not undergone further treatment. Sludge normally contains up to around 3% solids. Sewage sludge is regarded as having become biosolids once it has undergone further treatment to reduce disease causing pathogens and volatile organic matter significantly, producing a stabilised product suitable for beneficial use.

Biosolids, normally contain between 15% to 90% solids. Biosolids are carefully treated and monitored and they must be used in accordance with regulatory requirements.

What is in biosolids?

Biosolids are derived from wastewater sludge, mainly a mix of water and organic materials that are a by-product of the sewage treatment processes. Most wastewater comes from household kitchens, laundries and bathrooms. Biosolids may contain:

  • Macronutrients, such as nitrogen, phosphorus, potassium and sulphur and
  • Micronutrients, such as copper, zinc, calcium, magnesium, iron, boron, molybdenum and manganese

Biosolids may also contain traces of synthetic organic compounds and metals, including arsenic, cadmium, chromium, lead, mercury, nickel and selenium. These contaminants limit the extent to which biosolids can be used, with all applications regulated by appropriate government authorities in each State and federally. Australia has one of the strictest regulatory regimes for biosolids production and application in the world.

Treatment processes produce a stabilised product suitable for beneficial use. Biosolids normally contain between 15% and 90% solids.

In 2019 Australia produced approximately 371,000 dry tonnes of biosolids annually. Approximately 70% is applied to agricultural land and around 24% is used for landscaping or land rehabilitation. The remaining 6% is stockpiled, landfilled or discharged to the ocean.

What are biosolids used for?

Biosolids can be applied as a fertiliser to improve and maintain productive soils and stimulate plant growth. They are also used to fertilise gardens and parks and rehabilitate mining sites.

In Australia and New Zealand, biosolids have been used for:

  • Land application in agriculture (vine, cereal, pasture, olive)
  • Co-generation/power production/energy recovery
  • Road base
  • Land application in forestry operations
  • Land rehabilitation (including landfill capping)
  • Landscaping and topsoil
  • Composting
  • Oil from sludge (experimental).

Other uses include:

Biosolids are graded according to chemical composition and the level of pathogens remaining after production. Not all biosolids can be used for all applications. Lower qualities are typically used for road bases and mine site rehabilitation. Only the highest grade of biosolids can be used to grow crops for human consumption. Regulators, such as State departments of Health and Environment strictly control the production, quality and application of biosolids.

How are biosolids made?

What you flush down the toilet and plug-holes in your house eventually requires treatment at a sewage treatment plant. A typical sewage treatment plant uses a range of treatment processes to produce recycled water and biosolids. This is demonstrated in the figure below.

Figure 1: Processes in a typical wastewater (sewage) treatment plant which produce wastewater sludge for processing into biosolids

Figure 1: Sewage Treatment Process

Biosolids are produced primarily from the treatment of sewage. Sewage consists of used water from household activities such as washing dishes and clothes, taking a shower, flushing the toilet and even cleaning your teeth.

Industry also discharges into the sewerage system. This discharge is usually regulated and limits are set so that any potentially dangerous compounds are not allowed in the sewer at levels that might cause harm to the environment or people.

During sewage treatment, microorganisms digest (eat) the sewage, completely breaking down the original organic solids that have been discharged into the sewerage system. This leaves a low solids effluent and a solids component known as sewage sludges. The water content of the solids is then reduced, usually by passing through mechanical processes. The resultant product is biosolids.

Biosolids comprise dead micro-organisms, a small portion of active microorganisms, and any inert solids such as sand which have come down the sewer.

The final quality of the biosolids produced depends on the quality of the sewage entering the treatment plant and the treatment process.

Strict state and national guidelines in Australia and New Zealand specify the way in which specific biosolids can be used. The Australian and New Zealand water industries use some of the most advanced wastewater treatment and biosolids production technology and quality assurance programs in the world. This goes toward ensuring the safe and sustainable management of biosolids.

Figure 2: Five typical production systems for biosolids with possible alterative productions pathways

Figure 2: Biosolids Treatment Process

Figure 3: Nutrient flows through urban environment

Figure 3: Nutrient Flows