Solids recycling

Our role in protecting the environment doesn't end with recycling water.

We aim to beneficially re-use as many of the by-products of wastewater treatment as possible.

In some cases, we can even turn waste into valuable products like biosolids and renewable energy.

Crops grown with biosolids

Biosolids are nutrient rich and help crops grow.

What are biosolids?

We produce organic solids (sludge) during wastewater treatment.

This sludge is collected and processed to convert it into a safe, stable, nutrient rich fertiliser product called biosolids.

Did you know?
We produce around 180,000 wet tonnes of biosolids from Sydney’s wastewater in a year. That’s over 9,000 full bus loads.

How do we make biosolids?

  1. Separation - we collect the sludge (a mixture of scum and sludge) using a separation technique called sedimentation.
  2. Thickening - we use mechanical and chemical methods to reduce the water content and thicken the sludge. Some treatment plants use gravity thickeners, centrifuges or dissolved air flotation.
  3. Digestion - we use either aerobic or anaerobic digesters to break down and stabilise the sludge. This makes nutrients available to plants, reduces pathogens to low levels and reduces odour.
  4. Dewatering - we use high speed centrifuges or belt press machines to remove water from the biosolids. This helps make it lighter for easy transport in trucks.

Are biosolids safe?

NSW Health and NSW Environmental Protection Authority (EPA) have confirmed biosolids are safe if they are made and used following the EPA’s biosolids guidelines.

The National Association of Testing Authorities accredit the laboratories that test all our biosolids for contaminant levels and stability. This determines the grade of product and how the biosolids should be used.

What can you do to help?

Biosolids are applied to fruit trees.

Biosolids can be used on farms.

You can help us recycle our waste into biosolids by:

  • putting only pee, poo and toilet paper in the toilet
  • scraping food leftovers into the compost or bin before you wash up
  • using environmentally friendly detergents and cleaning products
  • dispose of household chemicals using your council's Household Chemical CleanOut services.

What are the benefits of producing biosolids?

Sheep and cattle graze on biosolids pasture

Biosolids can be used on pastures.

Producing biosolids helps us reduce our environmental footprint by:

  • minimising the discharge of solids to our oceans and rivers
  • recycling a valuable resource that helps minimise disposal to landfill
  • reducing the amount of chemical fertilisers used on farms
  • producing carbon neutral biogas used to generate renewable energy to power our treatment plants.

What are the benefits of using biosolids?

Uses Benefits
Agriculture and forestry
  • Nutrients from biosolids are released slowly during plant growth.
  • Increases crop production.
  • Biosolids hold water in soil better, increasing drought resistance.
  • Replaces nutrients removed from soil by harvesting.
  • Biosolids can be mixed with other organic materials such as green waste, shredded timber and sawdust and allowed to compost naturally.
  • This material can be mixed into the soil to improve soil structure and help hold water.
Land rehabilitation
  • Biosolids are used to restore degraded mine, construction or agricultural sites.

How are biosolids used?

We use some biosolids directly without further processing. Some biosolids go on to be processed further for indirect uses.

Direct use

Around 73% of the biosolids produced from our treatment plants are directly applied to agricultural soils.

Over 40 farms across the central west and south west of NSW use our biosolids to help improve soil, mainly in broad-acre farms.

These large farms grow canola, wheat, oats, barley and pastures.

The biosolids are spread and mixed into the soil before sowing the crops. The harvested parts of these crops don’t come into direct contact with the soil/biosolids mixture.

Biosolids spreader

Farmers use special machinery to spread biosolids on the ground before sowing crops. 

Some animals, such as sheep and cattle, may graze on crops and pastures grown in biosolids. There are withholding periods for farm animals fed on biosolids-treated pastures.

Sheep graze on pastures grown with biosolids

Farmers can use biosolids to grow food for sheep.

Indirect use

We send the remaining 25% of our biosolids for further processing. Biosolids can be mixed with other materials such as green waste and further composted. These products are tested to ensure they're safe for use in the same way as any other composted product.

These composted biosolids are used in agriculture, horticulture, mine rehabilitation and gardens and parklands within Sydney.

How can we make renewable energy?

We can use cogeneration and co-digestion to make heat and energy from biosolids to help power the rest of the treatment plant.

Learn more about wastewater treatment and Energy management and climate change.


Cogeneration is the production of electricity and heat at the same time.

We can use the electricity and heat to reduce the overall energy demand of a wastewater treatment plant.

In a wastewater system, the anaerobic digestion of organic waste can be used to produce methane gas. 

This gas can be used to power a combustion engine that drives an electricity generator. 

The heat generated from the combustion engine can then be used to warm the digester and improve its efficiency.

We already use cogeneration at a number of plants. We're looking to increase our cogeneration capacity at Bondi, Malabar and North Head.


Cogeneration at Wollongong Water Recycling Plant.



Co-digestion takes liquid organic waste from restaurants and other sources and combines it with solids from the wastewater system.

This mix is then digested to produce methane gas and biosolids.

In late 2011, we began exploring the possibilities of co-digestion and the benefits of treating organic waste as a resource at Cronulla Wastewater Treatment Plant.

This work is still in the early stages, but it's producing some impressive results with a lot of promise for the future.

The benefits of using co-digestion:

  • It diverts organics, like food scraps, from landfill.
  • It uses existing infrastructure.
  • It generates renewable energy.
  • It has low carbon emissions.

Where can I see biosolids being used?

In 2020, we worked with Australian Native Landscapes to build a garden at our education centre within Penrith Water Recycling Plant. You can see our garden when you visit us. Request an excursion.

We used recycled composted biosolids from our treatment plants and planted drought tolerant, low maintenance native plants. We collect and use rainwater to conserve our drinking water. 

Biosolids garden

Garden bed with composted biosolids, mulch and plants.

rainwater tank

Rainwater tank and pump to water garden.

We'll be watching how the garden thrives over time. It's a great example of how we can contribute to liveable cities.