Solids recycling

We recycle more than water

Our role in protecting the environment doesn't end with water recycling. We aim to beneficially reuse as many of the by-products of wastewater treatment as possible. In some cases, we can turn waste into valuable products like biosolids and renewable energy.

Making biosolids

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.

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 accredits the laboratories that test all our biosolids for contaminant levels and stability. This determines the grade of the product and how the biosolids should be used.

Biosolids can be used on farms.

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 busloads.

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 water resource recovery facilities 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.

Learn more about How biosolids are produced.

What can you do to help?

You can help us recycle our waste into biosolids by:

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

Benefits of biosolids

What are the benefits of producing biosolids?

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 water resource recovery facilities.

Considering biosolids as a resource instead of a waste product is a part of our circular economy.

Biosolids can be used on pastures.

How are biosolids part of a circular economy?

A circular economy is a set of principles that values all things as resources rather than products and waste. It aims to eliminate waste, circulate products and materials so they're used more than once and regenerate nature.

We used to think of the solids left over from wastewater treatment as waste products to be disposed of. But, when we look closer we find that they're full of useful resources – things like organic matter, nutrients and healthy bacteria. Valuing and reusing resources found in wastewater, like biosolids, is part of circular economy thinking.

Can you think of other connections between water, materials and energy? It's something we're working on.

We can draw inspiration from the Earth's natural cycles, including the natural water cycle, nutrient cycles (like the nitrogen cycle in wastewater treatment) and energy cycles.




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.

Using biosolids

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 water resource recovery facilities 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.

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.

Farmers use special machinery to spread biosolids on the ground.

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.

Making renewable energy

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 water resource recovery facility.

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 water resource recovery facility.

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 facilities. We're looking to increase our cogeneration capacity at Bondi, Malabar and North Head.

Cogeneration at Wollongong Water Resource Recovery Facility.


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 Water Resource Recovery Facility.

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.

Seeing biosolids in action

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 Resource Recovery Facility

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

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

Biosolids garden

Rainwater tank

Teacher resources

Polymers in water and solids recycling – fact sheet

Glossary – definitions of keywords and industry terms


A solid substance formed from the by-products of wastewater treatment. These solids can be beneficially used by agriculture or forestry.


Material from something that was once living but has now decayed.


Solid matter that is removed during wastewater treatment. It can be processed into a material called biosolids.


Any material that floats to the surface of wastewater during treatment, usually removed in sedimentation tanks.


Sedimentation is a physical wastewater treatment process used to settle out suspended solids in water under the influence of gravity.


Requiring oxygen.


Able to live without oxygen.


All biological and chemical reactions within the biosolids have been reduced to a minimum standard.

Recycled composted biosolids

Biosolids further treated and composted with other recycled waste. Usually for indirect use.

Renewable energy

Renewable energy comes from natural resources that never run out.


Devices that uses centrifugal (outward rotating) force to separate various components of a fluid.

Dissolved air flotation

A chemical separation technique used to remove suspended solids and fats, oils and grease from the wastewater.