A solid substance formed from the by-products of wastewater treatment. These solids can be beneficially used by agriculture or forestry.
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.
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.
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.
Learn more about How biosolids are produced (167KB).
You can help us recycle our waste into biosolids by:
Producing biosolids helps us reduce our environmental footprint by:
Considering biosolids as a resource instead of a waste product is a part of our 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
We use some biosolids directly without further processing. Some biosolids go on to be processed further for indirect uses.
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.
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.
We can use cogeneration and co-digestion to make heat and energy from biosolids to help power the rest of the water resource recovery faciliy.
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.
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:
In 2020, we worked with Australian Native Landscapes to build a garden at our education centre within Penrith Water Resource Recovery Facility. You can see our garden when you visit us. Request an excursion.
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.
Want to visit one of our sites? We offer free excursions and technical tours to schools, universities and community groups.