Wastewater treatment

Large pool of wastewater

We treat Sydney’s wastewater to a very high standard to protect public health and the environment.

Wastewater is used water from our homes, schools, businesses and industries that goes down drains inside buildings. This includes sinks, baths, showers, laundries and toilets.

Wastewater may contain food scraps, soap, laundry detergent, human waste and other pollutants. It can also contain pollutants from industry.

Wastewater flows by gravity or is pumped through pipes to a wastewater treatment plant or a water recycling plant. It's then cleaned and safely returned to the environment or re-used.  

We operate 30 wastewater treatment and water recycling plants that treat over 1.5 billion litres of wastewater every day.

The wastewater treatment plants, water recycling plants, pumping stations and pipes are all part of the urban water cycle.

If all the pipes in our wastewater system were placed end to end, they would stretch for 25,085 km. That's more than half way around the world!

Learn more about our wastewater network.

Water treatment processes

Treating wastewater is about removing or breaking down what people have added to the water that leaves their home or businesses.

Most wastewater is 99% water. Less than one per cent is solid or dissolved waste.

Wastewater can go through three levels of treatment called primary, secondary and tertiary treatment to remove impurities.

Different plants treat wastewater to different levels. The type of treatment depends on several factors, including where the treated water is discharged to or if the treated water will be re-used.

Our North Head, Bondi and Malabar plants treat wastewater to primary level only. Other treatment plants, particularly those on inland waterways, treat wastewater to all three levels.

Three stages of wastewater treatment

Primary treatment

Primary treatment removes solid particles from wastewater.

Wastewater is filtered through fine screens to remove solid matter such as paper, wet wipes, cotton tips and plastic (called screenings).

Heavy particles like sand (grit) sink to the bottom of a grit tank and are removed.

The wastewater then flows into primary sedimentation tanks, where solids (sludge) settle to the bottom and oils and grease (scum) float to the top where they are collected. 

Secondary treatment

Secondary treatment involves the effluent from primary treatment flowing into tanks where good bacteria grow. These bacteria break down the solid waste and remove nutrients from the water.

The liquid then flows into clarifiers where heavy particles settle to the bottom for collection.

Tertiary treatment

Tertiary treatment comes after primary and secondary treatment.

Water goes through a final dual media filtration process before being disinfected with chlorine or ultra violet light (UV). This kills any remaining microorganisms.

Want to visit a wastewater treatment or water recycling plant?
We offer excursions and technical tours to schools, universities, delegations and community groups.
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We treat wastewater so clean water can be safely returned to the environment or re-used.

In Sydney, more than 80% of treated wastewater is released deep into the ocean. This means it's diluted and disinfected by sunlight and seawater.

We're always finding new ways to recycle the liquid and solid parts of wastewater. 

Treated wastewater (recycled water) can be used:

  • in homes and businesses to water gardens and flush toilets
  • in industry
  • to fight fires
  • to irrigate parks, farms and sports fields
  • to maintain river flow.

The grit and screenings are made into compost and the sludge collected is turned into a fertiliser called biosolids.

Biosolids are used in agriculture, forestry and rehabilitation. 100% of our biosolids are beneficially re-used, with at least 70% used in agriculture.

Find out more about solids recycling.

We treat the wastewater of more than 4.8 million people, so individual actions can add up to make a big difference.

If too many people put the wrong things into the wastewater system, it can block drains, affect the machinery at treatment plants and eventually pollute waterways.

Pollutants like nutrients, paint, oil, chemicals and rubbish are difficult and expensive to remove and treat.

Wet wipes cause 75% of our wastewater system blockages. These blockages can cause sewage overflows into homes or creeks. 

Wet pipes can also block your property's sewer pipes leading to costly plumbing bills.  Find out more about the need to Keep wipes out of the pipes.

How you can help us keep our wastewater system working

You'll help look after the wastewater system and protect the environment by:
  • putting used wet wipes in the bin
  • not putting rubbish down the sink or in the toilet – only human waste and toilet paper should go into the toilet
  • scraping plates and putting the scrapings in the bin or compost before you wash up
  • using a sink strainer to catch small pieces of food and putting them in the bin or compost
  • not putting fats, oils, milk or other liquids down the sink. Pour them into used containers with secure lids and put them in the garbage bin
  • never putting paint down the sink. Wash paint brushes in a bucket and pour the water in the garden away from drains
  • using the half flush on the toilet to reduce the wastewater you produce.

Play the sorting game to find out what things should and shouldn’t go into wastewater drains.

Do you know what happens after you flush the toilet? Check out our This living thing is called the wastewater system video to find out more.