Wastewater treatment

What is wastewater?

Wastewater is the water you've used in your homes, schools, businesses and industries. It goes down drains from sinks, baths, showers, laundries and toilets and other drains inside buildings.

Wastewater is 99% water. The remaining one per cent is made up of things you've added to water as you've used it.


Who makes wastewater?

You do! About 70% of greater Sydney's wastewater is made inside the home.

You're part of the urban water cycle. Your interaction with wastewater may seem small, but you're one of nearly five million people making wastewater every day.

Your interaction with wastewater connects to:

We're all connected, so what you do can make a big difference.

What kinds of things are in wastewater?

There are a lot of drains inside your home that take wastewater away. Do you know where all the drains are inside your home?

House graphic

What you put down the drain inside your home, school or business goes into wastewater.

Anything that goes down the drain ends up in wastewater. We need to take it back out, so we can return the treated wastewater to the environment.

Drains Common things in wastewater
  • Kitchen sink
  • Dishwasher
  • Washing up detergents, dishwasher tablets and cleaning products
  • Dirty washing up water
  • Food scraps
  • Fats, oil and grease from frying pans and dirty dishes
  • Cleaning chemicals
  • Rubbish that gets washed down sinks, like apple stickers
  • Toilet
  • Shower
  • Bath
  • Sink
  • Poo, pee, vomit
  • Toilet paper
  • Soaps, shampoos and conditioners
  • Cleaning chemicals
  • Dirt from washing your body
  • Hair
  • Grit, like sand
  • Wet wipes
  • Tissues
  • Cotton tips
  • Dental floss
  • Tampons, pads and their wrappers, condoms
  • Sink
  • Washing machine
  • Washing detergents and fabric softeners
  • Dirt from washing your hands and clothes
  • Grit, like sand
  • Cleaning chemicals like bleach, ammonia and disinfectants
  • Other strong chemicals, like paints and solvents
Did you know?
About 75% of all wastewater pipe blockages involve wet wipes. These blockages can cause wastewater (sewage) overflows into homes or creeks.

What can you do to contribute to sustainable water management?

We all have a role to play in sustainable water management. Your interaction with wastewater connects you to the urban water cycle and the environment.

Find out how much wastewater you make and what's in it by doing a wastewater audit and sharing your plan with friends and family.

Use the table below to learn how to prevent and dispose of problem substances.
    Problem substances Effects on wastewater systems and/or environment Pollution prevention and disposal options
    Pesticides (including flea rinses) and chemicals
    • Toxic for many aquatic species.
    • Make biosolids unusable.
    • Never tip unused pesticides or flea rinses down the drain.
    • Dispose of leftover products using your local council's Household Chemical CleanOut services.
    Food waste, oil and grease, milk or other liquid leftovers
    • Block wastewater pipes.
    • Increases biochemical oxygen demand (BOD). Increased BOD can corrode wastewater system infrastructure like concrete pipes, and increase the energy needed for treatment. Increased BOD also reduces the amount of oxygen in creeks and rivers, harming aquatic life.
    • Make sure all sinks have a sink strainer fitted to trap food waste. Add these to the compost or put them in the garbage bin.
    • Ensure oily products like food scraps, excess cooking oil, unused milk and other leftovers are placed in sealed containers and put in the garbage bin.
    • Trade waste from restaurants and cafes can be disposed of at greasy waste treatment depots through Sydney Water’s commercial trade wastewater program.
    Paint and paint-related products
    • Contain heavy metals toxic to aquatic life.
    • Effect biosolid quality.
    • Paint strippers can release flammable or explosive gases endangering workers and form unsightly slicks in waterways.
    • Wash paintbrushes and rollers in a bucket instead of allowing paint to enter the wastewater system. Empty paint in the garden or put in sealed containers in the garbage bin.
    • Dispose of unwanted paint and paint-related products through the Household Chemical CleanOut service, or ask your local council about other disposal services.
    Solvents and fuels
    • Toxic to bacteria used in wastewater treatment.
    • Toxic to aquatic life.
    • Forms unsightly slicks in waterways.
    • Can release flammable or explosive gases endangering workers.
    • Dispose of unwanted paint and paint-related products through the Household Chemical CleanOut service, or ask your local council about other disposal services.
    Solid items including plastics, rubber, fibres and organic material
    • Block wastewater pipes causing overflows.
    • Effect the efficient operation and cost of treatment processes including sorting, screening and transporting for disposal.
    • Can pass into waterways if small enough.
    • Make sure all sinks have a sink strainer to trap solid items
    • Never throw solid objects into the toilet. Use a bin for unwanted or used items including disposable nappies, condoms, cotton-buds, tampons and cigarette butts.

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

Three stages of wastewater treatment

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

Different plants treat wastewater to different levels. We treat the wastewater so it's fit for purpose. This means we treat wastewater to suit the environment (creek, river or ocean) that will receive it, or to suit how it will be re-used.

Primary treatment

Primary treatment removes large solids from wastewater.

Screens trap and remove things such as food scraps, wet wipes, cotton tips and plastic (called screenings) as wastewater flows through. Grit tanks cause small, heavy particles like sand (grit) to sink to the bottom of a tank and a scraper removes it.

Sedimentation tanks allow solids (sludge) to settle to the bottom and oils and grease (scum) float to the top. Scrapers at the top and bottom of the tanks remove the sludge and scum, which are then treated to produce biosolids.

Primary sedimentation tanks

Primary treatment is the first stage of treatment.

Secondary treatment

We add microorganisms (activated sludge) to the wastewater. The microorganisms break down nutrients (like nitrogen and phosphorous) and small organic solids.

Next, we separate the activated sludge from the treated wastewater. The treated wastewater flows to tertiary treatment. The activated sludge is turned into biosolids.

Secondary bioreactor

Secondary treatment uses bacteria to break down nutrients.

Tertiary treatment

We filter the water and disinfect it with chlorine or ultraviolet light (UV). This kills any remaining microorganisms. 

Tertiary clarifier

Tertiary treatment removes very small particles.


Treatment plants

You can learn more about some of our wastewater treatment and water recycling plants:

Want to visit a wastewater treatment or water recycling plant?
We offer excursions and technical tours to schools, universities and community groups. Request an excursion
Take a look at the video below and do an experiment to find out how household products break down.


Separating mixtures can be tricky. Use the video below to try and make a simple water filter and investigate what makes a good filter.


Treated water

We treat wastewater so clean water can be safely returned to the environment or re-used.

We're always finding new ways to recycle water. The 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.
Find out more about water recycling.

Some treated wastewater is returned to creeks, rivers and oceans around Sydney. These important ecosystems are each unique environments.

Environmental protection licences tell us:
  • what quality our treated wastewater needs to be before it's released into the environment
  • how and when we monitor water quality and report our results.
Our monitoring program is consistent with the Australian and New Zealand guidelines for fresh and marine water quality.

Solids and sludge

Tractor spreading biosolids onto farmland

We re-use biosolids on farms.

The grit and screenings captured in wastewater treatment are sent to landfill. 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.