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. It's 99% water. The remaining 1% is made up of things you've added to water as you've used it.

Where does wastewater come from?

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 5 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?

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

Investigate some of the common things we put down drains in our homes.


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


Common things in wastewater

  • 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 buds
  • Dental floss
  • Tampons, pads and their wrappers, condoms


Common things in wastewater

  • 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 help?

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.

One of the best things you can do is make sure you only flush pee, poo and toilet paper down the toilet!

Prevent and dispose of problem substances

Effects on wastewater systems and/or environment

  • Toxic for many aquatic species.
  • Make biosolids unusable.

Pollution prevention and disposal options

  • Never tip unused pesticides or flea rinses down the drain.
  • Dispose of leftover products using your local council's Household Chemical CleanOut services.

Effects on wastewater systems and/or environment

  • Block wastewater pipes.
  • Increase 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.

Pollution prevention and disposal options

  • Make sure all sinks have a sink strainer fitted to trap food waste. Add this to the compost or put it 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.

Effects on wastewater systems and/or environment

  • Contain heavy metals toxic to aquatic life.
  • Affect biosolid quality.
  • Paint strippers can release flammable or explosive gases endangering workers, and form unsightly slicks in waterways.

Pollution prevention and disposal options

  • Wash paintbrushes and rollers in a bucket instead of allowing paint to enter the wastewater system. Empty paint in the garden or put it 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.

Effects on wastewater systems and/or environment

  • Toxic to bacteria used in wastewater treatment.
  • Toxic to aquatic life.
  • Forms unsightly slicks in waterways.
  • Can release flammable or explosive gases, endangering workers.

Pollution prevention and disposal options

  • Dispose of unwanted paint and paint-related products through the Household Chemical CleanOut service, or ask your local council about other disposal services.

Effects on wastewater systems and/or environment

  • Block wastewater pipes causing overflows.
  • Affect the efficient operation and cost of treatment processes including sorting, screening and transporting for disposal.
  • Can pass into waterways if small enough.

Pollution prevention and disposal options

  • 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.

How is wastewater cleaned?

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

Stages of wastewater treatment

Wastewater can go through up to 4 levels of treatment – primary, secondary, tertiary and advanced – to remove waste.

Different sites 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 reused.

Primary treatment is the first stage of treatment.

Primary treatment

Primary treatment removes large solids from wastewater.

Screens trap and remove things such as food scraps, wet wipes, cotton buds 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.

Secondary treatment

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

Learn more about Removing nutrients in wastewater (192KB).

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 treatment uses bacteria to break down nutrients.

Tertiary treatment removes very small particles.

Tertiary treatment

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

Advanced treatment

At some sites, we use advanced processes to further treat the water.

We use membrane technology like microfiltration, ultrafiltration or reverse osmosisThis removes trace nutrients and dissolved salts in the water.

How are polymers used in water treatment?

Investigate how!


What happens to treated water and solids?

Treated water

We treat wastewater so clean water can be safely returned to the environment or reused.

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 and How we purify our recycled water.

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

We reuse 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 reused, with at least 70% used in agriculture.

Find out more about solids recycling.

We reuse biosolids on farms.

How can you learn more about water resource recovery facilities?

You can learn more about some of our water resource recovery facilities.

What experiments can you do?

Take a look at the videos below, and do an experiment at home or at school.

Teacher resources

What's in wastewater? (282KB) – fact sheet

Removing nutrients in wastewater (192KB) – fact sheet

Wastewater audit – think about how much wastewater you make and what's in it

Stormwater audit – investigate what happens to rain when it hits hard surfaces

Fats, oil and grease practical investigation (273KB) – find out what happens when fats, oil and grease go down the drain

Density practical investigation (273KB) – find out how we use the properties of substances to help separate mixtures

Wipes out of pipes practical investigation (235KB) – test the difference between toilet paper, wipes and other things we put down the toilet

Make a simple water filter (182KB) – experiment

Stage 4 Science Separating Mixtures Lesson Plan (196KB) – make a simple water filter

Stage 4 Science sedimentation and decanting lesson plan (198KB) – separate a mixture

Make mock water samples (460KB) – experiment

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.

Biochemical oxygen demand

A measure of the amount of oxygen used by microorganisms in water when breaking down nutrients such as nitrogen and phosphorus.


Living or growing in water.


Solid materials like plastic removed from wastewater by screens.


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


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.


Organisms that are too small to be seen without a microscope.


Invisible radiation present in the ultraviolet range of light.


Thin layers of material that allow only some substances from a solution to pass through them.


A process to remove tiny particles from a liquid or solution by passing it through a filter.


The process of removing invisible particles like bacteria from a liquid or solution by passing it through a filter.

Reverse osmosis

A process where a solution is forced under pressure through a semipermeable membrane, separating pure water from dissolved salts.