Wastewater treatment plants

Wastewater doesn't have to go to waste

The 16 wastewater treatment plants that we own and operate make sure that more than 1.3 billion litres of wastewater produced every day by over 1.8 million homes and businesses in Sydney, the Illawarra and the Blue Mountains doesn't create a hazard for Greater Sydney. We treat it and, where possible, recycle and re-use it.

What do our treatment plants do?

Our wastewater treatment plants treat the wastewater before it's re-used or discharged to rivers or oceans. They follow strict licence conditions issued by the NSW Environment Protection Authority (EPA), which monitors the effect of discharges on water quality and aquatic life. They also produce biosolids – the nutrient-rich material created from treating wastewater solids. Biosolids are a rich source of phosphorus and nitrogen, which can be used in agriculture, composting and land rehabilitation. 

What are wastewater treatment levels?

Treating wastewater is about removing or breaking down what people have added to the water that leaves their home or business. We use different processes to remove impurities from wastewater at our treatment plants. The type of treatment needed depends on:

  • the location of the plant
  • where the treated water will be discharged or re-used
  • the nature of the plant's catchment area, including wastewater quality.

There are 3 treatment levels: primary, secondary and tertiary.

Primary treatment

Our primary treatment includes screens, sedimentation and grit removal. Primary treatment methods include:

  • filtering wastewater through fine screens to remove items such as paper, cotton tips and plastic
  • removing sand and grit that has fallen to the bottom of aerated grit tanks
  • removing solids that have settled to the bottom of sedimentation tanks
  • removing oil and grease that floats to the top of tanks using scrapers.

Secondary treatment

Our secondary treatment process removes carbonaceous organic matter and nutrients (nitrogen and phosphorus) from wastewater. This involves converting soluble decomposable organic matter into biomass. After this, the clarification process separates the biomass and any other suspended material from the liquid stream.

We design our treatment process on the characteristics of the sewage and the nutrients we need to remove. We create the conditions so selective organisms grow (mainly bacteria) to help with the treatment process. Our treatment process can even reduce pathogens and heavy metals in some cases.


Biological reactor system

This system generally has some or all of the following 5 key stages:

Fermentation tank

Solids from the sedimentation tanks are broken down to produce a better carbon supply for microorganisms in the anoxic and aerobic zones. This makes it easier to remove phosphorus. 

Anaerobic zone

Wastewater from the primary treatment process flows into the anaerobic zones. Micro-organisms consume carbon into their cells and release phosphates.

Anoxic zone

No oxygen is available for microorganisms. They use carbon in the organic matter as a food source, converting nitrates to nitrogen gas which is released to the atmosphere.

Aeration zone

Air works with microorganisms to further break down the wastewater. A number of processes can occur in this zone like organic carbon removal, nitrogen and phosphorus removal. This depends on the concentration and activity of the biomass, water temperature and dissolved oxygen levels.

Secondary clarifiers

Remaining solids are settled in a tank. The settled solids can be returned to the anaerobic zone and the clear wastewater may be sent on for tertiary treatment.

Intermittently decanted aerated lagoons

An alternative secondary process is the intermittently decanted aerated lagoons (IDAL). 

Settled wastewater is pumped from the primary distribution structure to the IDAL anaerobic zone. Iron-rich spent pickle liquor is added to help remove phosphorus.

In the IDALs, wastewater goes through 3 stages in the one tank: aeration, settling and decanting.


Air is pumped into the IDAL through diffusers. The air works with microorganisms in the tank to break down:

  • into nitrates and water (nitrification)
  • organic matter, reducing the BOD.

Air is no longer pumped into the tank and the water is still. No longer supplied with oxygen, the microorganisms use carbon in the organic matter as a food source, converting nitrates to nitrogen gas. This is then released to the atmosphere.

The solid particles settle to the bottom. Some go to a thickening tank before being treated for biosolids production. The rest of the solids are returned to the IDAL to provide microorganisms for incoming wastewater.


After settling, the clear wastewater flows over weirs from the top of the lagoon into an equalising basin. This basin controls the flow to the tertiary treatment process.


Wastewater contains nutrient-rich solids. We treat these solids so they can be re-used as biosolids to improve soil for agriculture and gardens.

There are 4 key steps in the treatment process:

  • Solids are collected from the primary and secondary treatment tanks.
  • Settled solids may go into digestion tanks for further break down of organic matter.
  • Solids go through a range of different processes to remove water, for example using centrifuges. The matter left behind is known as biosolids.
  • Biosolids are ready for re-use in agriculture, forestry, land rehabilitation and landscaping.

Tertiary treatment

Our tertiary treatment includes filtering, disinfecting and preparing wastewater for recycling.

Filtering and disinfecting

Treated wastewater from a biological reactor and an intermittently decanted aerated lagoon (IDAL) are combined.

We add alum to help remove additional phosphorus particles and group remaining solids together for easy removal in the filters.

The treated wastewater then flows to sand filters. The wastewater sinks down through these filters where the sand traps particles.

Filtered water then flows to a chlorine contact tank for disinfection. After the water is disinfected, we remove any remaining chlorine before discharging the treated wastewater. Alternatively, we may use ultraviolet lamps for disinfection.

Preparing wastewater for recycling

Treated wastewater from biological reactors can be passed through deep sand filters where the sand traps any remaining particles. Then clear wastewater goes to a water recycling water plant where it is filtered through fine membranes to remove very small particles.

The water is pumped at high pressure through reverse osmosis membranes. This is the finest level of filtration – it removes molecules including bacteria, viruses and parasites.

The recycled water may also be treated with chlorine before it enters the recycled water distribution pipes. 

What are the treatment levels at each plant?

Treatment plant Treatment level Discharge
(ML/ day)
Discharge location
Bellambi* Primary
(includes screening, actiflow treatment and disinfection)
0 Bellambi Point during wet weather
Bondi Primary 126.5 Deepwater ocean outfall 2.2 km from shoreline, 63 m maximum water depth, 512 m diffuser zone.
Brooklyn Tertiary
(includes disinfection using ultra-violet light and membrane bioreactor technology)
0.2 Hawkesbury River, Brooklyn
Cronulla Tertiary
53 Potter Point, Kurnell
Fairfield* Primary
(includes chemically assisted sedimentation)
0 Orphan School Creek (to Georges River) during wet weather
Hornsby Heights Tertiary
(includes additional phosphorus and nitrogen removal and disinfection)
5.2 Calna Creek to Berowra Creek
Malabar Primary 488 Deepwater ocean outfall 3.6 km from shoreline, 82 m maximum water depth, 720 m diffuser zone
North Head Primary 336 Deepwater ocean outfall 3.7 km from shoreline, 65 m maximum water depth, 762 m diffuser zone
North Richmond Tertiary
(includes additional phosphorus removal and disinfection)
0.9 Redbank Creek to the Hawkesbury River
Port Kembla* Primary
(includes screening, de-gritting, primary sedimentation and disinfection)
0 Red Point during wet weather
Riverstone Tertiary
(includes additional phosphorus removal and disinfection)
1.8 Eastern Creek to South Creek
Shellharbour Secondary
(includes disinfection)
16.9 Offshore outfall 130 m from Barrack Point, with diffuser zone 
Wallacia Tertiary
(includes additional phosphorus and nitrogen removal and disinfection)
0.6 Warragamba River
Warriewood Secondary
(includes disinfection)
17.6 Turimetta Head
West Hornsby Tertiary
(includes additional phosphorus and nitrogen removal and disinfection)
11.9 Waitara Creek to Berowra Creek
Winmalee Tertiary
(includes additional phosphorus and nitrogen removal and disinfection)
16.5 Unnamed creek to the Nepean River

The smaller coastal treatment plants at Warriewood, Cronulla, Port Kembla, Shellharbour and Bellambi discharge treated effluent closer to the shore.

Less than 1% of Sydney's wastewater is discharged untreated to the ocean at Vaucluse, Diamond Bay and Diamond Bay South. The Refresh Vaucluse Diamond Bay project will ensure that all wastewater is treated at Bondi's Wastewater Treatment Plant in the future.

* Bellambi, Port Kembla and Fairfield are stormwater plants and only receive flow during storm events.