Cronulla wastewater treatment

Cronulla Wastewater Treatment Plant is one of 30 wastewater treatment and water recycling plants in greater Sydney.

Here, we treat wastewater to a tertiary standard. We use some of the treated water for on-site re-use and return the rest to the environment.

Industrial building on coast

Cronulla Wastewater Treatment Plant is on the Kurnell Peninsula.

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Location Captain Cook Drive, Kurnell
Population served 250,000 people
Area served 145 km2

Including the suburbs of Cronulla, Sutherland, Helensburgh, Menai, Bundeena and Maianbar
Amount of wastewater treated each day 53 million litres
Treatment level Tertiary
Recycled water applications We re-use some water on-site for industrial purposes like washing down equipment and filter backwashes
Environmental discharge We release the treated wastewater to the ocean at Potter Point, Kurnell
Amount of biosolids produced each year 19,000 tonnes
Operating licence and regulation We operate the plant under two sets of rules:

Primary treatment

Treatment flow diagram Cronulla

Treatment flow chart. Select image to see a larger version.

Primary wastewater treatment removes large solids using physical separation processes.

Most of the solids removed can be treated for beneficial re-use.


Screens trap and remove large solids as wastewater flows through.

Mechanical step screen at Cronulla

Large solids like wipes, food scraps, rubbish, cotton tips and plastic are caught on the screens.

Grit removal

We inject air into a tank, causing the water to spiral. The air flings the grit, such as sand and coffee grounds, to the edges. It collects in the bottom of the tank where a scraper removes it.


Sedimentation tanks allow solids to settle to the bottom of the tank while oil and grease float to the top. Scrapers at both the bottom and the top of the tanks remove the solids, oil and grease, which are then treated to produce biosolids.

At Cronulla, we’ve covered the top of the sedimentation tanks with a canvas material to control odour.

Primary sedimentation tanks at Cronulla

We use sedimentation tanks to remove floating scum and settled solids. We’ve covered the tanks to help control odour.

Secondary treatment

Secondary treatment removes nutrients such as phosphorous and nitrogen using physical and biological processes. Learn more about Removing nitrogen and phosphorous.


We add a high concentration of micro-organisms (activated sludge) to the wastewater. By varying the amount of air in different parts of the tank, we ensure different types of micro-organisms are able to break down nutrients (like nitrogen and phosphorous).

Bioreactor at Cronulla

Micro-organisms break down nutrients in the bioreactor.

Secondary clarifier

The activated sludge settles to the bottom of the clarifier where scrapers remove it. We recycle some of this sludge back into the bioreactor and treat the rest to produce biosolids.

The treated water from the top of the tank flows to tertiary treatment.

Secondary clarifier at Cronulla

We use clarifiers to separate the activated sludge from the treated wastewater.

Tertiary treatment

Tertiary treatment uses physical processes to remove very fine solids and disinfect the treated wastewater.


Filters made of layers of sand and coal trap remove any remaining floc and fine solids.

Tertiary filters at Cronulla

We use layers of sand and coal to filter the treated wastewater.


We use ultraviolet light to kill any remaining micro-organisms that can make us sick. We pass the water through a set of submerged ultraviolet lamps that destroy micro-organisms by damaging their DNA.

On-site re-use

At our plants, we use recycled water instead of drinking water wherever we can. Hoses, sprays and filter backwashes all use recycled water.

Environmental discharge

We release the treated wastewater into the ocean at Potter Point, Kurnell.

We're constantly looking for ways to reduce our carbon footprint and use alternate and renewable energy sources.


At Cronulla, we use cogeneration to produce up to 60% of the plant's total energy needs.

We capture methane gas (biogas) from the anaerobic digesters and use it to power a combustion engine that drives an electricity generator.

Cronulla cogeneration

We use cogeneration to produce renewable energy that we can use on site.


We're currently trialing co-digestion at Cronulla.

We collect pulped fruit and vegetable waste from local businesses to add to the digesters, which creates more biogas. We can then use the biogas to make extra renewable energy in the cogeneration engine.

Learn more about energy management & climate change.

Running the plant

Several staff manage, operate and maintain the plant. They collect and analyse water samples, do laboratory testing and manage special projects to keep the plant running safely and efficiently.
Men in high vis clothing  pointing at computer screens

Staff working at the plant monitor its performance.

Maintaining the plant

There are three types of maintenance required to keep the plant operating: preventative, planned and reactive.

Maintenance type Description Example
Preventative Prevents a break down Oiling a motor
Planned Replacing equipment as it reaches the end of its useful life, before a break down Replacing a worn motor
Reactive Fixing equipment that has unexpectedly broken down Repairing a motor