St Marys Water Recycling Plant

Recycling water

St Marys Water Recycling Plant is one of 30 wastewater treatment and water recycling plants in greater Sydney. It treats wastewater to tertiary standard. We use the recycled water for onsite re-use, irrigating a local golf course and environmental flows.

We're upgrading!

By 2040, we expect the number of customers in the area to more than double. So, we're introducing new technology to meet demand. It will also improve biosolids treatment and enable us to use biogas as a renewable energy resource.


By the end of the work, we'll have a new cogeneration facility that will reduce our reliance on fossil fuels.


While the upgrade happens, we've relocated tours to the nearby Penrith Water Recycling Plant. You can also still visit the St Marys Advanced Water Recycling Plant to learn about membrane treatment.

Facts and figures

Location: Links Road, St Marys

Population served: 160,000 people.

Area served: 84 km2, including the suburbs of Cambridge Park, Werrington Downs, Blackett, Mt Druitt, Minchinbury and St Clair.

Amount of wastewater treated: 33.5 million litres each day.

Treatment level: Tertiary.

Recycled water applications: We re-use some water on site for industrial purposes like washing down equipment and filter backwashes. Dunheved Golf Course uses our recycled water. They use up to 2 million litres a day to water the greens.

Environmental discharge: We release 4–8 million litres a day to a tributary of South Creek. We send the remainder to the St Marys Advanced Water Recycling Plant for advanced treatment before we release it to the Nepean River.

Amount of biosolids produced: 20,000 tonnes each year.

Operating licence and regulation: We operate the plant under 3 sets of rules: 

Technical resources

What's in wastewater? (279KB) – learn about common wastewater parameters.

Removing nutrients in wastewater (192KB) – fact sheet.

Flow chart

Primary treatment

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.

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.

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


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.

We use scrapers on the sedimentation tank to remove floating scum and settled solids.

Secondary treatment

Secondary treatment removes nutrients such as phosphorous and nitrogen using physical, biological and chemical processes. Learn more about Removing nutrients in wastewater (192KB).


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).

The bioreactor has different aeration zones.

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.

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

Tertiary treatment

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

Chemical mixing

We add chemicals that make the smallest particles ‘stick’ together forming larger ‘flocs’. This process is called flocculation.

Large paddles stir chemicals into the water to help remove tiny particles.

Tertiary clarifier

When the flocs become large enough they settle to the bottom of the clarifier and are removed.

The treated water flows from the top of the tank to the filters.


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


We add chlorine to kill any micro-organisms. We remove any residual chlorine before discharging the treated wastewater to the environment or recycling it.

Flocs settle to the bottom of the tertiary clarifier.

Re-using the water

On our plants, we use recycled water instead of drinking water where ever we can. Hoses, sprays and filter backwashes all use recycled water. We use the remaining recycled water in a few ways.

Local golf course

We supply the local golf course with as much recycled water as they need to irrigate the greens and fairways.

Environmental flow

We release some water into the local creek, a tributary of South Creek, to help keep the creek alive and healthy.

Advanced treatment

The rest of the water is sent to the St Marys Advanced Water Recycling Plant for further treatment using membrane technology. This water is released into the Hawkesbury–Nepean River for environmental flow.

Dunheved Golf Course uses recycled water instead of drinking water to irrigate the greens.

We release high quality recycled water for environmental flow at Penrith.

Operations and maintenance

Running the plant

Eight 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.

Maintaining the plant

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

See below for examples.

Staff monitor the plant to make sure it's working at its best.

Maintenance type




Prevents a break down

Oiling a motor


Replacing equipment as it reaches the end of its useful life, before a break down

Replacing a worn motor


Fixing equipment that has unexpectedly broken down

Repairing a motor

Come behind the scenes

Want to visit one of our sites? We offer free excursions and technical tours to schools, universities and community groups.


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


The larger particles formed when smaller particles stick together with the aid of a coagulant during the process of flocculation.


Solid matter that is removed during wastewater treatment. It can be processed into a material called biosolids.