St Marys Water Recycling Plant

St Marys Water Recycling Plant is one of 30 wastewater treatment and water recycling plants in greater Sydney.

Here, we treat wastewater to tertiary standard. We use the recycled water for onsite re-use, irrigation of the local golf course and environmental flows.

Aerial photo St Marys

St Marys Water Recycling Plant produces high quality recycled water.


We're upgrading the St Marys Water Recycling Plant to meet increased population growth

By 2040, we expect the number of customers in the area to more than double. So, we're introducing new technology to 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.

Request an excursion.
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 each day 33.5 million litres
Treatment level Tertiary
Recycled water applications
  • We reuse 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 each year 20,000 tonnes
Operating licence and regulation We operate the plant under three sets of rules:

Primary treatment

Treatment flow diagram St Marys

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.


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.


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.


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).


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

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.

Chemical mixing

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.

Tertiary clarifier

Flocs settle to the bottom of the tertiary clarifier.


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


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


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


We add chlorine to make sure the water is safe for the environment.

We use recycled water for a number of different things.

On site re-use

On our plants, we use recycled water instead of drinking water where ever we can. Hoses, sprays and filter backwashes all use recycled water.

Local golf course

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

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

Environmental flow

We release some water into the local 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.

Discharge at Penrith

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

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.

Production Officer working at St Marys

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

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