St Marys Advanced Water Treatment Plant

Advanced water recycling

St Marys Advanced Water Treatment Plant is one of the biggest water recycling facilities in Australia. It receives tertiary-treated effluent from 3 water resource recovery facilities and uses membrane technology to produce highly treated recycled water, which is discharged into the Hawkesbury-Nepean River.

Facts and figures

Location: Links Road, St Marys

Population served: 500,000 people

Recycled water treated: Average of 60 million litres per day

Treatment level: Advanced treatment using membrane technology

Environmental discharge: Tertiary-treated wastewater is recycled as part of the Replacement Flows Project. The treated recycled water is discharged into Boundary Creek at Penrith.

Operating licence and regulation: We operate the plant under 2 sets of rules.

Energy use: For every million litres produced, the plant uses 9673 MWh of electricity. One MWh is equal to 1,000 kilowatts of electricity used continuously for one hour.

Source of the water

The St Marys Advanced Water Treatment Plant produces up to 50 million litres of highly treated recycled water each day.

It receives tertiary treated wastewater from 3 water resource recovery facilities – Quakers Hill, St Marys and Penrith.

The plant greatly reduces the volume of nutrients, including nitrogen and phosphorous, discharged into the river. This improves water quality downstream of the Penrith Weir and helps reduce the growth of algae and water weeds.

Flow chart

Treatment steps

Feed balance tank

We pump highly treated recycled water from St Marys, Penrith and Quakers Hill to the feed balance tank. Then we add chlorine and ammonia to disinfect the water and prevent bacteria from growing. The plant needs to process 60 million litres of tertiary-treated recycled water to produce 50 million litres of final water.

The feed balance tank ensures there's a constant supply of water for the membranes.


We use strainers made of a fine metal mesh to filter out any aquatic weeds, algae or leaves that may be in the water.

The strainers are made of a fine mesh.


We pump the water into a tank filled with ultrafiltration membranes. A vacuum sucks the water through the semipermeable membranes.

The membranes block particals larger than 0.02 micrometres (µm), like suspended solids, bacteria and some viruses.

Reverse osmosis

We force the water at high pressure through reverse osmosis membranes.

The membranes remove particles larger than 0.0005 µm.

Once the water has passed through the reverse osmosis membranes, it is called permeate.


We pump the water through decarbonators to reduce the amount of dissolved carbon dioxide in the water.

Decarbonators reduce the amount of carbon dioxide in the water.

Releasing the water

The treated water is temporarily stored in the clear water balance tank.

We adjust the pH level and then pump the water to Penrith, where it's released into Boundary Creek. It flows into the Hawkesbury-Nepean River, just below Penrith Weir.

The water from Boundary Creek flows into the Hawkesbury-Nepean River.

Make a membrane model

Try this activity to make your very own reverse osmosis module. It's easy to do.


Did you know?

The osmosis principle

The reverse osmosis process works using the osmosis principle, where molecules in a solvent pass through a semipermeable membrane from a less concentrated solution into a more concentrated solution.

The Replacement Flows Project

The St Marys Advanced Water Treatment Plant is part of a recycled water initiative called the Replacement Flows Project. The project is designed to stop up to 18 billion litres of drinking water being released from Warragamba Dam as environmental flow each year. It does this by replacing the environmental flow with highly treated recycled water from the plant.

The recycled water is such high quality that it greatly reduces the volume of nutrients discharged into the river from Penrith, St Marys and Quakers Hill water resource recovery facilities.

This improves the water quality downstream of the Penrith Weir and helps reduce the growth of algae and water weeds.

We monitor water quality in the discharge zone and report the results to the NSW Environment Protection Authority.

Operations and maintenance

Running the plant

A team of staff manage, operate and maintain the plant. They collect and analyse water samples and manage the equipment on-site to keep it running safely and efficiently.

Maintaining the plant

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

See the table below for examples.

Staff at the plant maintain the reverse osmosis filters.

Maintenance type




Prevents a breakdown

Oiling a motor


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

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.


What wastewater becomes after it has been treated at a wastewater treatment plant.


A substance that provides nourishment, food or energy.