St Marys Advanced Water Recycling Plant

St Marys Advanced Water Recycling Plant is one of the biggest water recycling plants in Australia.

The plant receives tertiary treated effluent from:


It uses membrane technology to produce highly treated recycled water, which is discharged into the Hawkesbury-Nepean River.

Reverse osmosis membranes

St Marys Advanced Water Recycling Plant uses reverse osmosis membranes to treat the water.

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Location Links Road, St Marys
Population served 500,000 people
Average amount of recycled water treated per day 60 million litres
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 two sets of rules:
Energy use For every million litres produced, the plant uses 9673 MWh of electricity.

1 MWh is equal to 1,000 kilowatts of electricity used continuously for one hour. 
Flow of water

Select the image to see a larger version of the flow of source water to the plant.

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

It receives tertiary treated wastewater from three water recycling plants - at 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.

The plant needs to process 60 million litres of tertiary treated recycled water to produce 50 million litres of final water. View the treatment process diagram.

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 growing.

Large concrete tank

The feed balance tank ensures there is 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.
Large wire mesh strainers

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 semi-permeable membranes.

The membranes block partials larger than 0.02 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.

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.


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

Large cylindrical decarbonators

Decarbonators reduce the amount of carbon dioxide in the water.

Clear water balance tank

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.

Flowing water

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

St Marys Advanced Water Recycling Plant is part of a project called The Replacement Flows Project.

This 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 process used at St Marys treats wastewater to such a high standard that it greatly reduces the volume of nutrients, including nitrogen and phosphorous, discharged into the river from Penrith, St Marys and Quakers Hill wastewater treatment plants.

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.

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 the plant running safely and efficiently.

Staff looking at computer screens

Our staff monitor water quality at the plant.

Maintaining the plant

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

Maintenance type Description Example
Preventative Preventing break downs 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


Staff changing filters

Staff at the plant maintain the reverse osmosis filters.

Make a membrane model

Membrane model materials

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

Make a membrane model

Teacher lesson plan - Make a membrane model