How the Sydney Desalination Plant protects the ocean while securing clean, reliable drinking water

Clean reliable drinking water is at the heart of Sydney Desalination Plant’s mission. But just as important as water security is how that drinking water is made and the care taken to protect the marine environment.

For the Sydney Desalination Plant, there is no simple trade-off between reliable drinking water and ocean stewardship. With the right design, planning and monitoring, the two can work together.

Designed to minimise ocean impact

As part of the desalination process to produce high quality drinking water, seawater is pushed through reverse osmosis membranes at high pressure to separate fresh water from the salts. The portion of seawater that doesn’t become drinking water, around 58 per cent, is returned to the ocean as brine or seawater concentrate. This is simply seawater with a higher salt content because the freshwater has been removed. The brine is carried back to the ocean through an outlet tunnel and released via specially designed dispersion nozzles that rapidly dilute it back to normal seawater levels.

A key part of the Sydney Desalination Plant’s approach to minimise the impact on the ocean is its multi-port diffuser system, which returns brine back to the ocean in a way that promotes rapid mixing and dilution.

What makes the diffuser design so important is the way it works: it is built to maximise mixing and dilution as quickly as possible.

The diffuser system uses two key design features:

• Small-diameter nozzles, which force the brine to exit at high speed
• Eight nozzles on each of the two diffusers, which spread the discharge in multiple directions

This combination helps the brine mix rapidly with surrounding seawater rather than remaining concentrated in one place, reducing the chance of concentrated brine discharge settling on the seabed.

Before construction, extensive environmental studies were carried out along the Kurnell coast to help determine the best location and design for the outfall. That science-based planning is one of the reasons the Plant has been able to operate with a strong focus on environmental performance.

What “minimal impact” means in practice

The Plant operates under a strict environmental protection license from the NSW Environmental Protection Authority. This requires monitoring of key water quality parameters including salinity, pH, chlorine and suspended solids to ensure the Plant stays within its design limits.

Just as importantly, the Sydney Desalination Plant also carried out a six-year independent marine monitoring programme, undertaken by The University of NSW with a team that included scientists from NSW Fisheries Research and Southern Cross University, when it first operated. This was a first-of-its-kind study that tracked the local marine environment before, during and after plant operation. The programme focused on rocky reef habitats and marine invertebrates.

The findings were reassuring. Overall biodiversity was maintained and the Plant’s operation had no significant impact on the local marine environment.

A responsible approach to water security

As climate change increases pressure on water supplies, desalination plays an important role in helping secure drinking water for Sydney. The Sydney Desalination Plant’s experience shows that desalination can be delivered responsibly when it is designed, operated, and monitored with the marine environment in mind from the very beginning.

The result is a plant that supports Sydney’s water resilience while continuing to demonstrate strong environmental stewardship.