As Australia expands its desalination capacity to strengthen water security, understanding how these projects interact with marine ecosystems remains a critical part of project planning and operation.
Speaking at the Australian Water Association's Desalination Specialist Network Member Circle held earlier this year, environmental scientist Michelle Rhodes shared lessons from nearly two decades of monitoring at the Perth Seawater Desalination Plant, one of the first large-scale seawater desalination plants in the Southern Hemisphere.
During the member circle, Rhodes discussed the environmental assessment framework used for large-scale desalination projects, and outlined how baseline ecological surveys (benthic communities and habitats, marine fauna and marine environmental quality), sediment testing, hydrodynamic modelling and plume dispersion studies help predict environmental outcomes well before construction begins. Long-term monitoring programs then verify performance and ensure the operational license and approval conditions are met.
For Rhodes, the Perth plant provides a valuable real-world example of how environmental risks can be understood and managed through science, monitoring and careful design.
"The Perth Seawater Desalination Plant is an excellent case study because it was the first large-scale major desalination plant in the Southern Hemisphere," Rhodes said.
"It was built in a semi-enclosed embayment, and if we can put a desalination plant into an environment like this and look at it over 20 years and see that it's been sustainable, then there are certainly lessons we can bring to other desalination plants."
Located in Cockburn Sound near Kwinana, the plant was developed at a time when public concern about desalination's environmental impacts was particularly high.
"The concerns were that the seawater concentrate would head into the deeper basin of the sound, extend existing stratification periods, drop the dissolved oxygen levels and cause contaminants and nutrients to be released from the sediments, as the sediments had been subject to historical impacts from activities in the Sound," Rhodes said.
"The fear was that this would harm benthic communities and habitats, impact the water quality, seagrass and the marine fauna."
Public debate was intense, with some critics suggesting the project could fundamentally and permanently alter the marine environment.
"At the time, being the first, it gathered a lot of public attention," Rhodes said. "There were front-page newspaper articles talking about Cockburn Sound being turned into a salt lake. People were very concerned about the environmental impacts and also about the energy consumption."
According to Rhodes, these concerns drove one of the most comprehensive environmental assessment programs undertaken for an Australian desalination project at the time, and it exceeded international efforts to understand the risks.
"The Water Corporation did an excellent job in terms of communication. By committing to comprehensive scientific studies in the Sound the risks were defined and it has been proven that the desalination plant could be sustainable and that environmental impacts could be suitably mitigated," she said.
A major focus of the work was understanding how the discharge plume would behave once the plant became operational.
"Mixing is really the key. Every situation is unique. Whether you're on an active coastline or in a relatively quiet embayment like Cockburn Sound, you need to understand how that plume is going to perform and behave," Rhodes said.
Extensive baseline studies were undertaken before construction began. Seagrass meadows, benthic habitats and marine communities were mapped, while sediment samples were tested to better understand how environmental conditions might change under different scenarios.
"Working with Water Corporation and UWA we took sediment cores back to the laboratory and tested at what dissolved oxygen levels those sediments would begin releasing nutrients and contaminants into the water column," Rhodes said.
"It was a very conservative approach designed to make sure the desalination plant would not have catastrophic impacts on the marine environment."
Hydrodynamic modelling played a central role in predicting how seawater concentrate would disperse through the sound.
"The modelling was really focused in on the seasonal winds (mixing is heavily driven by wind in Cockburn Sound) and how the dissolved oxygen would behave because that's tightly linked to stratification, and what low dissolved oxygen events would mean," Rhodes said.
"The question was whether the desalination plant would extend natural stratification events."
To validate the modelling, researchers conducted detailed field studies, including dye tracing experiments that tracked the movement of water through Cockburn Sound.
"It was one of the most detailed modelling studies undertaken by the Water Corporation at that time," Rhodes said.
The findings were encouraging. While elevated salinity was detectable near the discharge point, monitoring showed the plume mixed rapidly with surrounding seawater.
"The studies showed impacts from stratification and dissolved oxygen in the deeper waters could be managed," Rhodes said. " The mixing was enhanced by the diffuser design and the shallow embankment (almost acting like and extended diffuser) at the point of discharge."
These findings were supported by an extensive monitoring program that continued after the plant became operational.
"The regulator required a real-time decision-making framework with contingency triggers," Rhodes said.
"Monitoring buoys were deployed throughout Cockburn Sound and Water Corporation plant operators and the regulator could log in at any time to see what was happening with dissolved oxygen, temperature and conductivity throughout the water column."
The program generated valuable lessons about both environmental performance and operational management.
"One of the key lessons we learned in data gathering was that instruments can foul quite quickly, particularly during summer months with diatoms," Rhodes said.
"Routine calibration, maintenance and understanding what's driving the fouling are important to ensure data remains accurate and defensible."
The Perth experience helped establish an environmental management framework that has informed subsequent desalination developments across Western Australia, including the Southern Seawater Desalination Plant at Binningup and the Alkimos Desalination Plant currently under construction.
For Rhodes, the broader lesson is that desalination is now an increasingly important component of Australia's water future.
"Without desalination, Perth would be in a water supply crisis," she said.
"It's a very important part of the Integrated Water Supply Scheme and will continue to play a vital role in providing fresh water to Australia."
However, she noted that future projects will face new challenges, including climate change and population growth, competing coastal developments, changing intake water qualities and increasing scrutiny around sustainability performance.
"We continue to see reduced long-term rainfall, longer droughts and higher average temperatures," Rhodes said.
"Desalination provides a climate-independent source of water, which is incredibly important, but we also need to continue improving how these plants are designed and operated."
That includes greater use of renewable energy, more efficient recovery systems and advances in membrane technologies.
"Sustainability has to be a foundation stone," Rhodes said. "We need to think about energy efficiency, embodied carbon, circular economy opportunities and how we continue to reduce our environmental impact."
After two decades of monitoring and operational experience, Rhodes believes the evidence demonstrates that desalination and a healthy marine ecosystems can coexist when projects are supported by robust science and ongoing environmental stewardship.
"By proving that it was sustainable and manageable, that was very important for the future of desalination in Australia," she said.
Are you interested in learning more about AWA's Desalination Specialist Network? Take a closer look here.