Major desalination study finds minimal marine impact

A world-first major study into the marine ecological impacts of a large-scale water desalination plant have been publicly released — and the results may surprise you.

The six-year study on the Sydney Desalination Plant looked at the effects on marine invertebrate recruitment of the pumping and ‘diffusing’ of high-concentration salt water back into the ocean. It was commissioned by Sydney Water — the original owner of the plant — and led by UNSW Sydney, with a team including scientists from NSW Fisheries Research and Southern Cross University.

The logistically challenging study took place at six underwater locations, at a depth of about 25 m, over a six-year period during which the plant was under construction, then operating and then idle. This enabled the team to rigorously monitor for impacts and recovery among marine life from the effects of pumping large volumes of hyper-saline water into the ocean.

Study lead Dr Graeme Clark said the results, published in the journal Water Research, were surprising as they debunked the prevailing understanding that high salt levels in the outfall brine would be toxic to marine life. The findings instead showed that the main effect occurred over a small area within 100 m of where the outlets were located and were likely the result of changes to water flow.

“The high-pressure diffusers that return the high-concentrate salt water to the ocean at a high velocity are so effective at diluting the brine that concentrations were almost at background levels within 100 m of the outfall,” he said. “This is the result of good engineering and good modelling behind the diffuser design.

“And within this small 100 m impact zone, some invertebrates increased in numbers — the ones that benefit from high-flow conditions such as barnacles — and some were reduced in quantity or did not recover after shutdown of the outflow, and these were the species with slow swimming larvae, such as tubeworms, lace corals and sponges.”

Senior author Professor Emma Johnston said the results are timely as they coincide with renewed focus on desalination, with the plants in Sydney and Melbourne preparing to start operations due to rapidly decreasing water supplies.

“Increasingly, frequent and severe climate- and population-driven water shortages are projected to accelerate the growth in desalination in the coming years,” said Prof Johnston, who serves as UNSW Dean of Science.

“We do need reliable and high-quality drinking water and we need to understand the potential for ecological damage from the act of extracting it from the ocean. There is a real possibility that the Sydney desalination plant could be expanded to meet demand if storage gets critical, with the original design having the potential for a doubling of its size in the future should the need arise.

“The good news is that our comprehensive study shows the hyper-saline outflow from this modern plant is not having a major impact on the NSW coast.”

Prof Johnston did, however, stress that the results do not mean the urgent need for more sustainable water use can be ignored, noting, “Water saving and water re-use practices should be the first strategies considered by any state. Our study simply shows that the potential marine ecology impacts from a well-designed and well-located desalination plant should not immediately prevent us from considering desalination for the supply of fresh drinking water in a period of severe, prolonged drought.”