Concerns are escalating about the quality of treated effluent released from sewage treatment plants (STPs) in the catchment area supplying drinking water to Sydney. Urban water scientist Associate Professor Ian Wright from the University of Western Sydney is urging immediate action following a study he conducted with PhD student Catherine Warrick. Their research indicates that pollution levels in some areas are approaching crisis thresholds, despite assurances that Sydney’s tap water remains safe.
Dr. Wright’s study, published in the Urban Science Journal, involved water sampling at various points near STP outfalls, including Goulburn, Lithgow, Mittagong, Bowral, and Moss Vale. Samples were collected over four occasions from October 2023 to February 2024. Effluent from these locations flows directly into the Warragamba Dam catchment, responsible for supplying approximately 90 percent of the drinking water for over 5 million residents in Greater Sydney.
The research revealed alarming nutrient levels in the water. Upstream of the STP outfalls, samples showed an average concentration of total nitrogen at 486 micrograms per litre, nearly double the recommended WaterNSW catchment river benchmark. Downstream, this figure soared to an average of 2,820 micrograms per litre—about ten times the guideline. Similarly, total phosphorus levels were concerning, with upstream concentrations averaging 41 micrograms per litre and downstream readings reaching 102 micrograms per litre.
Dr. Wright emphasized the need for sewage treatment facilities to adhere to the highest standards, highlighting the increasing population pressures contributing to the problem. He has called on the NSW EPA to enforce stricter nutrient level limits for effluent discharged from STPs.
WaterNSW, in response to the findings, stated that it employs advanced monitoring technologies to ensure the best quality water from Warragamba is supplied for treatment and distribution. A spokesperson noted that while blue-green algae outbreaks can occur, they are manageable due to the dam’s multi-level offtake capacity, which selects water from various depths to avoid contamination. The last significant blue-green algae issue occurred in 2007.
While Sydney’s drinking water is deemed safe, Dr. Wright argues that the catchment area should maintain “industry-best” standards to protect public health. He pointed out that the Mittagong STP produced some of the study’s worst results, with nitrogen levels about 20 times higher than the catchment guideline. This facility has been operating beyond its design capacity for two years, prompting the need for upgrades that were finally approved in May 2023.
The NSW EPA regulates the discharge from STPs and considers various factors when setting license limits. In a statement, the EPA mentioned that the Mittagong STP had not breached its nitrogen discharge limits for the past decade, despite the troubling findings from Dr. Wright’s study. He criticized the EPA’s allowable nitrogen level of 10,000 micrograms per litre, stating it is 40 times greater than the WaterNSW guideline of 250 micrograms per litre.
Professor Stuart Khan, head of the School of Civil Engineering at the University of Sydney, acknowledged the study’s validity in demonstrating that STPs contribute to waterway pollution. However, he cautioned against directly comparing EPA regulations with catchment guidelines for regulatory purposes.
In response to the study’s findings, the Wingecarribee Shire Council, which operates the Mittagong STP, noted that it has already started implementing improvements to enhance the removal of nitrogen and phosphorus. Further reductions are anticipated as ongoing plant upgrades are completed.
The research was initially prompted by the discovery of persistent chemicals in the liver of a deceased platypus found in the Wingecarribee River. In light of the study, the NSW EPA has stated that it reviews all Environment Protection Licences at least every five years to ensure robust protection for sensitive waterways like Warragamba.
The situation calls for urgent attention to safeguard the quality of Sydney’s drinking water and the health of its surrounding ecosystems.
