Concerns about sewage discharge in Sydney's drinking water catchment
University of Western Sydney Associate Professor Ian Wright said his plea for "urgent action" followed a study he did with PhD student Catherine Warrick.
Dr Wright said that although Sydney's drinking water was safe to drink, some of the pollution he detected was "close" to crisis levels.
"We need sewage treatment plants that have the highest possible standard … and we need to better manage the effluent created by a growing population," he said.
The study, which was peer-reviewed and published in the Urban Science Journal, involved taking water samples around the Goulburn, Lithgow, Mittagong, Bowral and Moss Vale STP outfalls on four occasions between October 2023 and February 2024.
Effluent discharged from those locations flows into the Warragamba Dam catchment, which, under the management of WaterNSW, supplies 90 per cent of its water to more than 5 million Greater Sydney residents.
Samples were collected at varying distances from the STP outfalls, depending on accessibility, with downstream sites between 25 metres and 2 kilometres away.
Upstream samples were collected at sites 50m to 750m from the various outfalls.
The researchers found the concentration of total nitrogen in samples taken upstream of STP outfalls was, on average, 486 micrograms per litre — almost twice the recommended WaterNSW catchment river benchmark guideline.
Downstream from the outfalls, the average was 2,820 micrograms per litre — about 10 times the guideline.
Total phosphorous concentration upstream was, on average, double the guideline at 41 micrograms a litre, while the average reading downstream was 102 micrograms a litre.
Dr Wright said the situation could lead to a series of problems across the catchment.
A WaterNSW spokesperson said in a statement that it utilised advanced monitoring technology to identify the best quality water in Warragamba to supply to Sydney Water for treatment and distribution.
"When blue green algae (BGA) does occur, it can be managed using Warragamba's multi-level offtake capacity, which can select water from various depths to avoid the BGA."
It said Warragamba "rarely" experienced problematic levels of BGA, with the most serious outbreak in 2007.
Dr Wright said while Sydney's tap water was safe to drink, the catchment should have "industry-best" standards.
The Mittagong STP in the Southern Highlands, where the sampling sites were closest to the outfall (50 metres upstream and 25 metres downstream), produced some of the study's "worst" results, according to Dr Wright.
He said the total nitrogen levels detected both upstream and downstream from the plant were, on average, about 20 times higher than the catchment guideline.
The NSW EPA regulates discharge into zones where effluent from a pipe or outlet is released into a wider waterway.
A statement from the EPA said it considered a "range of factors, including the environmental values of the receiving waters", when setting or reviewing licence limits.
It said that, in accordance with its licence, the Mittagong STP had not recorded any breaches of its nitrogen discharge limits in the last 10 years.
Dr Wright said the EPA's permitted concentrations were not strict enough.
"Mittagong STP's licence allows nitrogen to be released at 10,000 micrograms per litre, which is 40 times higher than the WaterNSW catchment guideline of 250 micrograms per litre."
University of Sydney School of Civil Engineering head Professor Stuart Khan said the study was a "perfectly valid" demonstration that STPs were a source of waterway pollution.
But he said comparing EPA licence regulations with catchment guidelines might not be the best approach.
"You could certainly do so for the purpose of acquiring information and undertaking risk assessment," he said.
"But I don't think it's an appropriate comparison for regulatory purposes."
Wingecarribee Shire Council, which operates the Mittagong STP, said in a statement that it had already implemented improvements to enhance nitrogen and phosphorus removal, and "further reductions are expected as current plant upgrades are completed".
The NSW EPA declined the ABC's offer for an interview.
However, a spokesperson said in a statement that "all EPLs [Environment Protection Licence] are reviewed at least every five years … this approach provides robust protection for waterways, particularly sensitive catchments such as Warragamba".
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