Source of ‘disgusting’ grime balls that closed Sydney beaches revealed

The state’s environmental watchdog has blamed Sydney Water’s ageing sewerage network as the likely source of thousands of grime balls that forced the closure of several Sydney beaches last October.

The greasy black spheres about the size of tennis balls were first spotted on Coogee Beach in October, affecting 11 beaches from Bondi to Botany Bay, and sparked a multi-agency investigation to determine their source.

Black balls washed ashore at Gordons Bay in Sydney in October 2024.Credit: Janie Barrett

Analysis by the University of NSW found the balls were a “disgusting” mix of cooking oil, soap scum, faeces, medication and recreational drugs, and plastics.

The NSW Environment Protection Authority announced on Friday its investigation, which included later incidents on the northern beaches and the South Coast and took samples from visiting cruise ships, concluded that Sydney Water’s land-based sewage treatment network was the likely source.

The technical investigation found similarities between the make-up of the debris balls and samples taken from several major wastewater treatment networks, including those at Malabar and Bondi.

EPA operations director Adam Gilligan said in a statement that the EPA had issued an investigation notice to Sydney Water to try to find the precise source or sources.

UNSW scientists studied the composition of the black balls that washed up on Sydney’s beaches.Credit: Rhett Wyman

“This is a significant step forward in our investigation, but there is still work to do,” Gilligan said.

“While we are yet to determine exactly what caused the pollution incidents to occur when they did, we can say the composition and the characteristics of the debris balls are consistent with a land-based sewage source.”

The investigation notice required Sydney Water to undertake oceanographic modelling of the dispersion of the balls, complete a sampling and analysis program at its sewage treatment plants, and assess its deep ocean outfall systems at Bondi and Malabar and its sewage pipe network to find the fault and prevent a recurrence.

The EPA has established a wastewater expert panel to provide independent scientific advice to the EPA and other agencies, to be chaired by Professor Stuart Khan, head of civil engineering at the University of Sydney.

Smaller white debris balls washed up on the northern beaches in January.Credit: James Brickwood

Sydney Water’s acting executive general manager of water and environment services, Louise Beer, said in a statement that it was important to note all coastal treatment facilities were operating normally and the organisation was compliant with regulatory standards.

“As we could not find any faults with our system, we conducted widespread sampling and analysis of the debris balls at Sydney Water’s laboratories and appointed an independent oceanographer to determine the potential geographic origin of the debris balls,” Beer said.

Sydney Water’s statement says the balls probably came from 10 or more separate discharge events from various locations rather than a single origin point.

The state-owned corporation suggested the debris balls might have formed due to an “increased load of fats, oils and greases in the wastewater system over time” and reminded Sydneysiders to keep them out of drains and put them in the bin instead.

Sydney Water operates two massive wastewater treatment plants, one at Bondi and one at Malabar, which discharge sewage effluent two to four kilometres offshore. The waste receives primary treatment – or what Sydney Water calls “advanced primary treatment” – to remove large solids, but not secondary treatment that would break down remaining organic matter.

Khan said that if the grease balls had come from the treatment plants, then upgrading to secondary treatment would help, though it would cost billions of dollars.

However, he said the material might actually be released before the treatment plant because of overflows during wet weather.

“If it turns out that they’re being released further up in the system, before the sewage treatment plant, then it doesn’t actually matter what you do at the sewage treatment plant, you’re not going to solve the problem,” Khan said.

Ian Wright, a water scientist at Western Sydney University, said the Malabar ocean outfall discharges the largest volume of treated sewage in Australia – the combined output of more than 1.5 million people.

“The EPA will need to tighten the environmental regulation of the sewage treatment plants, and then Sydney Water will need to upgrade their processes,” Wright said.

“It is likely that improved sewage treatment will need to be installed. The scale of the challenge is enormous.”

The current EPA licence for Malabar authorises Sydney Water to release an annual load of many pollutants to the ocean, including 9261 tonnes of oil and grease. Wright said that “under the right conditions that could make a massive number of ‘tar balls’.”

Beer said the network was 136 years old and under additional pressure from housing growth and climate change. Sydney Water was considering options to upgrade wastewater treatment plants and networks over the next decade, she said.

Avoid touching debris balls or other pollution on beaches and report new finds to your local council or EPA’s Environment Line on 131 555.