Beach grime ball probe points to fatbergs that re-form after treatment
Sewage treatment should destroy oil mountains. But the grim conclusion of an investigation into the source of the dark balls of debris that washed up on Sydney beaches a year ago is that it was only a temporary disruption.
University of Sydney wastewater engineering expert professor Stuart Khan, who chaired the NSW Environmental Protection Agency panel investigating the dirt balls, believes the most likely source of the pollution is deep ocean currents connected to the Malabar wastewater treatment plant.
Like Bondi and North Head, the Malabar plant carries out primary treatment, which only removes solids but does not treat wastewater. Secondary treatment will remove fats, oils and grease through the process of biodegradation. Tertiary purification will add another layer of filtration.
Scientists examined the composition of black balls that washed up on Sydney beaches last October.Credit: Rhett Wyman
Fat bergs form when fats, oils and grease freeze into solids in colder water and combine with debris such as wet wipes. Sydney Water says 20,000 oil bergs and wipes are blocked in its wastewater network every year, at a cost of $27 million.
“The screening process is pretty good, so something the size of a ball that we saw shouldn’t be going through the sewer into the treatment plant,” Khan said. “It seems more likely that the balls of debris falling onto ocean beaches actually came from a deep-water ocean outlet.”
The most likely theory, Khan said, is that oils, oils and grease suspended in fine particles in the water after filtration freeze to form round balls in the four-kilometer-long deep ocean outlet tunnel, which is shaped by the constant warping of the sides of cylindrical pipes. But since the current is below sea level, it was difficult to confirm the theory.
The EPA requested that Sydney Water conduct a number of studies and investigations. The panel evaluated these along with reports from consultants, independent tests of the composition of the balls and hydrodynamic modeling to see how the material would move through the network and along the shoreline, Khan said.
The panel settled on Malabar as the likely source because the debris balls contained hydrocarbons consistent with heavy industry in the basin, Khan said. Smaller primary treatment plants at Bondi and North Head dealt mostly with residential waste.
“Malabar discharges wastewater from Western Sydney to Fairfield,” Khan said. “There’s a lot more industry and the wastewater is actually quite different. Chemical analysis of the tar balls seems to point quite clearly to the Malabar system.”
