In the first half of the 1900s, the mantra \u201cthe solution to pollution is dilution\u201d ruled. The idea was that harmful chemicals and pollutants could be dealt with by spreading them out in the environment.<\/p>\n
Now, that approach is derided as outdated and, often, dangerous.<\/p>\n
But that approach is still how Sydney<\/a> deals with most of the 1.5bn litres of sewage the city produces each day.<\/p>\n Sydney is one of the few cities in the world that deals with its sewage through what is known as fast primary treatment, a process that involves removing solids before it is pumped 2-4km out to sea via the three deepwater ocean outfalls (DOOFs) at Malabar, Bondi and North Head.<\/p>\n The eastern Australian current then dilutes the sewage and, generally, carries it south.<\/p>\n But the system might be reaching its limits.<\/p>\n As Guardian Australia revealed on the weekend<\/a>, a huge fatberg of fats, oils and grease in the Malabar treatment plant likely caused the \u201cdebris balls\u201d that washed up on Sydney\u2019s beaches in late 2024 and 2025. It can\u2019t be cleared because it\u2019s in an \u201cinaccessible dead zone\u201d.<\/p>\n Sydney Water\u2019s North Head wastewater treatment plant. Composite: AAPThe primary problem<\/p>\n The University of Sydney\u2019s Prof Stuart Khan says the primary treatment used at Sydney\u2019s sewage plants is a purely \u201cphysical process\u201d.<\/p>\n \u201cYou have a very large tank, and the raw sewage comes in at one end and flows through,\u201d Khan, who chairs the New South Wales government\u2019s independent water advisory panel, says.<\/p>\n \u201cWhen sewage is flowing quite slowly and gently, some of the solid material that\u2019s suspended in it will start to sink to the bottom of the tank, just under gravity.\u201d<\/p>\n Sign up: AU Breaking News email<\/a><\/p>\n The remaining \u201ceffluent\u201d is discharged into the ocean. Khan says the material scraped off the bottom of the tank, known as \u201csludge\u201d, nowadays receives further treatment and is used in soil remediation in forestry and agricultural areas.<\/p>\n Secondary treatment, used elsewhere in Australia and around the world, involves \u201cbiological processes\u201d such as using bacteria to break down organic material, or more sophisticated forms that remove nitrogen and nutrients from effluent, Khan says.<\/p>\n There is also tertiary treatment, which Khan describes as an \u201cadditional polishing step\u201d. \u201cUsually it\u2019s some kind of filtration through sand.\u201d<\/p>\n A brief history of Sydney\u2019s sewers<\/p>\n In the late 19th century, sewage was pumped into the harbour itself. \u201cThe sewers directed straight down towards Circular Quay, some of it off Bennelong Point where the Opera House is, so the harbour was becoming polluted and filthy,\u201d Khan says.<\/p>\n In the early 20th century, pumping stations were built to direct sewage straight to cliff face outfalls. The Bondi, Malabar and North Head primary treatment plants were constructed in the middle of last century.<\/p>\n After that, turds, sometimes seen floating in the water at Bondi beach, were colloquially known as \u201cBondi cigars\u201d. The ocean beaches became infamously polluted.<\/p>\n \u201cIncidences of ear infections were not uncommon for people swimming at Bondi. Some beaches like Malabar, which was completely unswimmable \u2026 you wouldn\u2019t go in the water,\u201d Khan says.<\/p>\n When Sydney was pondering what to do next, there were two main options, Khan says. One was to upgrade the treatment plants to conduct secondary treatment. The alternative was to build deepwater ocean outfalls and move the problem farther offshore.<\/p>\n The latter option won out, mainly due to cost considerations, and DOOFs extending up to 4km to sea were built in the 1980s. Malabar opened first in 1990.<\/p>\n Environmentalist Richard Gosden was part of a group called Stop the Ocean Pollution<\/a> (STOP), which campaigned unsuccessfully in the 1980s to add secondary treatment to the Malabar, Bondi and North Head treatment plants.<\/p>\n He says Sydney\u2019s use of primary treatment was unusual, even then.<\/p>\n \u201cThey had some pretty rough outfalls in Britain, where they only had primary treatment, but I couldn\u2019t find anyone in the world who was doing it like Sydney Water<\/a> was,\u201d Gosden says.<\/p>\n \u201cIt wasn\u2019t the level of treatment, it was the speed. The primary treatment is just sedimentation tanks [and] the longer it\u2019s left in the sedimentation tanks, the more they get out. But because of the high rate \u2026 most of the solid matter was going out with the sewage.\u201d<\/p>\n When the \u201cmystery balls\u201d appeared in October 2024, Gosden and former campaigners \u201cknew straight away\u201d what they were. \u201cWe were laughing over the phone. You know? What\u2019s the mystery?\u201d<\/p>\n The poo balls likely formed by the Malabar fatberg were being ejected 2.3km out to sea and carried back to shore by waves and wind.<\/p>\n What\u2019s the solution?<\/p>\n The obvious question is, why doesn\u2019t Sydney upgrade the three coastal treatment plants to include secondary, or even tertiary, treatment?<\/p>\n \u201cThe problem is that all of Sydney\u2019s sewers point to the sea,\u201d Godsen says. \u201cWhen it gets down to the sea, the real estate down there is very expensive, so they don\u2019t have room \u2026 to expand their treatment plants.\u201d<\/p>\n Khan agrees, noting: \u201cIt would be very difficult for a government to win community support to build a secondary sewage treatment plant in Bondi, for example. The houses that you\u2019d have to reclaim, the land that you\u2019d have to take up, the impact to the amenity would actually be very significant.\u201d<\/p>\n But, he says, if more sewage can be treated inland, emerging technologies could allow secondary treatment with a smaller footprint using \u201cmembrane bioreactors\u201d, for example.<\/p>\n A Sydney Water report from 2016<\/a> noted \u201cthe sewerage system \u2026 uses gravity as the fundamental means for moving wastewater and is highly efficient in the energy it expends\u201d.<\/p>\n Khan says the key is more treatment plants in the city\u2019s west \u201cso we don\u2019t wait for the sewage to run all the way \u2026 from Liverpool down to Malabar\u201d.<\/p>\n Sydney does have smaller secondary and tertiary plants, many of which discharge into the Hawkesbury-Nepean system, but 80% of effluent is still ejected through the three deepwater ocean outfalls.<\/p>\n Darren Cleary, the managing director of Sydney Water, says secondary treatment at the coastal plants isn\u2019t needed because the ocean outfalls are operating well overall \u2013 and decades of monitoring show no negative environmental impacts.<\/p>\n He notes that water quality at Sydney beaches is generally excellent, and the debris balls were \u201csurprising\u201d. \u201cThat\u2019s why we are undertaking these investigations of the cause,\u201d he says.<\/p>\n Adding secondary treatment now would be expensive because the sites are \u201cconstrained\u201d.<\/p>\n