In the first half of the 1900s, the mantra “the solution to pollution is dilution” ruled. The idea was that harmful chemicals and pollutants could be dealt with by spreading them out in the environment.
Now, that approach is derided as outdated and, often, dangerous.
But that approach is still how Sydney deals with most of the 1.5bn litres of sewage the city produces each day.
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.
The eastern Australian current then dilutes the sewage and, generally, carries it south.
But the system might be reaching its limits.
As Guardian Australia revealed on the weekend, a huge fatberg of fats, oils and grease in the Malabar treatment plant likely caused the “debris balls” that washed up on Sydney’s beaches in late 2024 and 2025. It can’t be cleared because it’s in an “inaccessible dead zone”.
The primary problem
The University of Sydney’s Prof Stuart Khan says the primary treatment used at Sydney’s sewage plants is a purely “physical process”.
“You have a very large tank, and the raw sewage comes in at one end and flows through,” Khan, who chairs the New South Wales government’s independent water advisory panel, says.
“When sewage is flowing quite slowly and gently, some of the solid material that’s suspended in it will start to sink to the bottom of the tank, just under gravity.”
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The remaining “effluent” is discharged into the ocean. Khan says the material scraped off the bottom of the tank, known as “sludge”, nowadays receives further treatment and is used in soil remediation in forestry and agricultural areas.
Secondary treatment, used elsewhere in Australia and around the world, involves “biological processes” such as using bacteria to break down organic material, or more sophisticated forms that remove nitrogen and nutrients from effluent, Khan says.
There is also tertiary treatment, which Khan describes as an “additional polishing step”. “Usually it’s some kind of filtration through sand.”
A brief history of Sydney’s sewers
In the late 19th century, sewage was pumped into the harbour itself. “The 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,” Khan says.
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.
After that, turds, sometimes seen floating in the water at Bondi beach, were colloquially known as “Bondi cigars”. The ocean beaches became infamously polluted.
“Incidences of ear infections were not uncommon for people swimming at Bondi. Some beaches like Malabar, which was completely unswimmable … you wouldn’t go in the water,” Khan says.
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.
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.
Environmentalist Richard Gosden was part of a group called Stop the Ocean Pollution (STOP), which campaigned unsuccessfully in the 1980s to add secondary treatment to the Malabar, Bondi and North Head treatment plants.
He says Sydney’s use of primary treatment was unusual, even then.
“They had some pretty rough outfalls in Britain, where they only had primary treatment, but I couldn’t find anyone in the world who was doing it like Sydney Water was,” Gosden says.
“It wasn’t the level of treatment, it was the speed. The primary treatment is just sedimentation tanks [and] the longer it’s left in the sedimentation tanks, the more they get out. But because of the high rate … most of the solid matter was going out with the sewage.”
When the “mystery balls” appeared in October 2024, Gosden and former campaigners “knew straight away” what they were. “We were laughing over the phone. You know? What’s the mystery?”
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.
What’s the solution?
The obvious question is, why doesn’t Sydney upgrade the three coastal treatment plants to include secondary, or even tertiary, treatment?
“The problem is that all of Sydney’s sewers point to the sea,” Godsen says. “When it gets down to the sea, the real estate down there is very expensive, so they don’t have room … to expand their treatment plants.”
Khan agrees, noting: “It 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’d have to reclaim, the land that you’d have to take up, the impact to the amenity would actually be very significant.”
But, he says, if more sewage can be treated inland, emerging technologies could allow secondary treatment with a smaller footprint using “membrane bioreactors”, for example.
A Sydney Water report from 2016 noted “the sewerage system … uses gravity as the fundamental means for moving wastewater and is highly efficient in the energy it expends”.
Khan says the key is more treatment plants in the city’s west “so we don’t wait for the sewage to run all the way … from Liverpool down to Malabar”.
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.
Darren Cleary, the managing director of Sydney Water, says secondary treatment at the coastal plants isn’t needed because the ocean outfalls are operating well overall – and decades of monitoring show no negative environmental impacts.
He notes that water quality at Sydney beaches is generally excellent, and the debris balls were “surprising”. “That’s why we are undertaking these investigations of the cause,” he says.
Adding secondary treatment now would be expensive because the sites are “constrained”.
Sydney Water plans to spend about $32bn to improve the sewerage system over the next 15 years to reduce the amount of sewage sent to the coastal plants.
New facilities will be built stretching from Arncliffe to Quakers Hill.
Treatment plants at Glenfield and Liverpool will be updated so that more recycled water is produced inland. In the short term, the water will be used for datacentres that need cooling and other industrial uses, to take the pressure off drinking supplies.
Sydney Water also wants to upgrade its Fairfield plant to take out more solids.
The Total Environment Centre’s Jeff Angel said last week: “The [deepwater] outfalls are old school technology and our sewerage system needs to be modernised. This should mean a higher level of treatment, but also and importantly, much more recycling.”
What happens in other cities?
Elsewhere, cities don’t dump primary-treated sewage offshore.
During the 1990s, Singapore made major investments in advanced wastewater infrastructure.
All used water is collected through a network of deep sewer tunnels and channelled to water reclamation plants as a result of a SGD$10bn investment.
After undergoing treatment, the water is channelled to factories that produce highly filtered water that’s safe for drinking. Any excess is discharged into the sea.
Singapore now gets up to 40% of its water from the plants.
In the US, Khan says, the Clean Water Act “effectively says that wastewater can’t be discharged without at least secondary treatment”.
Other Australian cities have embraced recycling.
When rainfall patterns changed in Western Australia in the 1990s and flows into metropolitan dams dropped, Perth faced a stark choice: adapt or run dry.
In 2017, Perth commissioned the groundwater replenishment scheme. The Beenyup water recycling plant treats wastewater to a level that exceeds drinking water guidelines.
The water is recharged into underground aquifers, which serve as natural, climate-independent reservoirs. The water remains underground for months or years before being extracted, treated, and supplied to homes.
Melbourne has long had secondary treatment of its effluent but has been upgrading its systems to recycle more water.
The city’s eastern treatment plant treats about 40% the city’s sewage and uses advanced methods like filtration, UV light, and ozone to produce high-quality recycled water, suitable for irrigation, toilet flushing and industrial uses.
The western treatment plant uses large lagoons with mechanical aerators to speed up natural treatment – making it energy-efficient. Werribee lake has become an important wetland for bird life. Tertiary treatment was added in 2004 and recycled water is now used by agriculture.
During the millennium drought, Sydney looked at similar recycling options, before it invested in a desalination plant in the 2000s.
“Sydney’s definitely out of step with the other [Australian] cities,” Khan says.
• This article was amended on 21 January 2026. An earlier version incorrectly stated that Melbourne’s western treatment plant additionally used land filtration.
