A fatberg is a massive, rock-hard mass of congealed fat, oil, grease, and non-biodegradable waste that forms inside sewer systems. These blockages can grow to enormous sizes, with the largest on record stretching 250 meters (about 820 feet) and weighing over 130 tonnes. They form when cooking fats solidify underground and bond with items like wet wipes, sanitary products, and cotton buds that should never have been flushed.
How Fatbergs Form
The process starts with cooking fat. When you pour used oil or grease down the drain, it cools as it travels through the pipes and begins to solidify. Used cooking oil is especially problematic because frying breaks down the oil and increases the concentration of fatty acids, particularly palmitic acid, which solidifies easily at lower temperatures.
Once those fats reach the sewer, a chemical reaction called saponification kicks in. Metal ions from food waste, wastewater, and dishwashing detergents react with the fatty acids, causing them to clump together and harden. In concrete sewer pipes, this problem gets worse: calcium ions leach out of the pipe walls and accelerate the reaction, essentially turning liquid grease into a soap-like solid that clings to the pipe interior.
As this greasy layer builds up, it traps anything that passes through: wet wipes, diapers, condoms, dental floss, and other items that don’t break down. Over weeks and months, the mass grows denser and harder, eventually forming a blockage that can fill an entire sewer tunnel. The resulting fatberg has the consistency of concrete and often has to be broken apart with high-pressure water jets, pickaxes, or even specialized machinery.
What’s Inside a Fatberg
A 2018 forensic examination of the famous Whitechapel fatberg in London revealed what scientists and sewer workers had long suspected: these masses are biologically hazardous. Researchers found potentially infectious bacteria including listeria, campylobacter, and E. coli living inside the fatberg material. The warm, nutrient-rich environment of a sewer provides ideal conditions for these pathogens to thrive.
Beyond bacteria, fatbergs contain a stew of personal waste products. Wet wipes are the single biggest non-fat contributor, since even products labeled “flushable” rarely break down fast enough to avoid getting trapped. Sanitary products, cotton buds, and food waste round out the mix, all bound together by hardened grease into a foul-smelling, pale grey mass.
The Largest Fatbergs on Record
The most famous fatberg was discovered under Whitechapel in East London in September 2017. It stretched 250 meters, roughly the length of two and a half football fields, and weighed over 130 tonnes. Removing it took weeks of round-the-clock work by teams of sewer engineers.
In December 2018, sewer workers in Sidmouth, Devon found a 64-meter fatberg, notable partly because Sidmouth is a small seaside town with a population of only about 13,000. That discovery made clear that fatbergs aren’t just a big-city problem. Any sewer system receiving cooking grease and non-biodegradable waste is vulnerable.
Environmental and Public Health Risks
When a fatberg blocks a sewer, wastewater has nowhere to go. It backs up through the system and can overflow into streets, basements, and waterways. In 2017, a fatberg in Baltimore caused a sewer overflow that discharged 1.2 million gallons of untreated wastewater into the local Jones Falls waterway. Because this wastewater contains raw human sewage, it creates a direct public health hazard and can spread disease through contaminated water.
Backups don’t just affect private homes. They can hit apartment buildings, restaurants, businesses, and healthcare facilities, all places where exposure to sewage-contaminated water carries serious risk. For sewer workers, the danger is even more direct. Breaking apart a fatberg means spending hours in confined spaces surrounded by material harboring harmful bacteria, sharp debris, and toxic gases.
Turning Fatbergs Into Fuel
The Whitechapel fatberg didn’t just end up in a landfill. After a cleanup operation lasting more than three weeks, Thames Water sent roughly 50 tonnes of the material to Argent Energy, a waste-to-power firm, to be converted into biodiesel.
The conversion process is intensive. The raw fatberg material goes into a large reception pit with strainers to remove solid debris. It then passes through multiple filtration and dewatering stages before the chemical processing begins. The fat undergoes a reaction called transesterification, which converts the fatty acids into usable fuel oil. Argent’s process includes double distillation to remove impurities, producing a high-purity biodiesel that can be blended with conventional diesel fuel. The company’s ability to handle fat with extremely high free fatty acid content, far higher than typical biodiesel feedstocks, is what makes processing fatberg material feasible at all.
How to Prevent Fatbergs
Fatbergs are almost entirely preventable. The two main contributors are cooking grease poured down drains and non-biodegradable items flushed down toilets. Letting cooking fat cool and scraping it into the trash eliminates the primary binding agent. Wiping greasy pans with paper towels before washing them makes a measurable difference to what enters the sewer system.
For flushing, the rule is straightforward: only human waste and toilet paper belong in a toilet. Wet wipes, even those marketed as flushable, do not disintegrate quickly enough to pass safely through sewer infrastructure. Cotton buds, dental floss, sanitary products, and condoms should all go in the bin. These are small individual choices, but fatbergs are built one flush at a time.