A fatberg is a massive, rock-hard lump that forms inside sewer pipes when cooking fats, oils, and grease combine with non-biodegradable items like wet wipes, sanitary products, and tissues. The most famous example, discovered beneath Whitechapel in London in 2017, stretched 250 meters long (roughly the length of Tower Bridge) and weighed an estimated 130 tonnes, about as heavy as 11 double-decker buses.
These blockages aren’t just gross curiosities. They cost the UK alone up to £200 million a year to clear, and when left untreated, they can force raw sewage to overflow into streets and waterways.
How Fatbergs Form
It starts with what goes down the drain. When you pour leftover cooking oil into the kitchen sink or flush wet wipes down the toilet, those materials enter the sewer system and begin interacting with each other in ways that create something far worse than either would alone.
Cooking oil doesn’t stay liquid once it cools. Used cooking oil contains high concentrations of free fatty acids, particularly palmitic acid, which form during the frying process. Once those fatty acids reach the sewer, they react with calcium and other metal ions dissolved in the wastewater, a chemical process called saponification. This is essentially the same reaction used to make soap, except in a sewer it produces a hard, calcium-based grease deposit that clings to pipe walls. Concrete sewer pipes make this worse because calcium leaches directly from the pipe material, accelerating the buildup.
Wet wipes and facial tissues act as the structural skeleton. Their large, porous fibers trap sewer solids and grease, dramatically increasing the overall mass. Lab research has shown that adding wet wipes to used cooking oil nearly doubles the amount of solid deposits compared to oil alone. Over weeks and months, layers of grease, wipes, food particles, and other flushed debris compact together into a dense, concrete-like mass that narrows or completely blocks the pipe.
What’s Actually Inside One
The bulk of a fatberg is congealed fats, oils, and grease (often abbreviated FOG in the water industry), but that’s only part of the picture. Embedded throughout are wet wipes, cotton buds, sanitary products, condoms, food scraps, and anything else people flush or wash down drains. Microbiological analysis of fatberg samples has found bacteria typically associated with the human gut, including species that can cause infections in people with weakened immune systems. Researchers have identified potentially harmful bacterial strains within fatberg material, raising concerns about the health risks these masses pose to sewer workers who have to break them apart by hand.
Why They Keep Growing
Products labeled “flushable” are a major contributor. Unlike toilet paper, which disintegrates in water within minutes, so-called flushable wipes do not break down in sewer conditions. They travel intact through the system, snagging on pipe joints and existing grease deposits, giving the fatberg more material to grow around.
The problem is self-reinforcing. A small grease deposit narrows the pipe, which slows water flow, which makes it easier for more debris to accumulate. Once a fatberg reaches a certain size, normal water pressure can’t dislodge it.
How Crews Remove Them
Small grease deposits can sometimes be treated with industrial degreasers that liquefy the fat and allow it to flush through the system. But once a fatberg solidifies into a large mass, removal becomes a physical job.
The primary tool is hydro jetting: high-pressure water nozzles blast the fatberg into chunks, which are then suctioned out by vacuum trucks. For the largest blockages, workers may need to enter the sewer and break the mass apart manually. The 2017 Whitechapel fatberg took weeks of round-the-clock work to clear. When a new 100-meter, 100-tonne fatberg was discovered in the same Whitechapel area in late 2025 (dubbed “the grandchild” of the original), Thames Water warned that extraction could again take weeks.
The costs add up fast. Beyond the direct expense of crews and equipment, fatberg removal means disrupted roads, diverted traffic, and the risk of sewage overflows during the process.
Environmental Damage From Blockages
When a fatberg blocks a sewer pipe, wastewater has nowhere to go. The result is sewer overflows, where untreated sewage spills into streets, basements, or nearby rivers and streams. In Ireland, water authorities responded to roughly 10,000 sewer blockages in a single year, many caused by grease and non-flushable items.
Raw sewage entering waterways depletes dissolved oxygen, which can kill fish and other aquatic life. It also introduces harmful bacteria into environments where people swim, fish, or draw drinking water. Urban flooding from backed-up sewers brings sewage into direct contact with homes and public spaces, creating immediate public health risks.
How to Prevent Fatbergs at Home
The single most effective thing you can do is keep fats, oils, and grease out of your drains. After cooking, let oil and grease cool in the pan, then scrape it into your compost bin or trash. Wipe greasy pans with a paper towel before washing them. Never pour cooking oil down the sink, into the garbage disposal, or into the toilet.
Chemical drain cleaners that claim to dissolve grease don’t actually solve the problem. They simply move the grease further down the pipe, where it solidifies in a section you can’t reach. Some municipalities have explicitly banned these products for this reason.
For wipes, the rule is straightforward: only toilet paper should be flushed. Everything else, including products marketed as flushable, belongs in the trash. If a product doesn’t disintegrate in a glass of water within a few minutes the way toilet paper does, it will survive the journey through your pipes and contribute to the next fatberg forming downstream.