Fatbergs in London Sewers: What They Are and What They Mean for Homeowners
Fatbergs make headlines when they are discovered blocking major London sewers, but the same process that creates them starts in the kitchens and bathrooms of individual homes. Understanding how fatbergs form helps homeowners protect their own drains and avoid contributing to a citywide problem.
What Is a Fatberg
A fatberg is a solid mass formed in a sewer when fats, oils, and grease from cooking combine with non-biodegradable materials flushed down toilets and sinks — principally wet wipes, cotton wool, sanitary products, and condoms. The grease acts as a binding agent, coating the flushed materials and causing them to adhere to each other and to the walls of the sewer pipe. Over time, successive layers accumulate and harden through saponification — a chemical reaction between the fat and alkaline materials in the sewage — producing a substance that has the consistency of concrete and can be extremely difficult to remove.
Fatbergs are not a new phenomenon, but they have grown dramatically in size and frequency as the use of single-use wet wipes has increased. The largest fatberg ever found in the UK was discovered beneath Whitechapel in east London in 2017 and weighed approximately 130 tonnes, extending for 250 metres through the sewer. It took engineers several weeks to remove. Fatbergs have since been found across many London boroughs and in cities throughout the UK.
How Fatbergs Form: The Role of Individual Households
A fatberg does not form suddenly. It develops incrementally from contributions made by hundreds or thousands of individual households over months or years. Each time a pan of cooking fat is rinsed down a kitchen sink, a wet wipe is flushed down a toilet, or cotton pads are disposed of via a bathroom waste, a small contribution is made to the conditions in which fatbergs form. No single household creates a fatberg, but every household in a street or neighbourhood is a potential contributor.
The chemistry of fatberg formation means that temperature plays an important role. London has relatively cold winters, and the ground temperature around shallow sewer runs drops significantly in cold weather. Fats that remain liquid at household temperatures solidify rapidly in cold sewers and adhere more readily to pipe walls and to other materials. This is why fatbergs tend to grow fastest in autumn and winter and why fatberg-related blockages peak during and after cold spells.
Older parts of the London sewer network are particularly susceptible to fatberg formation because brick-lined sewers have a rougher internal surface than modern smooth-bore pipes, providing more surface area for fat to adhere to and more ledges and crevices for non-degradable materials to catch on. The Victorian sewers in inner London boroughs — Southwark, Lambeth, Islington, Tower Hamlets, and many others — carry disproportionate fatberg risk compared with more recently constructed suburban sewers.
What Fatbergs Mean for Your Home Drainage
A fatberg forming in the public sewer beneath a street can cause backflows into the drainage systems of individual properties. When the sewer is obstructed, sewage backs up through the drain runs connecting properties to the main sewer. This can result in foul water appearing in ground-floor toilets, in gulley drains in back gardens, or in internal inspection chambers. In severe cases it can cause sewage flooding in basements or at ground level. Thames Water is responsible for clearing blockages in public sewers, but the disruption caused to affected properties — and the potential for property damage from sewage flooding — is borne by the homeowner.
Even short of a full fatberg in the public sewer, fat and grease build-up in the private drain lateral connecting a property to the sewer is a common cause of internal drain blockages. This section of drain — from the property boundary to the building — is the responsibility of the homeowner to maintain. A partial grease blockage in the private lateral can cause slow drainage throughout the property, intermittent gurgling from drains, and eventually a full blockage requiring professional clearance.
How to Avoid Contributing to Fatbergs
The preventive steps are simple but require a change in household habits. Cooking fats, oils, and grease should never be poured down any drain — kitchen sink, outside gully, or otherwise. They should be cooled, collected in a sealed container such as an old jar or tin, and disposed of in the general household waste or at a dedicated fat recycling point. Food scraps should be scraped from plates into a food waste bin rather than rinsed down the sink. Wet wipes — even those labelled as flushable — should be disposed of in a bin, not flushed. Only toilet paper should go down the toilet; sanitary products, cotton wool, cotton buds, and nappies must go in the bin.
Prestige Engineers carry out drain jetting and clearance work across all London boroughs and can remove grease and fat build-up from private drain laterals before it contributes to larger blockages in the public sewer. Regular preventive jetting of kitchen waste pipes is a cost-effective way for London homeowners and landlords to maintain free-flowing drainage and avoid emergency blockage call-outs.