Image by/from Lord Belbury
A fatberg is a congealed mass in a sewer system formed by the combination of flushed non-biodegradable solid matter, such as wet wipes, and congealed grease or cooking fat. Fatbergs became a problem in the 2010s in Britain, because of ageing Victorian sewers and the rise in usage of disposable (so-called “flushable”) cloths.
Fatbergs form at the rough surfaces of sewers where the fluid flow becomes turbulent. In pipes and tubes with smooth inner linings, fluid near the containing wall flows only slightly slower than fluid in the central channel of the pipe; thus, the whole volume of fluid flows smoothly and freely. When fluid encounters an obstruction, a resulting swirl of water starts trapping debris.
In some areas, such as London, fat blocked in a sewer can react with the lining of the pipe and undergo saponification, converting the oil into a solid, soap-like substance.
Grease and fat blockages can cause sanitary sewer overflows, in which sewage is discharged into the environment without treatment. In the United States, almost half of all sewer blockages are caused by grease. By using a grease trap, the amount of FOG (fat, oil and grease) reaching the sewer from commercial hot food premises, such as restaurants is greatly reduced.
Fatbergs have been considered as a source of fuel, specifically biogas. Most of the fatberg discovered in Whitechapel in London in 2017, weighing 130 tonnes (130,000 kg) and stretching more than 250 metres (820 ft), was converted into biodiesel.
Curated with thanks from Wikipedia.