The blockage was discovered on July 17 when residents of the affluent southwestern suburb of Kingston upon Thames reported trouble with flushing their toilets.
A video posted online Tuesday shows the moment a remote camera discovered the enormous lump, which weighed as much as two fully grown African elephants.
"The sewer was almost completely clogged with over 15 tons of fat," said Gordon Hailwood, water contracts supervisor for the local water utility, Thames Water.
Workers used a high-pressure water jet to remove the blockage from the Victorian-era sewer, according to Graeme Sanderson of CountyClean Environmental Services, the company that performed the work.
"The mound of fat had reduced the 70-by-48-centimetre sewer to just 5 per cent of its normal capacity," Sanderson wrote online.
Fatbergs build up in sewer systems as fat, oil and grease are transported along with raw sewage.
Once a fat deposit forms on a sewer wall, other fat tends to build up on the site, causing wrongly flushed items such as wet wipes and sanitary items to stick to it.
Just as cholesterol can slowly constrict human arteries, the fat buildups can go unnoticed until they constrict the sewer, with catastrophic, or at least disgusting, consequences.
"If we hadn't discovered it in time, raw sewage could have started spurting out of manholes across the whole of Kingston," Hailwood said.
The sewer was damaged by the enormous deposit, Hailwood said, and repairs would take six weeks.
Fatbergs often form after special occasions such as Christmas, when many households simultaneously pour fat and oil from a large meal down the drain.
Although fatbergs pose problems for sewer systems, they are also a potential source of energy that would otherwise go to waste.
In April, Thames Water signed a 20-year deal with energy firm 20C to generate renewable electricity from fats and oils retrieved from the Beckton treatment works in east London.
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