The most disgusting thing you’ve never heard of is haunting sewers around the world. Fatbergs, as they’re called, are deposits of fat and grease mixed with non-dissolvable waste. Each one is a profile of what urban denizens are improperly flushing down toilets and pouring down sinks. Increasingly, they are trouble for sanitation systems, which bear the burden of removing them so sewage can flow freely to water-treatment plants.

1. Where’d the term come from?

London sewage workers are said to have combined “fat” and “iceberg” to coin the term. London, with a sewage system dating back to the late 1800s, has been ground zero for the fatberg problem. The American Dialect Society named fatberg one of the most outrageous words of 2013. Australians call the accumulations “sewage sheep.”

2. What’s in a fatberg?

An autopsy on the so-called Whitechapel fatberg in London showed that cooking fat poured down the drain was the main ingredient. Other common items stuck in the grease included sanitary napkins, drugs and wet wipes. The concoction was rife with antibiotic-resistant bacteria. A piece of the specimen is now in the Museum of London, where you can watch live on a “FatCam” how it is growing a type of toxic mold. A debate is ongoing in the U.S. about how much wet wipes contribute to such blockages and what portion of them are the type marketed as flushable.

3. How big can fatbergs get?

London’s sewers have grown some real whoppers. One specimen located in 2013 in the Kingston-upon-Thames area was about the size of a bus. It was bested by the Whitechapel colossus, discovered in 2017, which was about 820 feet (250 meters) long and weighed well over 100 tons. The South Bank Fatberg, found in 2018, was even longer — 750 meters, or almost a half-mile.

4. Why are they a problem?

They can cause sewer clogs and overflows, which spill bacteria-laden sewage water, creating a public-health risk. A fatberg under Baltimore was blamed for an overflow that sent 1.2 million gallons of sewage into a stream in 2017. The Washington-based National Association of Clean Water Agencies says that clearing the clogs diverts sanitation resources from improving water quality and sewer infrastructure. Workers in Macomb County, Michigan, used hand saws, shovels and a vacuum truck to remove a 19-ton fatberg, a process that cost about $100,000. Such labor exposes workers to dangers, as they risk contact with raw sewage and sharp objects like hypodermic needles. In the New Zealand town of Dannevirke, workers had to repair not just a sewer pipe that failed because of a fatberg but also a sinkhole in the roadway above it filled with rats feasting on the grease.

5. Why are fatbergs drawing attention now?

People have been pouring cooking fat down the sink for as long as they’ve had sinks, rather than disposing of it properly in a sealed container in the garbage. And they’ve long flushed things down the toilet that they shouldn’t, including baby wipes, face-cleaning towelettes and Q-tips. In recent years, however, so-called flushable wipes have been increasingly used as an alternative to toilet paper. A recent study commissioned by New York City’s Department of Environmental Protection and obtained by Bloomberg has showed that unlike toilet paper, flushable wipes don’t break down in water. The International Nonwovens and Disposables Association, an industry group, says its own tests, which use different standards, show flushable wipes do break down.

6. Are fatbergs good for anything?

They can be used to generate energy. In London, energy company 2OC is using fats, oils and greases derived from fatbergs to produce biofuel, which it then sells back to the water utility. Canadian researchers are testing a method to turn fatbergs into fuel without having to remove them from sewers first. Despite the energy potential, sewage authorities say they would much prefer to prevent the obstacles from building up in the first place.

To contact the reporter on this story: Tiffany Kary in New York at tkary@bloomberg.net

To contact the editors responsible for this story: Anne Riley Moffat at ariley17@bloomberg.net, Lisa Beyer, Jonathan Roeder

©2019 Bloomberg L.P.