Sewer Sludge

Last September, workers discovered a massive, smelly blob of congealed fat clogging a sewer in London, England. The mass, called a fatberg, was nearly as long as a city block and weighed as much as a blue whale.

Fatbergs are a big problem for cities like London with aging sewer systems and growing populations. The gunky globs form when fats and oils wash down kitchen sinks and react with the element calcium in wastewater. The calcium causes the fat to solidify. As it hardens, it traps muck swept into sewers. “[Fatbergs] potentially trap everything that’s being flushed down the toilet,” says Joel Ducoste, an environmental engineer at North Carolina State University. In addition to looking nasty, he says, “they stink!”

Fatbergs adhere to sewer walls and grow as more and more fat sticks to their surfaces. Eventually, they become large enough to block sewers. When that happens, pipes can back up and flood buildings with reeking sewage. To break up the fatbergs, workers must blast them with high-pressure water hoses. It costs London $1.4 million each month to clear its sewers.

Engineers now have a better way for people to dispose of fat—before it can collect in sewers. They’re installing traps in restaurants to catch grease. It can be used to create biodiesel. This fuel, made from vegetable and animal fats, can power sewage treatment plants and even homes and businesses. The whale-sized fatberg, for example, contained enough fat to have made 10,000 liters (2,641 gallons) of biodiesel.