Lurking in darkness under our city’s streets is a silent and growing threat to human health. Cooking fats and grease tipped down the sink in homes and businesses are mixing with solids in our sewers and forming giant lumps of solid fat, sometimes the size of cars, sometimes hundreds of metres long. They cause a media frenzy when photos of them appear online, but in their most extreme form, ‘fatbergs’ can damage or block or sewers, causing serious disruption. “Annually, there are around 366,000 sewer blockages in the UK”. “Eight times every hour, a Thames Water customer suffers a blockage caused by sewer abuse. As well as being costly in terms of money, there is also a human cost of these fatbergs, as we often have to close a road to dig down and clear them, often causing delays and inconveniences.” As destructive as they are, utility companies are beginning to take advantage of some surprising opportunities to be found by reprocessing fatbergs. At a site on England’s North-west coast, Argent Energy has started turning fatbergs from wastewater facilities around the country into a biodiesel. In a simplified version of the process the untreated fats, oils and greases – FOG – arrives at the plant, gets heated to separate oil and grease, has excess solids removed and becomes biodiesel with the addition of a few chemicals, polished off with distillation.