In Many countries, restaurants deposit the fat from all the cooking in barrels. UK, it seems, is still behind in this. Restaurants dump the fat straight down the drains, many have no working grease traps, leading to huge deposits in the sewer lines, which people have come to call Fatbergs (readers may have thought we were talking about Hamburgers).
The above image is from a 130 tonne fatberg, found under Whitechapel Road in east London.
FYI, in China, they just suction the stuff out of the sewer and sell it as gutter oil.