Half eaten kebabs, a broken high heel, a puddle of sick in a shop doorway - typical remnants of a Saturday night out in the city. The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) though, measure a much less visible indicator of a city's partying habits. Every year, they analyse waste water samples around the continent for traces of illicit drugs such as cocaine and MDMA.Looking at traces of cocaine found at the weekend, the otherwise unassuming English city of Bristol is at the top of the scale with 1,139mg of the drug per one thousand people flowing through the drains. In second is party city Amsterdam with 1,040mg and rounding off the top three is the Swiss banking hub of Zurich with 971mg.Note: Bristol is the largest UK city in the test. The most recent data for London is for 2016 (999mg).