Drug Testing Entire Cities at Once

Researchers have developed a technique for determining what illicit drugs people might be consuming in a given area, by testing a sample from the local sewage treatment plant. As little as a teaspoonful of untreated wastewater can reveal drug use patterns in a given community. Obviously, any drugs found can't be tied to any specific user, but how much longer until the drug warriors want to deploy automatic sampling units farther upstream of the sewage treatment plant?