A teaspoon of sewage is all that's needed to test an entire city for drug use.
Imagine how this could be used...do we need Constitutional protection for the sanctity of our excretions? Will overzealous prosecutors "tap" your sewage line?
And...what does this mean for the animals on the receiving end of our sewage, since most of the drugs survive sewage treatment? Perhaps this explains poor performance of fish on most standardized achievement tests, and the lackadaisical attitude to authority figures that seems to prevail among sea lions?