In a way it seems so obvious: find out what kinds of things people are putting into their bodies by studying what comes out the other end. These do sound like muddy waters, but with some clever science researchers are able to draw some clear insights about the habits of different demographics of the population. And with their methods improving all the time, some see a bright future for this dark corner of science that involves real-time health monitoring, predicting disease outbreaks ahead of time and even tracking the rise of antibiotic resistance.
Source: The science of sewage: What your wastewater could reveal about you
Insight Sciences 