When it doesn't make sense to build an entire sewage collection and treatment system, septic systems are the go-to option. Simple but effective, these combinations of tank and soil remove solids from waste and send wastewater into the ground, where filtration, microbial activity and time eventually render clean water. It all seems a bit miraculous, given what we're working with. But for all the impressive work septic systems do, few people - even some of their users - really know how they work.