These are actually mini-wastewater treatment plants not unlike the huge plants that might serve a city.  The technology is really very simple.  In this system, the matter is broken down in two different tanks.  The first tank is the settling tank just like the tank on the older style systems.  The material is broken down without oxygen and then moves from the first tank to the second tank.  The second tank is has a pvc line run into the bottom which is attached to an air compressor at the surface.  The air compressor is constantly forcing oxygen into the tank, thus breaking down the matter "aerobically" or "with oxygen".  There is also a third tank, which is simply a holding tank.  When the third tank reaches a certain height, a sprinkler pump within the tank pumps the water out to the sprinkler lines.  Between the second and third tank is a pipe which can be easily accessed from the surface.  About once a month, chlorine tablets are dropped into this pipe.  The treated water slowly washes by the chlorine tablets and is chemically sanitized.