Septic tank aerators?

I don't know but this house was sold to me and it is obvious that the aerator has been dead for some time, so I guess the county did not inspect the septic system.

The basic idea I'm reading is that a large level yard with 3 ft of good soil can use an old-fashioned anaerobic septic system. However if you have a smaller yard, or a yard that is not level, or a yard that has less than 3 feet of good soil above rock or hardpan then at a minimum you will need an aerobic system with an aerator. Apparently the motor-shaft type aerators are what came first and the air-pump type aerators are the more modern approach. Some systems have a chlorinator on the final tank, and I guess there are even more complex systems.