What is the difference between a conventional and aerobic septic system?

A conventional septic system separates solids in the tank and distributes liquid effluent to a drain field where soil naturally filters it.

An aerobic treatment unit (ATU) uses an air pump to inject oxygen, accelerating bacterial breakdown and producing a cleaner effluent — which is required in areas where soil or lot conditions don't support conventional drain fields.

Aerobic systems are more effective but require more maintenance: in Illinois, they must have a licensed service contract with inspections every 6 months.