Aerobic Sewer Systems
An aerobic treatment system uses an aerobic (increasing oxygen) process for digestion rather than just the anaerobic (reducing oxygen) process used in septic systems. Aerobic systems may be used for a single residence or for a small group of homes. These treatment systems produce a high quality effluent. This allows much greater flexibility in the placement of the leach field.
Aerobic System Cont.
The ATS process generally consists of the following phases: 
1 Pre-treatment stage to remove large solids and other undesirable substances.
2 Aeration stage, where aerobic bacteria digest biological wastes.
3 Settling stage allows undigested solids to settle. This forms a sludge that must be periodically removed from the system.
4 Disinfecting stage, where chlorine or similar disinfectant is mixed with the water, to produce an antiseptic output. Another option is UV disinfection, where the water is exposed to UV light inside of a UV disinfection unit.
The disinfecting stage is optional, and is used where a sterile effluent is required, such as cases where the effluent is distributed above ground. Excessive amounts of items such as bleach or antibiotics can damage the ATS environment and reduce treatment effectiveness.