Aerobic units are used to collect and treat household waste water including the water from toilets, bathtubs, showers, sinks and laundry. The aerobic units are available in various sizes and shapes like rectangular, conical etc.
 
Suspended growth is the process that is used by most aerobic units to treat waste water. The main compartment in this unit is called the aeration chamber in which air is mixed with the waste water. An air blower or compressor forces air into the aeration chamber as most of the home aerobic units are buried underground. The air that mixes with the waste water in the aeration chamber supports the growth of aerobic bacteria that digests the solids in the waste water. The mixed waste water and oxygen is called mixed liquor. Suspended growth is the treatment occurring in the mixed liquor as the bacteria that grows in the liquid is unattached to any surface. The undigested solids in the mixed liquor settle out as sludge. A secondary chamber is included in many aerobic units and is called a settling chamber or clarifier where excess solids can settle. In other designs the sludge settles at the bottom of the tank. The sludge returns to the aeration chamber in aerobic units with a separate settling chamber. The sludge contains bacteria which help in the treatment process. The built up sludge should be pumped out at least once a year to prevent clogging of the unit.
 
The Kochi apartments where the septic tank is failing or is not suitable use the aerobic systems.