An alternate on-lot sewage disposal system is known as At-grade and shallow At-grade system. The at-grade system is generally used in sites with restrictive soil conditions like limiting zones at less than 48 inches. Hence instead of placing an elevated sand mound on some sites, the at-grade systems can be used as an alternative.
 
The at-grade system consists of five components: a treatment tank, a filtration unit which is optional, a disinfection unit, a dose tank, and an at-grade soil absorption area constructed on the soil surface. The waste water from the house first reaches the treatment tank. It is generally a two chamber septic tank. The septic tank allows the heavier solids to settle to the bottom while the lighter scum floats to the surface. The rest of the liquid is passed to the next stage of the system. A properly sized aerobic treatment tank can be used instead of the septic tank. In the aerobic tank a stream of air containing oxygen is forced through the waste water allowing the biological breakdown of the carbonaceous waste material.
 
Depending on the depth to the site’s limiting zone the filtration requirements vary. The septic tank effluent is conveyed to a filtration unit and the waste water is filtered to remove and biodegrade a large portion of the waste water’s biological components in cases where the depth to the limiting zone is less than 48 inches. A secondary treatment unit is required if the limiting zone is 20 to 48 inches. An advanced treatment unit is required if the limiting zone is less than 20 inches below the land surface.
 
Then the water enters a single chamber tank called a dose tank from where it is transferred to the absorption area.
 
It is very important to choose the right method of sewage treatment in large apartments. Hence builders Cochin are using innovative and new technologies of sewage treatment that suit their needs.