Your septic system boils down to two basic working parts:
The septic tank itself, and the leaching facility.
The Image below shows the basic parts of your septic system and how it works.
The Septic Tank
Waste water flows from the house into the septic tank. Here, heavy solids settle and are partially decomposed by bacteria in the tank to form sludge. Light solids and grease float to the top, forming a scum layer. Up to fifty percent of the solids retained in the tank decompose. The remainder accumulates in the tank. Biological and chemical additives are not needed, or recommended to aid or accelerate decomposition.
As the septic system is used, sludge continues to accumulate in the bottom of the septic tank. Usually, tanks have enough space for up to three years safe accumulation of sludge and scum. When the sludge and scum levels increase beyond this point, sewage has less time to settle properly before leaving the tank. As the sludge level increases, more solids escape into the absorption area. If sludge accumulates for too long, no settling occurs before the sewage escapes directly to the soil absorption area. To prevent this, the tank must be pumped periodically. The material pumped out is known as “septage”.
The Leaching Facility
Partially treated waste water is discharged from the septic tank into a leaching facility. Here, the water is further purified by filtration and decomposition by micro-organisms in the soil. This is the last line of defense to prevent polluted water from entering surface and groundwater.