A septic system has two basic working parts..
Wastewater flows from the house into the septic tank. Here heavy solids settle and are partially decomposed by bacteria to form sludge. Light solids and grease float to the top forming a scum layer.
Partially treated wastewater is discharged from the septic tank though perforated pipes into an absorption field. Here, the water is further purified by filtration and decomposition by microorganisms in the soil. This is the last line of defense to prevent polluted water from entering lakes, streams and groundwater.