Septic tanks are widely used in rural areas where there is no sewage collection system and it is necessary to treat domestic  wastewater.  Untreated  wastewater  from  the  house  or  group  of houses flows into the septic tank where the solids  separate from the liquids. The majority of the solids settle to the bottom where they are liquefied slowly. Lighter solids such as grease, fats and soaps float to  the  surface  and  form  the  scum  layer.  The  septic  tank is  designed  to  retain  the solids  in  the  tank.  The  liquid  passes  out  of  the  septic  tank  and  into  a  percolation  system  or soakaway.

The degradative activity in a septic tank is based on a natural anaerobic (oxygen -free) biological process.  This  activity helps  to  liquefy solids,  digest  paper  and  other fibres  and  breakdown  fats and grease. The liquid waste flows to the soakaway where further degradation occurs.