Septic tanks are alternatives in areas (urban and rural) where there is no sanitary sewage service, requiring the implementation of some means of final disposal of treated sewage in order to avoid contamination of soil and water.
In general, the septic tank is a chamber whose purpose is to retain the sanitary sewers for an established period of time, in order to allow the sedimentation of the solids and the retention of the fats contained in the sewage, transforming them biochemically into substances and compounds more simple. The main trench arrangements are: single chamber; and chambers in series.
It is important to note that the septic tank has treatment limitations, since the average efficiency of BOD removal is 30% and suspended solids, 50%. Because of this limitation, the septic tank effluent can be potentially contaminating the soil and water, with unpleasant odor and appearance, requiring, for these reasons, an adequate final disposition. Among the processes adopted for the provisions, the most usual, are: sink; infiltration trench; filter ditch; and sand filter.