A septic tank is a pretreatment structure designed to retain the decanting materials present in the wastewater (W.C.) from the dwelling houses.

The settling matters are measured thanks to the visible volume accumulated at the bottom of an Imhoff cone (or a 1-litre measuring cylinder) after a 30 minutes settling period.

Wastewater treatment is mainly the result of biological reactions in the upper water layer and sedimentation of settling materials at the bottom of the basins (Figure 10).