Collection and treatment of household sewage in the system of independent sewerage. Septic tanks are used in the construction of closed systems that are not connected to the central sewage collection system. Depending on the number of chambers in the septic tank, sewage is purified mechanically (sedimentation of impurities) and biologically (disintegration of organic impurities by bacteria). Septic tanks also have a system of soil post-treatment - the liquid is drained into the soil and when passing through a layer of soil, gets rid of the remaining impurities.