A septic tank takes raw sewage in, allows the solids to settle (sludge) and allows the remaining liquid to flow into the surrounding soil by means of a soakaway. Scum on the surface is also prevented from leaving the tank. Microorganisms in the anaerobic environment in the tank digest the sludge and scum. The system consists of several stages, supply to the tank, the tank itself and the soak field. Septic tanks take sewage (grey water - washing and household waste and black water - sewage from latrines,) but not rainwater. Sludge volume is reduced by microbial action but still needs periodic emptying. Septic tanks provide partial treatment of wastewater. The soakfield provides secondary treatment in the form of subsoil infiltration. Septic tanks are suitable for conditions where the wastewater can drain away and be absorbed into the soil without contaminating ground water where it is extracted. Sealed solid waste storage is an option if soil is unsuitable or the water table is too high. Cesspits are another option.