The septic tank is normally an underground, watertight container made of concrete, fiberglass, or other durable material that provides primary wastewater treatment unit(settling of solids). It is connected to the standard drain field constructed with a series of parallel, underground, perforated pipes that allow septic tank effluent to percolate into the surrounding soil in the vadose (unsaturated) zone where it is expected that most of the residual nutrients may be assimilated. Several types of effluent distribution are applicable in standard drain field systems. These include gravity systems, low-pressure dosed systems, and drip irrigation systems. Some systems require an additional pump. Through various physical, chemical, and biological processes, most bacteria, viruses, and nutrients in wastewater are expected to be consumed or filtered as the wastewater passes through the soil. The oxygen released by photosynthesis is available to meet the needs of oxidizing organic matter stabilised by anaerobic bacteria.