Septic tank
A septic tank is an underground chamber made of concrete, fiberglass or plastic through which domestic wastewater (sewage) flows for basic treatment. Settling and anaerobic processes reduce solids and organics, but the treatment efficiency is only moderate (referred to as "primary treatment"). Septic tank systems are a type of simple onsite sewage facility (OSSF). They can be used in areas that are not connected to a sewerage system, such as rural areas. The treated liquid effluent is commonly disposed in a septic drain field which provides further treatment. However, groundwater pollution may occur and can be a problem.
Water supply and sanitation in Nigeria
Adequate sanitation is typically in the form of septic tanks, as there is no central sewerage system, except for Abuja and some areas of Lagos. A 2006 study estimated that only 1% of Lagos households were connected to sewers. Lagos has four wastewater treatment plants which have been rehabilitated around 2010. As of 2011, the state planned to build ten new "mega wastewater treatment plants" over the next five years with the help of private investors. These have not yet been completed.