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.
Atlanta metropolitan area
Sewerage is also handled by the water utilities, but the various water and sewer networks may not conform to the same boundaries, resulting in interbasin water transfers. This is for practical reasons, because the area is hilly and divided by several watersheds, because the area has developed irregularly and erratically, and because water treatment plants are usually not near sewage treatment plants. Septic tanks are still used in the older homes of some exurbs.