The Water Pollution Control Act required states to do something about non-point pollution but little has been accomplished. In 1995, the EPA developed a discharge trading policy, similar to policies used for air pollution, which allows polluting industries to sell water pollution credits. Treatment Methods One way is to use the filtering properties of soils combined with settling tanks. Septic systems receive household sewage. In the septic tank, heavier substances settle on the bottom and remaining liquid gets pumped over a drainage field.
Soil bacteria decompose biodegradable material.
Found in most rural areas. In urban areas, the volume of sewage is simply too great for the area they occupy. Sewage gets sent to treatment plants. Sewage is carried by pipes and culverts. Some have combined systems because it is cheaper to run one line than two.