TREATMENT OF POLLUTANTS (Cont)
Septic systems are generally used in rural areas to handle household wastes. They usually use a large tank buried in the ground to contain and break down household sewage. Attached to the tank is a series of perforated pipes that are buried in a drain field and are usually surrounded by crushed rock or gravel to facilitate drainage. Fats, oils, and grease, as well as large waste particles, are stored and later pumped out of the holding tank, while the water and suspended solids in the water flow into the soil through the perforated pipes. The soil around the septic system filters many harmful compounds, and bacteria break down organic matter.