Wastewater treatment involves a multi-step process to clean sewage and remove contaminants before water is disposed or reused. Sewage from homes and industries contains human waste, chemicals, bacteria and other pollutants. It is carried through pipes to a treatment plant where it passes through screens to remove large solid objects, settles in tanks to separate out sand, grit and solids, and is treated with oxygen and bacteria to break down organic matter. The cleaned water is then disinfected and released while solid waste is further dewatered and dried. Proper wastewater treatment is needed to prevent water pollution and disease.