The process of handling sewage to safely return it to the environment. The process consists of 1. preliminary treatment to remove solid particles, 2. primary treatment to create a substance void of free oxygen to allow anaerobic bacteria to digest all organic materials, 3. secondary treatment to allow the effluent to come into contact with air so that aerobic bacteria q.v. can oxidize putrescible material, 4. chlorination to decontaminate the effluent, and 5. final disposal of the liquid end product, usually into a large body of water.