Homes, industry, schools, and businesses all generate sanitary waste, or sewage. Sewage treatment is a multistage process that cleans up wastewater before discharge or reuse. In the final step of the treatment, disinfectants are added to kill disease-causing organisms. Common disinfectants are chlorine gas and sodium hypochlorite. Chlorine dosage levels are designed to leave almost no residual in the wastewater after treatment.
Image credit: "PB120079" killbox © 2006, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/