In older sewage-treatment plants, the water after primary treatment is often chlorinated to kill pathogens and then discharged into a natural waterway. The discharged water at this stage still contains a large amount of oxygen-consuming wastes, which may deplete dissolved oxygen in the water way and cause eutrophicationIn older sewage-treatment plants, the water after primary treatment is often chlorinated to kill pathogens and then discharged into a natural waterway. The discharged water at this stage still contains a large amount of oxygen-consuming wastes, which may deplete dissolved oxygen in the water way and cause eutrophication