In the past, the biochemical processes that take place in water bodies were relied upon to neutralize sewage. Aerobic, or oxygen-requiring, bacteria feed on the organic material in sewage, decomposing it. However, this process may use up the available oxygen that is dissolved in water. Frequently, the concentration of organic waste is so great that the biochemical oxygen demand (BOD) depletes the water's oxygen supply, killing fish and plants. The BOD measures of the amount of oxygen needed (in milligrams per liter or parts per million) by bacteria and other microorganisms to oxidize the organic matter present in a water sample over a period of 5 days. The BOD of drinking water should be less than 1, while raw sewage may run to several hundred.
Sewage treatment is classified as primary,
secondary, or tertiary, depending on the degree to which the effluent is
purified. All treatment facilities that
discharge to waters of the United States must comply with the secondary treatment standards. Primary
treatment is removal of floating and suspended solids. Secondary
treatment uses biological methods such as digestion. Tertiary
treatment removes all but a negligible portion of bacterial and organic
matter.
Primary and secondary treatment together can remove up to 90% of the BOD. After undergoing chlorination to remove its bacterial component, the effluent from secondary sewage treatment is returned to the local surface water. This combination of primary and secondary treatment removes most of the organic matter in sewage and thus lowers the BOD. However, most of the nitrogen and phosphorus in sewage still remains in the effluent from secondary treatment. These inorganic nutrients can cause eutrophication of surface water receiving the effluent causing blooms of algae. To avoid this, a few communities add a third stage of treatment called tertiary or advanced waste treatment.
During this final process, the nearly purified water flows from the post-secondary sedimentation tanks into a large basin, where it is chlorinated to kill any remaining potential pathogens. This tertiary effluent then undergoes an aeration process to de-chlorinate the water as it flows down a series of steps prior to its final discharge from the sewage treatment plant. Discharge water must be free of odors, suspended solids, and objectionable bacteria. (Coliform bacteria, which inhabit the lower intestines of mammals, while not pathogenic of themselves, are taken as an index of contamination of watercourses.)