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Wastewater treatment is the process throughwhich contaminants are removed from household,municipal, or industrial sewage and water is madesafe for release into the environment. In naturalsettings water purification is accomplishedthrough dilution and decomposition; wastewatertreatment is an industrialized and standardizedvariation of natural processes that uses acombination of physical, chemical, and biologicalprocedures. There are five components towastewater treatment, though the extent of theprocessing in any given system will vary bylocation and government. The process ofwastewater treatment includes: pretreatment,primary, secondary, and tertiary treatment, andpolishing.
Pretreatment allows large debris like trash or treelimbs to be removed from wastewater prior toentering the more sensitive mechanized stages ofprocessing.
The first stage of treatment is the physical removalof biological solid wastes from wastewater.Biological solids include such items as hair, food,or feces. This first stage of treatment collectswastewater for a regulated period of time duringwhich solid materials will settle to the bottom; fatsand oils rise to the top. Physical implements thenphysically separate the wastewater from bothsolids and oils.
Secondary treatment is typically a biologicalprocess and closely approximates what occurs innature. Various native bacteria and microbes arecultivated to decompose the organic compounds inwastewater. This stage can be accomplishedthrough the use of aerated basins, constructedwetlands, or soil biotechnology.
Tertiary treatment is most frequently used in areaswhere there is a concern of excess nutrients fromtreated wastewater entering the environment. Thislevel of processing removes excess phosphorousand nitrogen from water through chemical orbiological means. An excess of nutrients couldresult in eutrophication, i.e. algal bloom, andcause impaired environmental conditions for manyoxygen-sensitive species.
Polishing is a fifth level of wastewater treatment.It is chemical disinfection, often via chlorination.