Wastewater treatment plant
A wastewater treatment plant is a physical plant where various physical, biological or chemical processes are used to change the properties of the wastewater (e.g. by removing harmful substances) in order to turn it into a type of water (also called effluent) that can be safely discharged into the environment or that is usable for a certain reuse purpose. In the case of the latter, the term "reclaimed water" is commonly used. The treatment of wastewater belongs to the over-arching term sanitation, just like the management of human excreta, solid waste and stormwater (drainage).  
Sewage treatment plants
City wastewater collection and drainage systems called sewers became increasingly common through the 19th century. The drainage from these sewers became known as sewage; and treatment of sewage evolved through the 20th century into a standardized sequence of primary treatment, followed by secondary treatment, and ending with disinfection. A typical municipal sewage treatment plant includes primary treatment to remove solid material, secondary treatment to remove dissolved and suspended organic material, and disinfection to kill disease-causing micro-organisms. Larger municipalities often include factories discharging industrial wastewater into the municipal sewer system. Sewage treatment plants may be called wastewater treatment plants when the standard municipal sewage treatment plant sequence of primary treatment, secondary treatment, and disinfection is presumed adequate for treating the combined sanitary sewage and industrial wastewater collected by the sewer.
Industrial wastewater treatment plants
Industrial wastewater treatment plants are required where municipal sewage treatment plants are unavailable or cannot adequately treat specific industrial wastewaters. Industrial wastewater plants may reduce raw water costs by converting selected wastewaters to reclaimed water used for different purposes. Industrial wastewater treatment plants may reduce wastewater treatment charges collected by municipal sewage treatment plants by pre-treating wastewaters to reduce concentrations of pollutants measured to determine user fees.
Although economies of scale may favor use of a large municipal sewage treatment plant for disposal of small volumes of industrial wastewater, industrial wastewater treatment and disposal may be less expensive than correctly apportioned costs for larger volumes of industrial wastewater not requiring the conventional sewage treatment sequence of a small municipal sewage treatment plant.
An industrial wastewater treatment plant may include one or more of the following rather than the conventional primary, secondary, and disinfection sequence of sewage treatment:
- An API oil-water separator, for removing separate phase oil from wastewater.
- A clarifier, for removing solids from wastewater.
- A roughing filter, to reduce the biochemical oxygen demand of wastewater.
- A carbon filtration plant, to remove toxic dissolved organic compounds from wastewater.
Agricultural wastewater treatment plants
Agricultural wastewater treatment for continuous confined animal operations like milk and egg production may be performed in plants using mechanized treatment units similar to those described under industrial wastewater; but where land is available for ponds, settling basins and facultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles.
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