Effluent iswastewater from sewers or industrial outfalls that flows directly intosurface waters, either untreated or after beingtreated at a facility.[1] The term has slightly different meanings in certain contexts, and may contain variouspollutants depending on the source.[2]
Effluent is defined by theUnited States Environmental Protection Agency (EPA) as "wastewater–treated or untreated–that flows out of a treatment plant, sewer, or industrial outfall. Generally refers to wastes discharged into surface waters".[1] TheCompact Oxford English Dictionary defines effluent as "liquid waste or sewage discharged into a river or the sea".[3] Wastewater is not usually described as effluent while beingrecycled, re-used, or treated until it is released to surface water. Wastewaterpercolated orinjected intogroundwater may not be described as effluent ifsoil is assumed to perform treatment byfiltration orion exchange;[4] although concealed flow through fracturedbedrock,lava tubes,limestone caves,[5] orgravel in ancientstream channels[6] may allow relatively untreated wastewater to emerge assprings.[7]
Effluent in the artificial sense is in general considered to bewater pollution, such as the outflow from asewage treatment facility or anindustrial wastewater discharge. An effluentsump pump, for instance, pumps waste from toilets installed below a main sewage line. In the context ofwaste water treatment plants, effluent that has been treated is sometimes calledsecondary effluent, ortreated effluent. This cleaner effluent is then used to feed thebacteria inbiofilters.[8]
In the context of athermal power station and other industrial facilities, the output of the cooling system may be referred to as the effluent cooling water, which is noticeably warmer than the environment and is calledthermal pollution.[9]: 375 Inchemical engineering practice, effluent is the stream exiting achemical reactor.[10]
Effluent may carry pollutants such as fats, oils and greases; solvents, detergents and other chemicals; heavy metal; other solids; and food waste.[2] Possible sources include a wide range of manufacturing industries, mining industries, oil and gas extraction, and service industries.[11]
There are several kinds of wastewater which are treated at the appropriate type of treatment plant. Domestic wastewater (also called municipal wastewater orsewage) is processed at asewage treatment plant. For industrial wastewater, treatment either takes place in a separateindustrial wastewater treatment facility, or in a sewage treatment plant (usually after some form of pre-treatment). Other types of wastewater treatment plants includeagricultural wastewater treatment andleachate treatment plants.
Treating wastewater efficiently is challenging, but improved technology allows for enhanced removal of specific materials, increasedre-use of water, and energy production from waste.[2][12][13][14]
In the United States, theClean Water Act requires all direct effluent discharges to surface waters to be regulated with permits under theNational Pollutant Discharge Elimination System (NPDES).[15][16] Indirect dischargers–facilities which send their wastewater to municipal sewage treatment plants–may be subject to pretreatment requirements.[17] NPDES permits require discharging facilities to limit or treat effluent to the levels that result from using the most effectivetreatment technologies possible at a practical cost to mitigate the effects of discharges on the receiving waters.[16] EPA has published technology-based regulations, called "effluent guidelines", for 59 industrial categories.[11] The agency reviews the standards annually, conducts research on various categories, and makes revisions as appropriate.[16] Noncompliance with these standards and all other conditions in the permits is punishable by law.[18] Each year, effluent guidelines regulations prevent billions of pounds of contaminants from being released into bodies of water.[19]
EPA regulations require effluent limitations to be expressed as mass-based limits (rather than concentration-based limits) in the permits, so that discharging facilities will not use dilution as a substitute for treatment. In cases where setting mass-based limits are infeasible, the permit authority must set conditions in the permit that prohibit dilution.[20]
The U.S. "Secondary Treatment Regulation" is the national standard for municipal sewage treatment plants.[21]
