Effluent is wastewater from sewers or industrial outfalls that flows directly into surface waters either untreated or after being treated at a facility.[1] The term has slightly different meanings in certain contexts, and may contain various pollutants depending on the source.[2] Treating wastewater efficiently is challenging, but improved technology allows for enhanced removal of specific materials, increased re-use of water, and energy production from waste.[2][3][4][5]

Wastewater is considered effluent as it is released to surface water.


Effluent is defined by the United 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] The Compact Oxford English Dictionary defines effluent as "liquid waste or sewage discharged into a river or the sea".[6] Wastewater is not usually described as effluent while being recycled, re-used, or treated until it is released to surface water. Wastewater percolated or injected into groundwater may not be described as effluent if soil is assumed to perform treatment by filtration or ion exchange;[7] although concealed flow through fractured bedrock, lava tubes, limestone caves,[8] or gravel in ancient stream channels[9] may allow relatively untreated wastewater to emerge as springs.[10]


Effluent in the artificial sense is in general considered to be water pollution, such as the outflow from a sewage treatment facility or an industrial wastewater discharge. An effluent sump pump, for instance, pumps waste from toilets installed below a main sewage line. In the context of waste water treatment plants, effluent that has been treated is sometimes called secondary effluent, or treated effluent. This cleaner effluent is then used to feed the bacteria in biofilters.[11]

In the context of a thermal 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 called thermal pollution.[12]: 375  In chemical engineering practice, effluent is the stream exiting a chemical reactor.[13]

Effluent may carry pollutants such as fats/oils/greases, chemicals, detergents, heavy metal rinses, solids, and food waste.[2] Possible sources include a wide range of manufacturing industries, mining industries, oil and gas extraction, and service industries.[14]

Pollution control regulationEdit

United States effluent guidelinesEdit

In the United States, the Clean Water Act requires all direct effluent discharges to surface waters to be regulated with permits under the National 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 effective treatment 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.[14] 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]


When pretreating effluent, it is difficult to reduce expenses, increase productivity, adhere to stringent standards, and prevent discharge of byproducts.[2] Furthermore, depletion of clean, fresh, accessible water is a global issue, but evolving technology can help prevent this because improving treatment methods allows for re-use of water.[4] Additionally, waste-to-energy technology transforms organic matter into usable electricity, heat, or fuel.[5] Treatments also exist to remove certain elements, compounds, salts, and minerals from effluent.[3]

See alsoEdit


  1. ^ a b "Terms of Environment". Washington, D.C.: United States Environmental Protection Agency (EPA). February 1993. p. 10. EPA 175-B-93-001.
  2. ^ a b c d Tuser, Cristina (2021-10-27). "What is Effluent?". Water & Wastes Digest. Endeavor Business Media, LLC.
  3. ^ a b Brandt, Malcolm J.; Johnson, K. Michael; Elphinston, Andrew J.; Ratnayaka, Don D. (2017-01-01), Brandt, Malcolm J.; Johnson, K. Michael; Elphinston, Andrew J.; Ratnayaka, Don D. (eds.), "Chapter 10 - Specialized and Advanced Water Treatment Processes", Twort's Water Supply (Seventh Edition), Boston: Butterworth-Heinemann, pp. 407–473, doi:10.1016/b978-0-08-100025-0.00010-7, ISBN 978-0-08-100025-0, retrieved 2022-11-30
  4. ^ a b Jegatheesan, V.; Shu, L.; Visvanathan, C. (2011), "Aquaculture Effluent: Impacts and Remedies for Protecting the Environment and Human Health", Encyclopedia of Environmental Health, Elsevier, pp. 123–135, doi:10.1016/b978-0-444-52272-6.00340-8, ISBN 9780444522726, retrieved 2022-11-30
  5. ^ a b (Firm), ProQuest (2018). ENERGY FROM TOXIC ORGANIC WASTE FOR HEAT AND POWER GENERATION. WOODHEAD. ISBN 978-0-08-102529-1. OCLC 1096233045.
  6. ^ "AskOxford: effluent". Ask Oxford.com. Oxford University Press. 2010. Retrieved 2010-06-09.[dead link]
  7. ^ Rich, Linville G. (1980). Low-Maintenance Mechanically Simple Wastewater Treatment Systems. New York: McGraw-Hill Book Company. pp. 181–186. ISBN 0-07-052252-9.
  8. ^ Linsley, Ray K.; Franzini, Joseph B. (1972). Water-Resources Engineering (Second ed.). New York: McGraw-Hill Book Company. pp. 84 & 85. ISBN 0-07-037959-9.
  9. ^ Harr, M.E. (1962). Groundwater and Seepage. New York: McGraw-Hill Book Company. p. 26. ISBN 0-07-026740-5.
  10. ^ Steel, E.W.; McGhee, Terence J. (1979). Water Supply and Sewerage (Fifth ed.). New York: McGraw-Hill Book Company. pp. 81–82. ISBN 0-07-060929-2.
  11. ^ George Tchobanoglous, Franklin L. Burton, H. David Stensel, Metcalf & Eddy (2003). Wastewater engineering: Treatment and reuse (4th ed.). Boston: McGraw-Hill. ISBN 0-07-041878-0. OCLC 48053912.{{cite book}}: CS1 maint: uses authors parameter (link)
  12. ^ Laws, Edward A. (2017). Aquatic Pollution: An Introductory Text (4th ed.). Hoboken, NJ: John Wiley & Sons. ISBN 9781119304500.
  13. ^ Fogler, H. Scott (2006). Elements of Chemical Reaction Engineering. Hoboken, NJ: Prentice Hall. p. 43. ISBN 9780131278394.
  14. ^ a b "Industrial Effluent Guidelines". EPA. 2022-11-07.
  15. ^ "NPDES Permit Limits". National Pollutant Discharge Elimination System. EPA. 2021-10-11.
  16. ^ a b c "Learn about Effluent Guidelines". EPA. 2022-09-09.
  17. ^ "National Pretreatment Program Overview". NPDES. EPA. 2022-10-03.
  18. ^ "Effluent Guidelines Implementation & Compliance". EPA. 2022-09-09.
  19. ^ "Effluent Guidelines Plan". EPA. 2022-11-18.
  20. ^ EPA. "Calculating NPDES permit conditions." Code of Federal Regulations, 40 CFR 122.45