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Wastewater treatment
Converting wastewater into an effluent for return to the water cycle

Wastewater treatment removes contaminants from wastewater, converting it into an effluent that safely returns to the water cycle or can be reused through water reclamation. Treatment occurs in specialized plants such as sewage treatment for domestic wastewater, or industrial wastewater treatment for industrial effluent. Common processes include phase separation and biological oxidation. Sludge is a main by-product, sometimes producing biogas via anaerobic treatment. Proper disposal or reuse options guide the treatment method, ensuring safe effluent release or reuse. The terms “wastewater treatment” and “sewage treatment” are often used interchangeably.

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Types of treatment plants

Wastewater treatment plants may be distinguished by the type of wastewater to be treated. There are numerous processes that can be used to treat wastewater depending on the type and extent of contamination. The treatment steps include physical, chemical and biological treatment processes.5

Types of wastewater treatment plants include:

Sewage treatment plants

This section is an excerpt from Sewage treatment.[edit]

Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges.6 Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. There are a high number of sewage treatment processes to choose from. These can range from decentralized systems (including on-site treatment systems) to large centralized systems involving a network of pipes and pump stations (called sewerage) which convey the sewage to a treatment plant. For cities that have a combined sewer, the sewers will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter (measured as biological oxygen demand) from sewage,  using aerobic or anaerobic biological processes. A so-called quaternary treatment step (sometimes referred to as advanced treatment) can also be added for the removal of organic micropollutants, such as pharmaceuticals. This has been implemented in full-scale for example in Sweden.7

A large number of sewage treatment technologies have been developed, mostly using biological treatment processes. Design engineers and decision makers need to take into account technical and economical criteria of each alternative when choosing a suitable technology.8: 215  Often, the main criteria for selection are: desired effluent quality, expected construction and operating costs, availability of land, energy requirements and sustainability aspects. In developing countries and in rural areas with low population densities, sewage is often treated by various on-site sanitation systems and not conveyed in sewers. These systems include septic tanks connected to drain fields, on-site sewage systems (OSS), vermifilter systems and many more. On the other hand, advanced and relatively expensive sewage treatment plants may include tertiary treatment with disinfection and possibly even a fourth treatment stage to remove micropollutants.9

At the global level, an estimated 52% of sewage is treated.10 However, sewage treatment rates are highly unequal for different countries around the world. For example, while high-income countries treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.11

Industrial wastewater treatment plants

This section is an excerpt from Industrial wastewater treatment.[edit]

Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater (or effluent) may be reused or released to a sanitary sewer or to a surface water in the environment. Some industrial facilities generate wastewater that can be treated in sewage treatment plants. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters into sewers or into rivers, lakes or oceans.12: 1412  This applies to industries that generate wastewater with high concentrations of organic matter (e.g. oil and grease), toxic pollutants (e.g. heavy metals, volatile organic compounds) or nutrients such as ammonia.13: 180  Some industries install a pre-treatment system to remove some pollutants (e.g., toxic compounds), and then discharge the partially treated wastewater to the municipal sewer system.14: 60 

Most industries produce some wastewater. Recent trends have been to minimize such production or to recycle treated wastewater within the production process. Some industries have been successful at redesigning their manufacturing processes to reduce or eliminate pollutants.15 Sources of industrial wastewater include battery manufacturing, chemical manufacturing, electric power plants, food industry, iron and steel industry, metal working, mines and quarries, nuclear industry, oil and gas extraction, petroleum refining and petrochemicals, pharmaceutical manufacturing, pulp and paper industry, smelters, textile mills, industrial oil contamination, water treatment and wood preserving. Treatment processes include brine treatment, solids removal (e.g. chemical precipitation, filtration), oils and grease removal, removal of biodegradable organics, removal of other organics, removal of acids and alkalis, and removal of toxic materials.

Agricultural wastewater treatment plants

This section is an excerpt from Agricultural wastewater treatment.[edit]

Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or irrigation water. Agricultural wastewater treatment is required for continuous confined animal operations like milk and egg production. It may be performed in plants using mechanized treatment units similar to those used for industrial wastewater. 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.16: 6–8  Animal slurries are usually treated by containment in anaerobic lagoons before disposal by spray or trickle application to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal wastes.

Nonpoint source pollution includes sediment runoff, nutrient runoff and pesticides. Point source pollution includes animal wastes, silage liquor, milking parlour (dairy farming) wastes, slaughtering waste, vegetable washing water and firewater. Many farms generate nonpoint source pollution from surface runoff which is not controlled through a treatment plant.

Leachate treatment plants

Main article: Leachate § Treatment

Leachate treatment plants are used to treat leachate from landfills. Treatment options include: biological treatment, mechanical treatment by ultrafiltration, treatment with active carbon filters, electrochemical treatment including electrocoagulation by various proprietary technologies and reverse osmosis membrane filtration using disc tube module technology.17

Unit processes

See also: List of wastewater treatment technologies

The unit processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such oxidation or anaerobic treatment. Some wastewaters require specialized treatment methods. At the simplest level, treatment of most wastewaters is carried out through separation of solids from liquids, usually by sedimentation. By progressively converting dissolved material into solids, usually a biological floc or biofilm, which is then settled out or separated, an effluent stream of increasing purity is produced.1819

Phase separation

Phase separation transfers impurities into a non-aqueous phase. Phase separation may occur at intermediate points in a treatment sequence to remove solids generated during oxidation or polishing. Grease and oil may be recovered for fuel or saponification. Solids often require dewatering of sludge in a wastewater treatment plant. Disposal options for dried solids vary with the type and concentration of impurities removed from water.20

Sedimentation

Further information: Sedimentation (water treatment) and Sewage treatment § Primary treatment

Solids such as stones, grit, and sand may be removed from wastewater by gravity when density differences are sufficient to overcome dispersion by turbulence. This is typically achieved using a grit channel designed to produce an optimum flow rate that allows grit to settle and other less-dense solids to be carried forward to the next treatment stage. Gravity separation of solids is the primary treatment of sewage, where the unit process is called "primary settling tanks" or "primary sedimentation tanks".21 It is also widely used for the treatment of other types of wastewater. Solids that are denser than water will accumulate at the bottom of quiescent settling basins. More complex clarifiers also have skimmers to simultaneously remove floating grease such as soap scum and solids such as feathers, wood chips, or condoms. Containers like the API oil-water separator are specifically designed to separate non-polar liquids.22: 111–138 

Biological and chemical processes

Oxidation

Oxidation reduces the biochemical oxygen demand of wastewater, and may reduce the toxicity of some impurities. Secondary treatment converts organic compounds into carbon dioxide, water, and biosolids through oxidation and reduction reactions.23 Chemical oxidation is widely used for disinfection.24

Biochemical oxidation (secondary treatment)

This section is an excerpt from Secondary treatment.[edit]

Secondary treatment (mostly biological wastewater treatment) is the removal of biodegradable organic matter (in solution or suspension) from sewage or similar kinds of wastewater.25: 11  The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.
Chemical oxidation

Main article: Advanced oxidation process

Advanced oxidation processes are used to remove some persistent organic pollutants and concentrations remaining after biochemical oxidation.26: 363–408  Disinfection by chemical oxidation kills bacteria and microbial pathogens by adding hydroxyl radicals such as ozone, chlorine or hypochlorite to wastewater.27: 1220  These hydroxyl radical then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, and salts.28

Anaerobic treatment

Anaerobic wastewater treatment processes (for example UASB, EGSB) are also widely applied in the treatment of industrial wastewaters and biological sludge.

Polishing

Further information: Sewage treatment § Fourth treatment stage

Polishing refers to treatments made in further advanced treatment steps after the above methods (also called "fourth stage" treatment). These treatments may also be used independently for some industrial wastewater. Chemical reduction or pH adjustment minimizes chemical reactivity of wastewater following chemical oxidation.29: 439  Carbon filtering removes remaining contaminants and impurities by chemical absorption onto activated carbon.30: 1138  Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.

See also

  • Water portal
  • Media related to Wastewater treatment at Wikimedia Commons

References

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  2. Metcalf & Eddy Wastewater Engineering: Treatment and Reuse (4th ed.). New York: McGraw-Hill. 2003. ISBN 0-07-112250-8. 0-07-112250-8 ↩

  3. Takman, Maria; Svahn, Ola; Paul, Catherine; Cimbritz, Michael; Blomqvist, Stefan; Struckmann Poulsen, Jan; Lund Nielsen, Jeppe; Davidsson, Åsa (2023-10-15). "Assessing the potential of a membrane bioreactor and granular activated carbon process for wastewater reuse – A full-scale WWTP operated over one year in Scania, Sweden". Science of the Total Environment. 895: 165185. Bibcode:2023ScTEn.89565185T. doi:10.1016/j.scitotenv.2023.165185. ISSN 0048-9697. PMID 37385512. S2CID 259296091. https://doi.org/10.1016%2Fj.scitotenv.2023.165185 ↩

  4. Tchobanoglous, George; Burton, Franklin L.; Stensel, H. David (2003). Metcalf & Eddy Wastewater Engineering: Treatment and Reuse (4th ed.). McGraw-Hill. ISBN 978-0-07-112250-4. 978-0-07-112250-4 ↩

  5. Salai, Ramasamy (1 May 2025). "Wastewater Treatment Plants and Processes – A Complete Guide". Pervel.in. Archived from the original on 3 June 2025. Retrieved 1 June 2025. https://pervel.in/wastewater-treatment-plants-processes-a-complete-guide/ ↩

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  7. Takman, Maria; Svahn, Ola; Paul, Catherine; Cimbritz, Michael; Blomqvist, Stefan; Struckmann Poulsen, Jan; Lund Nielsen, Jeppe; Davidsson, Åsa (2023-10-15). "Assessing the potential of a membrane bioreactor and granular activated carbon process for wastewater reuse – A full-scale WWTP operated over one year in Scania, Sweden". Science of the Total Environment. 895: 165185. Bibcode:2023ScTEn.89565185T. doi:10.1016/j.scitotenv.2023.165185. ISSN 0048-9697. PMID 37385512. S2CID 259296091. https://doi.org/10.1016%2Fj.scitotenv.2023.165185 ↩

  8. Von Sperling, M. (2007). "Wastewater Characteristics, Treatment and Disposal". Water Intelligence Online. 6. doi:10.2166/9781780402086. ISSN 1476-1777. Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License https://www.iwapublishing.com/books/9781843391616/wastewater-characteristics-treatment-and-disposal ↩

  9. Takman, Maria; Svahn, Ola; Paul, Catherine; Cimbritz, Michael; Blomqvist, Stefan; Struckmann Poulsen, Jan; Lund Nielsen, Jeppe; Davidsson, Åsa (2023-10-15). "Assessing the potential of a membrane bioreactor and granular activated carbon process for wastewater reuse – A full-scale WWTP operated over one year in Scania, Sweden". Science of the Total Environment. 895: 165185. Bibcode:2023ScTEn.89565185T. doi:10.1016/j.scitotenv.2023.165185. ISSN 0048-9697. PMID 37385512. S2CID 259296091. https://doi.org/10.1016%2Fj.scitotenv.2023.165185 ↩

  10. Jones, Edward R.; van Vliet, Michelle T. H.; Qadir, Manzoor; Bierkens, Marc F. P. (2021). "Country-level and gridded estimates of wastewater production, collection, treatment and reuse". Earth System Science Data. 13 (2): 237–254. Bibcode:2021ESSD...13..237J. doi:10.5194/essd-13-237-2021. ISSN 1866-3508. https://essd.copernicus.org/articles/13/237/2021/ ↩

  11. Jones, Edward R.; van Vliet, Michelle T. H.; Qadir, Manzoor; Bierkens, Marc F. P. (2021). "Country-level and gridded estimates of wastewater production, collection, treatment and reuse". Earth System Science Data. 13 (2): 237–254. Bibcode:2021ESSD...13..237J. doi:10.5194/essd-13-237-2021. ISSN 1866-3508. https://essd.copernicus.org/articles/13/237/2021/ ↩

  12. Tchobanoglous G, Burton FL, Stensel HD (2003). Metcalf & Eddy Wastewater Engineering: treatment and reuse (4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0. 0-07-041878-0 ↩

  13. George Tchobanoglous; Franklin L. Burton; H. David Stensel (2003). "Chapter 3: Analysis and Selection of Wastewater Flowrates and Constituent Loadings". Metcalf & Eddy Wastewater engineering: treatment and reuse (4th ed.). Boston: McGraw-Hill. ISBN 0-07-041878-0. OCLC 48053912. 0-07-041878-0 ↩

  14. Von Sperling, M. (2007). "Wastewater Characteristics, Treatment and Disposal". Water Intelligence Online. 6. doi:10.2166/9781780402086. ISSN 1476-1777. Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License https://www.iwapublishing.com/books/9781843391616/wastewater-characteristics-treatment-and-disposal ↩

  15. "Pollution Prevention Case Studies". Washington, D.C.: U.S. Environmental Protection Agency (EPA). 2021-08-11. https://www.epa.gov/p2/pollution-prevention-case-studies ↩

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  21. Gupta, Ashok; Yan, Denis, eds. (2016-01-01), "Chapter 16 - Gravity Separation", Mineral Processing Design and Operations (Second Edition), Amsterdam: Elsevier, pp. 563–628, doi:10.1016/B978-0-444-63589-1.00016-2, ISBN 978-0-444-63589-1, retrieved 2020-11-30 978-0-444-63589-1 ↩

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  23. BERGENDAHL, JOHN. "Applications of Advanced Oxidation for Wastewater Treatment" (PDF). Dept. Of Civil & Environmental Engineering, WPI. Archived (PDF) from the original on 2017-08-29. https://web.wpi.edu/Images/CMS/NEABC/wastewatersummary.pdf ↩

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  25. Wastewater engineering : treatment and reuse. George Tchobanoglous, Franklin L. Burton, H. David Stensel, Metcalf & Eddy (4th ed.). Boston: McGraw-Hill. 2003. ISBN 0-07-041878-0. OCLC 48053912.{{cite book}}: CS1 maint: others (link) 0-07-041878-0 ↩

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