Wastewater treatment

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Sewage treatment plant (a type of wastewater treatment plant) in Cuxhaven, Germany

Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called water reclamation).[1] The treatment process takes place in a wastewater treatment plant. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. For domestic wastewater (also called municipal wastewater or sewage), the treatment plant is called a sewage treatment plant. For industrial wastewater, treatment either takes place in a separate industrial wastewater treatment plant, or in a sewage treatment plant (usually after some form of pre-treatment). Further types of wastewater treatment plants include agricultural wastewater treatment plants and leachate treatment plants.

Processes commonly used include phase separation (such as sedimentation), biological and chemical processes (such as oxidation) or polishing. The main by-product from wastewater treatment plants is a type of sludge which is usually treated in the same or another wastewater treatment plant.[2]:Ch.14 Biogas can be another by-product if anaerobic treatment processes are used.

Some wastewater may be highly treated and reused as reclaimed water. The main purpose of wastewater treatment is for the treated wastewater to be able to be disposed or reused safely. However, before it is treated, the options for disposal or reuse must be considered so the correct treatment process is used on the wastewater.

The term "wastewater treatment" is in the literature often used to mean "sewage treatment".[3] Strictly speaking, wastewater treatment is broader than sewage treatment.

Types of treatment plants[edit]

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.

Types of wastewater treatment plants include:

Sewage treatment plants[edit]

Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage. Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated effluent) that is safe enough for release into the environment. A by-product of sewage treatment is a semi-solid waste or slurry, called sewage sludge. The sludge has to undergo further treatment before being suitable for disposal or application to land. The term "sewage treatment plant" is often used interchangeably with the term "wastewater treatment plant".[4]

For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant that has usually received pre-treatment at the factories to reduce the pollutant load. If the sewer system is a combined sewer, then it will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage is conveyed in sewerage which comprises the drains, pipework and pumps to convey the sewage to the treatment works inlet. The treatment of municipal wastewater is part of the field of sanitation. Sanitation also includes the management of human waste and solid waste as well as stormwater (drainage) management.[5]

At the global level, an estimated 52% of municipal wastewater is treated.[6] However, wastewater treatment rates are highly unequal for different countries around the world. For example, while high-income countries treat approximately 74% of their municipal wastewater, developing countries treat an average of just 4.2%.[6] Wastewater that is discharged untreated into the environment can cause water pollution.[7]

Industrial wastewater treatment plants[edit]

Wastewater from an industrial process can be converted at a treatment plant to solids and treated water for reuse.

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. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have onsite 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.[8]:1412 Industrial wastewater treatment plants are required where municipal sewage treatment plants are unavailable, do not have sufficient capacity or cannot adequately treat specific industrial wastewaters.

Most industries produce some wastewater. Recent trends have been to minimize such production or to recycle treated wastewater within the production process. Sources of industrial wastewater include battery manufacturing, electric power plants, food industry, iron and steel industry, mines and quarries, nuclear industry, oil and gas extraction, organic chemicals manufacturing, petroleum refining and petrochemicals, pulp and paper industry, smelters, textile mills, industrial oil contamination, water treatment, 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, removal of toxic materials.

Agricultural wastewater treatment plants[edit]

Anaerobic lagoon for treatment of dairy wastes

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.[9]: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[edit]

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.[10]


Diagram of a typical surface-aerated basin for wastewater treatment.

The processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such as aerated lagoons, activated sludge, or bio-films in trickling filters.

To be effective, wastewater must be conveyed to a treatment plant by appropriate pipes and infrastructure, and the process itself must be subject to regulation and controls. 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, which is then settled out, an effluent stream of increasing purity is produced.[2][11]

Phase separation[edit]

Clarifiers are widely used for wastewater treatment.

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.[12]

Primary settling tank of wastewater treatment plant in Dresden-Kaditz, Germany


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."[13] 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.[14]:111–138

Biological and chemical processes[edit]


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.[15] Chemical oxidation is widely used for disinfection.

Aeration tank of an activated sludge process at the wastewater treatment plant in Dresden-Kaditz, Germany
Biochemical oxidation (secondary treatment)[edit]
This small secondary clarifier at a rural sewage treatment plant is a typical phase separation mechanism to remove biological solids formed in a suspended growth or fixed-film bioreactor.
Secondary treatment is a treatment process for wastewater (for example for sewage but also for some types of industrial wastewaters) to achieve a certain degree of effluent quality by using a sewage treatment plant with physical phase separation to remove settleable solids and a biological process to remove dissolved and suspended organic compounds. After this kind of treatment, the wastewater may be called as secondary-treated wastewater. Secondary treatment is the portion of a sewage treatment sequence removing dissolved and colloidal compounds measured as biochemical oxygen demand (BOD). Secondary treatment is traditionally applied to the liquid portion of sewage after primary treatment has removed settleable solids and floating material. Secondary treatment is usually performed by microorganisms in a managed aerobic habitat (however, it can also be an anaerobic process). 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 by biochemical oxidation of dissolved and colloidal organic compounds is widely used in sewage treatment and is applicable to some agricultural and industrial wastewaters.
Chemical oxidation[edit]

Advanced oxidation processes are used to remove some persistent organic pollutants and concentrations remaining after biochemical oxidation.[14]:363–408 Disinfection by chemical oxidation kills bacteria and microbial pathogens by adding hydroxyl radicals such as ozone, chlorine or hypochlorite to wastewater.[2]:1220 These hydroxyl radical then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, and salts.[16]

Anaerobic treatment[edit]

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


Polishing refers to treatments made following the above methods. These treatments may also be used independently for some industrial wastewater. Chemical reduction or pH adjustment minimizes chemical reactivity of wastewater following chemical oxidation.[14]:439 Carbon filtering removes remaining contaminants and impurities by chemical absorption onto activated carbon.[2]:1138 Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.


Some facilities such as oil and gas wells may be permitted to pump their wastewater underground through injection wells. Wastewater injection has been linked to induced seismicity.[17]

See also[edit]


  1. ^ "wastewater treatment | Process, History, Importance, Systems, & Technologies". Encyclopedia Britannica. October 29, 2020. Retrieved 2020-11-04.
  2. ^ a b c d Metcalf & Eddy, Inc. (2003). Wastewater Engineering: Treatment and Reuse (4th ed.). New York: McGraw-Hill. ISBN 0-07-112250-8.
  3. ^ Tchobanoglous, George; Burton, Franklin L.; Stensel, H. David; Metcalf & Eddy, Inc. (2003). Wastewater Engineering: Treatment and Reuse (4th ed.). McGraw-Hill. ISBN 978-0-07-112250-4.
  4. ^ Metcalf & Eddy (2014). Wastewater engineering : treatment and resource recovery. George Tchobanoglous, H. David Stensel, Ryujiro Tsuchihashi, Franklin L. Burton, Mohammad Abu-Orf, Gregory Bowden (Fifth ed.). New York, NY. ISBN 978-0-07-340118-8. OCLC 858915999.
  5. ^ "Sanitation". Health topics. World Health Organization. Retrieved 2020-02-23.
  6. ^ a b 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. doi:10.5194/essd-13-237-2021. ISSN 1866-3508.
  7. ^ WWAP (United Nations World Water Assessment Programme) (2017). The United Nations World Water Development Report 2017. Wastewater: The Untapped Resource. Paris. ISBN 978-92-3-100201-4. Archived from the original on 8 April 2017.
  8. ^ Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc (4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.CS1 maint: multiple names: authors list (link)
  9. ^ Reed, Sherwood C. (1988). Natural systems for waste management and treatment. E. Joe Middlebrooks, Ronald W. Crites. New York: McGraw-Hill. ISBN 0-07-051521-2. OCLC 16087827.
  10. ^ "Landfills Effluent Guidelines". EPA. 2018-03-16.
  11. ^ Primer for Municipal Waste water Treatment Systems (Report). Washington, DC: US Environmental Protection Agency (EPA). 2004. EPA 832-R-04-001..
  12. ^ Ajay Kumar Mishra Smart Materials for Waste Water Applications ,Wiley-Scrivener 2016 ISBN 111904118X https://onlinelibrary.wiley.com/doi/book/10.1002/9781119041214
  13. ^ Gupta, Ashok; Yan, Denis, eds. (2016-01-01), "Chapter 16 - Gravity Separation", Mineral Processing Design and Operations (Second Edition), Amsterdam: Elsevier, pp. 563–628, ISBN 978-0-444-63589-1, retrieved 2020-11-30
  14. ^ a b c Weber, Walter J. (1972). Physicochemical processes for water quality control. New York: Wiley-Interscience. ISBN 0-471-92435-0. OCLC 389818.
  15. ^ BERGENDAHL, JOHN. "Applications of Advanced Oxidation for Wastewater Treatment" (PDF). Dept. Of Civil & Environmental Engineering, WPI.
  16. ^ Deng, Yang; Zhao, Renzun (2015-09-01). "Advanced Oxidation Processes (AOPs) in Wastewater Treatment". Current Pollution Reports. 1 (3): 167–176. doi:10.1007/s40726-015-0015-z. ISSN 2198-6592.
  17. ^ van der Baan, Mirko; Calixto, Frank J. (2017-07-01). "Human-induced seismicity and large-scale hydrocarbon production in the USA and Canada". Geochemistry, Geophysics, Geosystems. 18 (7): 2467–2485. Bibcode:2017GGG....18.2467V. doi:10.1002/2017gc006915. ISSN 1525-2027.