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'''Sewage treatment''' (or '''domestic wastewater treatment''', '''municipal wastewater 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.<ref name="
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.<ref name="Marcos2" />{{rp|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 country|developing countries]] and in rural areas with low population densities, sewage is often treated by various [[Sanitation#Onsite sanitation|on-site sanitation]] systems and not conveyed in sewers. These systems include [[septic tank]]s connected to [[Septic drain field|drain fields]], [[Onsite sewage facility|on-site sewage systems]] (OSS), [[vermifilter]] systems and many more. On the other hand, advanced and relatively expensive sewage treatment plants in cities that can afford them may include tertiary treatment with disinfection and possibly even a fourth treatment stage to remove micropollutants.
At the global level, an estimated 52% of sewage is treated.<ref name="
The treatment of sewage is part of the field of [[sanitation]]. Sanitation also includes the management of [[human waste]] and [[solid waste]] as well as [[stormwater]] (drainage) management.<ref>{{cite web|title=Sanitation|url=https://www.who.int/topics/sanitation/en/|access-date=2020-02-23|work=Health topics|publisher=World Health Organization}}</ref> The term ''sewage treatment plant'' is often used interchangeably with the term ''wastewater treatment plant''.<ref name="Metcalf2014">{{Cite book|last=Metcalf & Eddy|url=https://www.worldcat.org/oclc/858915999|title=Wastewater engineering : treatment and resource recovery|date=2014|others=George Tchobanoglous, H. David Stensel, Ryujiro Tsuchihashi, Franklin L. Burton, Mohammad Abu-Orf, Gregory Bowden|isbn=978-0-07-340118-8|edition=Fifth|location=New York, NY|oclc=858915999}}</ref><ref name="Marcos2" />
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==Terminology==
[[File:Marlborough East Wastewater Treatment Plant Aerial.JPG|thumb|Activated sludge sewage treatment plant in [[Massachusetts]], US]]
The term ''sewage treatment plant'' (STP) (or ''sewage treatment works'') is nowadays often replaced with the term ''[[wastewater treatment]] plant'' (WWTP).<ref name="Metcalf2014" /><ref name="
The terms ''water recycling center'' or ''water reclamation plants'' are also in use as synonyms.
== Purposes and overview ==
The overall aim of treating sewage is to produce an [[effluent]] that can be discharged to the environment while causing as little [[water pollution]] as possible, or to produce an effluent that can be [[Reclaimed water|reused]] in a useful manner.<ref name="
With regards to biological treatment of sewage, the treatment objectives can include various degrees of the following: to transform or remove organic matter, nutrients (nitrogen and phosphorus), pathogenic organisms, and specific trace organic constituents (micropollutants).<ref name="Metcalf2014" />{{rp|548}}
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=== Disposal or treatment options ===
There are other process options which may be classified as disposal options, although they can also be understood as basic treatment options. These include: [[Biosolids|Application of sludge]], [[irrigation]], [[Dry well|soak pit]], [[Septic drain field|leach field]], [[fish pond]], floating plant pond, water disposal/[[groundwater recharge]], surface disposal and storage.<ref name="
The application of sewage to land is both: a type of treatment and a type of final disposal.<ref name="Marcos2" />{{rp|189}} It leads to groundwater recharge and/or to evapotranspiration. Land application include slow-rate systems, rapid infiltration, subsurface infiltration, overland flow. It is done by flooding, furrows, sprinkler and dripping. It is a treatment/disposal system that requires a large amount of land per person.
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=== Process selection ===
When choosing a suitable sewage treatment process, decision makers need to take into account technical and economical criteria.<ref name="Marcos2" />{{rp|215}} Therefore, each analysis is site-specific. A [[Life-cycle assessment|life cycle assessment]] (LCA) can be used, and criteria or weightings are attributed to the various aspects. This makes the final decision subjective to some extent.<ref name="Marcos2" />{{rp|216}} A range of publications exist to help with technology selection.<ref name="Marcos2" />{{rp|221}}<ref name="
In [[Developed country|industrialized countries]], the most important parameters in process selection are typically efficiency, reliability, and space requirements. In [[Developing country|developing countries]], they might be different and the focus might be more on construction and operating costs as well as process simplicity.<ref name="Marcos2" />{{rp|218}}
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Over time, different treatment configurations for activated sludge processes have evolved to achieve high levels of nitrogen removal. An initial scheme was called the Ludzack–Ettinger Process. It could not achieve a high level of denitrification.<ref name="Metcalf2014" />{{rp|616}} The Modified Ludzak–Ettinger Process (MLE) came later and was an improvement on the original concept. It recycles mixed liquor from the discharge end of the aeration tank to the head of the anoxic tank. This provides nitrate for the facultative bacteria.<ref name="Metcalf2014" />{{rp|616}}
There are other process configurations, such as variations of the Bardenpho process.<ref name="
====Phosphorus removal====
Studies of United States sewage in the late 1960s estimated mean per capita contributions of {{convert|500|g}} in urine and feces, {{convert|1000|g}} in synthetic detergents, and lesser variable amounts used as corrosion and scale control chemicals in water supplies.<ref>{{Cite report |date=1976 |title=Process Design Manual for Phosphorus Removal |url=http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=20007TYZ.txt |pages=2–1 |publisher=EPA |id=EPA 625/1-76-001a}}</ref> Source control via alternative detergent formulations has subsequently reduced the largest contribution, but naturally the phosphorus content of urine and feces remained unchanged.
Phosphorus can be removed biologically in a process called [[enhanced biological phosphorus removal]]. In this process, specific bacteria, called [[polyphosphate-accumulating organisms]] (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass).<ref name="
Phosphorus removal can also be achieved by chemical [[Precipitation (chemistry)|precipitation]], usually with [[salt (chemistry)|salts]] of [[iron]] (e.g. [[ferric chloride]]) or [[aluminum]] (e.g. [[alum]]), or lime.<ref name="EPA Primer" />{{rp|18}} This may lead to a higher sludge production as hydroxides precipitate and the added chemicals can be expensive. [[Chemical phosphorus removal]] requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal. Another method for phosphorus removal is to use granular [[Laterite#Waste water treatment|laterite]] or [[zeolite]].<ref name="wood">{{cite journal|author1=Wood, R. B.|author2=McAtamney, C.F.|date=December 1996|title=Constructed wetlands for waste water treatment: the use of laterite in the bed medium in phosphorus and heavy metal removal|journal=Hydrobiologia|volume=340|pages=323–331|doi=10.1007/BF00012776|number=1–3|s2cid=6182870}}</ref><ref>{{cite journal|last1=Wang|first1=Shaobin|last2=Peng|first2=Yuelian|date=2009-10-09|title=Natural zeolites as effective adsorbents in water & wastewater treatment|url=http://ida-ore.com/wp-content/uploads/2020/02/Wang_Natural-zealots-as-effective-absorbents.pdf|journal=Chemical Engineering Journal|volume=156|issue=1|pages=11–24|doi=10.1016/j.cej.2009.10.029|access-date=2019-07-13}}</ref>
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Some systems use both biological phosphorus removal and chemical phosphorus removal. The chemical phosphorus removal in those systems may be used as a backup system, for use when the biological phosphorus removal is not removing enough phosphorus, or may be used continuously. In either case, using both biological and chemical phosphorus removal has the advantage of not increasing sludge production as much as chemical phosphorus removal on its own, with the disadvantage of the increased initial cost associated with installing two different systems.
Once removed, phosphorus, in the form of a phosphate-rich [[sewage sludge]], may be sent to landfill or used as fertilizer in admixture with other digested sewage sludges. In the latter case, the treated sewage sludge is also sometimes referred to as biosolids. 22% of the world's phosphorus needs could be satisfied by recycling residential wastewater.<ref name="
=== Fourth treatment stage ===
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=== Irrigation ===
{{See also|Sewage farm}}
Increasingly, people use treated or even untreated sewage for [[irrigation]] to produce crops. Cities provide lucrative markets for fresh produce, so are attractive to farmers. Because agriculture has to compete for increasingly scarce water resources with industry and municipal users, there is often no alternative for farmers but to use water polluted with sewage directly to water their crops. There can be significant health hazards related to using water loaded with pathogens in this way. The [[World Health Organization]] developed guidelines for safe use of wastewater in 2006.<ref name="
[[File:Circular secondary sedimentation tank.png|thumb|Circular secondary sedimentation tank at [[activated sludge]] sewage treatment plant at Arrudas Treatment Plant, [[Belo Horizonte]], Brazil ]]
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Before the 20th century in Europe, sewers usually discharged into a [[body of water]] such as a river, lake, or ocean. There was no treatment, so the breakdown of the [[human waste]] was left to the [[ecosystem]]. This could lead to satisfactory results if the [[assimilative capacity]] of the ecosystem is sufficient which is nowadays not often the case due to increasing population density.<ref name="Marcos2" />{{rp|78}}
Today, the situation in urban areas of [[Developed country|industrialized countries]] is usually that sewers route their contents to a sewage treatment plant rather than directly to a body of water. In many [[developing countries]], however, the bulk of municipal and industrial wastewater is discharged to rivers and the [[ocean]] without any treatment or after preliminary treatment or primary treatment only. Doing so can lead to [[water pollution]]. Few reliable figures exist on the share of the wastewater collected in sewers that is being treated in the world. A global estimate by [[UNDP]] and [[UN-Habitat]] in 2010 was that 90% of all wastewater generated is released into the environment untreated.<ref>{{cite book|last1=Corcoran, E., C. Nellemann, E. Baker, R. Bos, D. Osborn, H. Savelli (eds)|url=http://www.unep.org/pdf/SickWater_screen.pdf|title=Sick water? : the central role of wastewater management in sustainable development : a rapid response assessment|date=2010|publisher=UNEP/GRID-Arendal|isbn=978-82-7701-075-5|location=Arendal, Norway}}</ref> A more recent study in 2021 estimated that globally, about 52% of sewage is treated.<ref name="
The [[Joint Monitoring Programme for Water Supply and Sanitation|Joint Monitoring Programme (JMP)]] for Water Supply and Sanitation by WHO and UNICEF report in 2021 that 82% of people with sewer connections are connected to sewage treatment plants providing at least secondary treatment.<ref name="
=== Global targets ===
[[Sustainable Development Goal 6]] has a Target 6.3 which is formulated as follows: "By 2030, improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally."<ref name="SDGTracker6" /> The corresponding Indicator 6.3.1 is the "proportion of wastewater safely treated". It is anticipated that wastewater production would rise by 24% by 2030 and by 51% by 2050.<ref name="
Data in 2020 showed that there is still too much uncollected household wastewater: Only 66% of all household wastewater flows were collected at treatment facilities in 2020 (this is determined from data from 128 countries).<ref name="
== History ==
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===Europe===
In the European Union, 0.8% of total energy consumption goes to wastewater treatment facilities.<ref name="
In October 2021, [[United Kingdom|British]] [[Member of Parliament (United Kingdom)|Members of Parliament]] voted to continue allowing untreated sewage from combined sewer overflows to be released into waterways.<ref>{{cite news |date=7 September 2021 |title=Government says polluters can dump risky sewage into rivers as Brexit disrupts water treatment |work=The Independent |url=https://www.independent.co.uk/climate-change/brexit-raw-sewerage-water-treatment-b1915765.html}}</ref><ref>{{cite news |date=26 October 2021 |title=Why sewage is causing a political stink |work=The Week |url=https://www.theweek.co.uk/news/uk-news/954581/the-uproar-over-sewage-explained}}</ref>{{Excerpt|Urban Waste Water Treatment Directive#Description|paragraphs=1}}
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