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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="Khopkar-2004">{{cite book |last=Khopkar|first=S.M.|url=https://books.google.com/books?id=TAk21grzDZgC|title=Environmental Pollution Monitoring And Control|publisher=New Age International|year=2004|isbn=978-81-224-1507-0|location=New Delhi|page=299}}</ref> Sewage contains [[wastewater]] from households and businesses and possibly pre-treated [[Industrial wastewater treatment|industrial wastewater]]. There are a high number of sewage treatment processes to choose from. These can range from [[Decentralized wastewater system|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 [[Biochemical oxygen demand|biological oxygen demand]]) from sewage, using aerobic or anaerobic biological processes. A so-called quarternary 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
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 may include tertiary treatment with disinfection and possibly even a fourth treatment stage to remove micropollutants.<ref name=":1" />
At the global level, an estimated 52% of sewage is treated.<ref name="Jones-2021">{{Cite journal|last1=Jones|first1=Edward R.|last2=van Vliet|first2=Michelle T. H.|last3=Qadir|first3=Manzoor|last4=Bierkens|first4=Marc F. P.|date=2021|title=Country-level and gridded estimates of wastewater production, collection, treatment and reuse|url=https://essd.copernicus.org/articles/13/237/2021/|journal=Earth System Science Data|language=English|volume=13|issue=2|pages=237–254|doi=10.5194/essd-13-237-2021|bibcode=2021ESSD...13..237J|issn=1866-3508|doi-access=free}}</ref> However, sewage treatment rates are highly unequal for different countries around the world. For example, while [[World Bank high-income economy|high-income countries]] treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.<ref name="Jones-2021"/>
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[[File:Trickling filter sewage treatment plant in Brazil.png|thumb|[[Trickling filter]] sewage treatment plant at Onça Treatment Plant, [[Belo Horizonte]], Brazil]]
Examples for more low-tech, often less expensive sewage treatment systems are shown below. They often use little or no energy. Some of these systems do not provide a high level of treatment, or only treat part of the sewage (for example only the [[Blackwater (waste)|toilet wastewater]]), or they only provide pre-treatment, like septic tanks. On the other hand, some systems are capable of providing a good performance, satisfactory for several applications. Many of these systems are based on natural treatment processes, requiring large areas, while others are more compact. In most cases, they are used in rural areas or in small to medium-sized communities.
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For example, [[waste stabilization pond]]s are a low cost treatment option with practically no energy requirements but they require a lot of land.<ref name="Marcos2" />{{rp|236}} Due to their technical simplicity, most of the savings (compared with high tech systems) are in terms of operation and maintenance costs.<ref name="Marcos2" />{{rp|220–243}}
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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="Tilley etal-2014">{{cite book |
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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=== Population equivalent ===
The ''per person organic matter load'' is a parameter used in the design of sewage treatment plants. This concept is known as [[population equivalent]] (PE). The base value used for PE can vary from one country to another. Commonly used definitions used worldwide are: 1 PE equates to 60 gram of BOD per person per day, and it also equals 200 liters of sewage per day.<ref name="henze">{{Cite book |last1=Henze |first1=M. |url=http://iwaponline.com/ebooks/book/59/Biological-Wastewater-Treatment-Principles |title=Biological Wastewater Treatment: Principles, Modelling and Design |last2=van Loosdrecht |first2=M. C. M. |last3=Ekama |first3=G.A. |author-link3=G.A. Ekama |last4=Brdjanovic |first4=D. |date=2008
=== 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="Tilley etal-2014" /><ref>{{Cite journal |last1=Spuhler |first1=Dorothee |last2=Germann |first2=Verena |last3=Kassa |first3=Kinfe |last4=Ketema |first4=Atekelt Abebe |last5=Sherpa |first5=Anjali Manandhar|last6=Sherpa |first6=Mingma Gyalzen |last7=Maurer |first7=Max |last8=Lüthi |first8=Christoph |last9=Langergraber |first9=Guenter |date=2020 |title=Developing sanitation planning options: A tool for systematic consideration of novel technologies and systems |journal=Journal of Environmental Management |language=en |volume=271 |pages=111004 |doi=10.1016/j.jenvman.2020.111004 |pmid=32778289 |s2cid=221100596 |doi-access=free|bibcode=2020JEnvM.27111004S |hdl=20.500.11850/428109 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Spuhler |first1=Dorothee |last2=Scheidegger |first2=Andreas |last3=Maurer |first3=Max |date=2020 |title=Comparative analysis of sanitation systems for resource recovery: Influence of configurations and single technology components |journal=Water Research |language=en |volume=186 |pages=116281 |doi=10.1016/j.watres.2020.116281 |pmid=32949886 |bibcode=2020WatRe.18616281S |s2cid=221806742 |doi-access=free}}</ref>
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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=== Disinfection ===
[[Water disinfection|Disinfection]] of treated sewage aims to kill [[pathogen]]s (disease-causing microorganisms) prior to disposal. It is increasingly effective after more elements of the foregoing treatment sequence have been completed.<ref name="Metcalf & Eddy">{{cite book|author=Metcalf & Eddy, Inc.|title=Wastewater Engineering|publisher=McGraw-Hill|year=1972|isbn=978-0-07-041675-8|location=New York}}</ref>{{rp|359}} The purpose of disinfection in the treatment of sewage is to substantially reduce the number of pathogens in the water to be discharged back into the environment or to be reused. The target level of reduction of biological contaminants like pathogens is often regulated by the presiding governmental authority. The effectiveness of disinfection depends on the quality of the water being treated (e.g. [[turbidity]], pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Water with high turbidity will be treated less successfully, since solid matter can shield organisms, especially from [[ultraviolet light]] or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include [[ozone]], [[chlorine]], [[ultraviolet light]], or [[sodium hypochlorite]].<ref name="EPA Primer" />{{rp|16}} [[Monochloramine]], which is used for drinking water, is not used in the treatment of sewage because of its persistence.
[[Water chlorination|Chlorination]] remains the most common form of treated sewage disinfection in many countries due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be [[carcinogenic]] or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.
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== Global situation ==
[[File:
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}}
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