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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.<ref name=":1">{{Cite journal |last1=Takman |first1=Maria |last2=Svahn |first2=Ola |last3=Paul |first3=Catherine |last4=Cimbritz |first4=Michael |last5=Blomqvist |first5=Stefan |last6=Struckmann Poulsen |first6=Jan |last7=Lund Nielsen |first7=Jeppe |last8=Davidsson |first8=Åsa |date=2023-10-15 |title=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 |journal=Science of the Total Environment |volume=895 |pages=165185 |doi=10.1016/j.scitotenv.2023.165185 |pmid=37385512 |bibcode= 2023ScTEn.
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" />
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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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