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Moitas veces as augas pluviais mestúranse directamente coas augas residuais no mesmo sumidoiro. Nalgunhas áreas urbanas, as augas residuais circulan por separado polo seu propio sistema de sumidoiros sanitario, e a auga pluvial que corre polas rúas é recollida en drenaxes de tormenta. Nos sistemas combinados, onde se mesturan todas as augas, durante os períodos de elevada precipitación o grande volume de líquido pode facer que a capacidade de tratamento do sistema se vexa superada e se vertan ao ambiente augas residuais non tratadas. Isto pode supoñer un serio problema de saúde pública ou ambiental.
==TreatmentTratamento==
{{AP|Depuración de augas residuais}}
{{Further2|[[Sewage treatment]], [[Agricultural wastewater treatment]], and [[Industrial wastewater treatment]]}}
ThereHai arenumerosos numerousprocesos processesque thatpoden canutilizarse bepara usedlimpar to clean upas wastewatersaugas dependingresiduais ondependendo thedo typetipo ande extentgrao ofda contaminationcontaminación. ThereHai aredúas twoestratexias basic approachesbásicas: to use theconstruír wastezonas inhúmidas thepara waterverter as aaugas resourceou (suchtratalas asen constructedestacións wetlands)de ortratamento strictlyde asaugas a pollutionresiduais (such[[EDAR]]), asque theé majorityo ofmáis today's treatment plants)común. MostNas wastewaterestacións isde treatedtratamento inas industrial-scaleaugas energyson intensivesometidas [[sewagea treatment|wastewaterprocesos treatmentfísicos, plants]]químicos (WWTPs)e whichbiolóxicos includepara physical, chemical and biologicala treatmentsúa processesdepuración. HoweverPorén, theo useuso ofde [[septicfosa tank]]sséptica|fosas and other On-Site Sewage Facilities ([[OSSFsépticas]]) isou widespreadtanques insépticos ruralé areas,común servingen upzonas torurais 20illadas percentdos ofsistemas thede homes in the U.Ssumidoiros.<ref>U.S. Environmental Protection Agency, Washington, D.C. (2008). [http://water.epa.gov/aboutow/owm/upload/2009_06_22_septics_septic_systems_factsheet.pdf "Septic Systems Fact Sheet."] EPA publication no. 832-F-08-057.</ref>
TheO mostsistema importantde aerobictratamento treatmentaeróbico systemmáis isimportante theé o proceso de [[activatedlodos sludgeactivados]] process, basedbaseado onno themantemento maintenancee andrecirculación recirculationdunha ofcomplexa abiomasa complexcomposta biomasspor composedmicroorganismos bycapaces micro-organismsde ableabsorber to absorb ande [[adsorbadsorber]] thea organicmateria matterorgánica carriedpresente innas theaugas wastewaterresiduais. AnaerobicOs wastewaterprocesos treatmentde processestratamento anaeróbico (ver [[Upflowdixestión anaerobicanaeróbica]], sludgemétodos blanket digestion|[[UASB]], [[Expanded granular sludge bed digestion|EGSB]]) areaplícanse alsotamén widelymoito appliedpara ino thetratamento treatmentde ofaugas industrialresiduais wastewatersindustriais ande biologicalldodos sludgebiolóxicos. SomeAlgunhas wastewateraugas mayresiduais bepoden highlyser treatedintensamente andtratadas reusede asdespois [[reclaimedreutilizadas water]]como augas recicladas. Increasingly, for most wastewaters [[ecological]] approaches using [[reed bed]] systems such as [[constructed wetland]]s are being used. Tertiary treatment is being increasingly applied and most common technologies are [[micro filtration]] or [[synthetic membranes]]. After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality (without its minerals). [[Nitrate]]s can be removed from wastewater by natural processes in wetlands but also via intensive microbial [[denitrification]], for which a small amount of [[methanol]] is typically added to provide the bacteria with a source of carbon. Ozone wastewater treatment is also growing in popularity, and requires the use of an [[ozone generator]], which decontaminates the water as [[ozone]] bubbles percolate through the tank but is energy intensive.
Disposal of wastewaters from an industrial plant is a difficult and costly problem. Most [[petroleum refineries]], chemical and [[petrochemical]] plants<ref>{{cite book | author=Beychok, Milton R. | title=[[Aqueous Wastes from Petroleum and Petrochemical Plants]] | edition=1st | publisher=[[John Wiley & Sons]] | year=1967 | id= [[LCCN]] 67019834}}</ref><ref>{{cite book|author=Tchobanoglous, G., Burton, F.L., and Stensel, H.D.|title=Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc.|edition=4th|publisher=[[McGraw-Hill Book Company]]|year=2003|isbn=0-07-041878-0}}</ref> have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans. Constructed wetlands are being used in an increasing number of cases as they provided high quality and productive on-site treatment. Other industrial processes that produce a lot of waste-waters such as [[paper and pulp production]] has created environmental concern, leading to development of processes to recycle water use within plants before they have to be cleaned and disposed.<ref>J. F. Byrd, M. D. Ehrke, J. I. Whitfield. (1984) [http://www.jstor.org/stable/25042250 "New Bleached Kraft Pulp Plant in Georgia: State of the Art Environmental Control"] ''Water pollution control federation'' '''56'''(4): 378–385.</ref>
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