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'''Sewage sludge''' refers to the residual, [[quasi-solid|semi-solid]] material that is produced as a by-product during [[wastewater treatment]] of industrial or municipal wastewater.
When fresh sewage or wastewater
In the industrialized world, cities in particular have had a difficult time dealing with sewage waste.<ref>{{cite web|url=http://www.cefns.nau.edu/Projects/WDP/resources/History/History.htm}}</ref> In 1992, a United States ban on ocean dumping of human sewage sludge went into effect, leaving cities the expensive option of having to dispose sewage waste in landfills.<ref>{{cite news|title=Human sewage ocean dumping ban|newspaper=http://www.nytimes.com/1992/06/29/nyregion/ocean-dumping-ending-but-not-problems-new-york-can-t-ship-bury-burn-its-sludge.html?pagewanted=all&src=pm}}</ref>▼
Seeing as sewage contains not only heavy metals<ref>{{cite web|url=http://nepis.epa.gov/Exe/ZyNET.exe/9100FMSN.TXT?ZyActionD=ZyDocument&Client=EPA&Index=1976+Thru+1980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C76thru80%5CTxt%5C00000015%5C9100FMSN.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL}}</ref> and disease pathogens such as [[Clostridium difficile]]<ref>{{cite journal|url=http://aem.asm.org/content/78/18/6643.full}}</ref> but nutrients as well, companies such as Synagro embraced the nutrient idea and marketed sludge as "biosolids" to farmers as a free fertilizer.<ref>{{cite web|url=http://www.synagro.com/solutions/reusing/land-application/}}</ref>▼
▲When fresh sewage or wastewater is added to a [[settling]] [[Storage tank|tank]], approximately 50% of the suspended solid matter will settle out in an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be "fresh" before anaerobic processes become active. The sludge will become [[putrefaction|putrescent]] in a short time once anaerobic bacteria take over, and must be removed from the [[sedimentation tank]] before this happens.
This is accomplished in one of two ways. In an [[Imhoff tank]], fresh sludge is passed through a slot to the lower story or digestion chamber where it is [[decomposition|decomposed]] by [[anaerobic bacteria]], resulting in liquefaction and reduced volume of the sludge. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then [[landfill]]ing. More commonly with domestic <!-- Domestic to where? I assume this is referring to the USA. America != centre of reference for the world --> sewage, the fresh sludge is continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and, as a result, digest much more rapidly and efficiently.
Excess solids from biological processes such as [[activated sludge]] may still be referred to as "sludge", but "[[#Biosolids|biosolids]]" or "compost" are [[public relations]] terms sometimes used to refer to treated human waste.<ref>{{cite web|title=Biosolids = PR redefine human waste|url=http://www.perelandra-ltd.com/Health-Watch-3A-Sewage-Sludge-Biosolids-W288.aspx}}</ref><ref>{{cite web|title=Interview with author of 'Toxic Sludge is Good For You'|url=http://www.youtube.com/watch?v=iHsIjMPP2M8}}</ref><ref>{{cite web|title=Sludge rebranded as compost in latest PR campaign|url=https://www.commondreams.org/view/2012/03/18-2}}</ref> Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.
Sewage sludge is usually treated by one or several of the following treatment steps: thickening, dewatering, anaerobic digestion or composting. After treatment, sludge is either disposed of in landfills or applied to land where its fertilising properties can be used. However, this is only possible if the sludge is not polluted too much with heavy metals and other micro-pollutants. National regulations for land application of treated sewage sludge differ widely and e.g. in the US there are wide-spread disputes about this practice.
▲In the industrialized world, cities in particular have had a difficult time dealing with sewage waste.<ref>{{cite web|url=http://www.cefns.nau.edu/Projects/WDP/resources/History/History.htm}}</ref> In 1992, a United States ban on ocean dumping of human sewage sludge went into effect, leaving cities the expensive option of having to dispose sewage
▲Seeing as sewage contains not only heavy metals<ref>{{cite web|url=http://nepis.epa.gov/Exe/ZyNET.exe/9100FMSN.TXT?ZyActionD=ZyDocument&Client=EPA&Index=1976+Thru+1980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C76thru80%5CTxt%5C00000015%5C9100FMSN.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL}}</ref> and disease pathogens such as [[Clostridium difficile]]<ref>{{cite journal|url=http://aem.asm.org/content/78/18/6643.full}}</ref> but nutrients as well, companies such as Synagro embraced the nutrient idea and marketed sludge as "biosolids" to farmers as a free fertilizer.<ref>{{cite web|url=http://www.synagro.com/solutions/reusing/land-application/}}</ref>
== Environmental justice==
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