Editing Sewage sludge (section)

==Treatment process==
{{Main|Sewage sludge treatment}}

[[Sewage sludge treatment]] is the process of removing contaminants from [[wastewater]]. Sewage sludge is produced from the [[Wastewater Treatment|treatment of wastewater]] in [[sewage treatment plant]]s and consists of two basic forms — raw primary sludge and secondary sludge, also known as [[activated sludge]] in the case of the activated sludge process.

Sewage sludge is usually treated by one or several of the following treatment steps: lime stabilization, thickening, dewatering, drying, anaerobic digestion or [[composting]]. Some treatment processes, such as composting and alkaline stabilization, that involve significant amendments may affect contaminant strength and concentration: depending on the process and the contaminant in question, treatment may decrease or in some cases increase the bioavailability and/or solubility of contaminants.<ref name="jeq.scijournals.org">{{cite journal |first1=Brian K. |last1=Richards |first2=John H. |last2=Peverly |first3=Tammo S. |last3=Steenhuis |first4=Barry N. |last4=Liebowitz |year=1997 |title=Effect of Processing Mode on Trace Elements in Dewatered Sludge Products |journal=Journal of Environmental Quality |volume=26 |issue=3 |pages=782–8 |doi=10.2134/jeq1997.00472425002600030027x|bibcode=1997JEnvQ..26..782R }}</ref> Regarding sludge stabilization processes, anaerobic and aerobic digestion seem to be the most common used methods in EU-27.<ref name="Kelessidis and Stasinakis, 2012">Kelessidis and Stasinakis, 2012. COMPARATIVE STUDY OF THE METHODS USED FOR TREATMENT AND FINAL DISPOSAL OF SEWAGE SLUDGE IN EUROPEAN COUNTRIES, Waste Management, vol. 32, June 2012, p. 1186-1195. [https://www.sciencedirect.com/science/article/pii/S0956053X12000268?via%3Dihub Kelessidis and Stasinakis, 2012]</ref>

When fresh sewage or wastewater enters a primary [[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. Most commonly, the fresh sludge is continuously extracted from the bottom of a hopper-shaped tank by mechanical scrapers and passed to separate sludge-digestion tanks. In some treatment plants an [[Imhoff tank]] is used: sludge settles through a slot into the lower story or digestion chamber, where it is [[decomposition|decomposed]] by [[anaerobic bacteria]], resulting in liquefaction and reduced volume of the sludge. [[File:SEWAGE SLUDGE SETTLES TO BOTTOM OF BEAKER. SEWAGE TREATMENT PLANT - NARA - 543807.jpg|thumb|Sewage sludge in a beaker from a treatment plant]]The secondary treatment process also generates a sludge largely composed of bacteria and protozoa with entrained fine solids, and this is removed by settlement in secondary settlement tanks. Both sludge streams are typically combined and are processed by anaerobic or aerobic treatment process at either elevated or ambient temperatures. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then [[landfill]]ing.

Following treatment, sewage sludge is either landfilled, dumped in the ocean, incinerated, applied on agricultural land or, in some cases, retailed or given away for free to the general public.<ref>{{Cite web|url= http://www.sludgenews.org/about/sludgenews.aspx?id=5|title= Branded products containing sewage sludge|website= Sludge News|access-date= 16 January 2015|date= 2007-11-30}}</ref><ref>{{Cite news|url= http://www.prwatch.org/news/2013/05/12103/trade-group-offers-free-sewage-sludge-compost-community-gardens-million-tomato-ca|title= Trade Group Offers Free Sewage Sludge "Compost" to Community Gardens in "Million Tomato Campaign" for Food Banks|last= Wilce|first= Rebekah|date= 9 May 2013|website= [[Center for Media and Democracy|PRWatch]]|access-date= 16 January 2015}}</ref> According to a review article published in 2012, sludge reuse (including direct agricultural application and composting) was the predominant choice for sludge management in EU-15 (53% of produced sludge), following by incineration (21% of produced sludge). On the other hand, the most common disposal method in EU-12 countries was landfilling.<ref name="Kelessidis and Stasinakis, 2012" />