Chlorination 162748 222400565 2008-06-29T04:08:08Z Benbest 167797 /* Alternatives */ clarifying words {{refimprove|talk=y|date=April 2008}} {{Original research|article|date=April 2008}} :''This article is about a water treatment process. For chlorination in organic chemistry, see [[Halogenation]].'' '''Chlorination''' is the process of adding the element [[chlorine]] to [[water]] as a method of [[water purification]] to make it fit for human consumption as [[drinking water]]. Water which has been treated with chlorine is effective in preventing the spread of [[disease]]. The chlorination of public drinking supplies was originally met with resistance, as people were concerned about the [[health effects]] of the practice. The use of chlorine has greatly reduced the prevalence of waterborne disease as it is effective against almost all [[bacteria]] and [[virus]]es, as well as [[amoeba]]. Chlorination is also used to sterilize the water in [[swimming pool]]s and as a disinfection stage in [[sewage treatment]]. It can also apply to the addition of chlorine to other elements, such as gold in the formation of [[gold chloride]]. ==History== The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed in 1910 by [[U.S. Army]] [[Major]] (later [[Brigadier General|Brig. Gen.]]) [[Carl Rogers Darnall]] (1867-1941), Professor of Chemistry at the [[Army Medical School]]. Shortly thereafter, Major (later [[Colonel|Col.]]) [[William J. L. Lyster]] (1869-1947) of the [[Army Medical Department]] used a solution of [[calcium hypochlorite]] in a linen bag to treat water. For many decades, Lyster's method remained the standard for U.S. ground forces in the field and in camps, implemented in the form of the familiar [[Lyster Bag]] (also spelled Lister Bag). Darnall's work became the basis for present day systems of municipal water purification. ==Chemistry in Water== When chlorine is added to water, it reacts to form a [[pH]] dependent equilibrium mixture of chlorine, [[hypochlorous acid]] and [[hydrochloric acid]]: :Cl<sub>2</sub> + H<sub>2</sub>O → HOCl + HCl Depending on the pH, hypochlorous acid partly [[Dissociation (chemistry)|dissociates]] to hydrogen and [[hypochlorite]] ions: :HClO → H<sup>+</sup> + ClO<sup>-</sup> In acidic solution, the major species are Cl<sub>2</sub> and HOCl while in alkaline solution effectively only ClO<sup>-</sup> is present. Very small concentrations of ClO<sub>2</sub><sup>-</sup>, ClO<sub>3</sub><sup>-</sup>, ClO<sub>4</sub><sup>-</sup> are also found<ref>.Shunji Nakagawara, Takeshi Goto, Masayuki Nara, Youichi Ozaqa, Kunimoto Hotta and Yoji Arata, "Spectroscopic Characterization and the pH Dependence of Bactericidal Activity of the Aqueous Chlorine Solution", Analytical Sciences, <b>14</b>, 69, 1998.</ref>. ==Drawbacks== Disinfection by chlorination can be problematic, in some circumstances. Chlorine can react with naturally occurring [[organic compound]]s found in the water supply to produce dangerous compounds, known as [[disinfection byproduct]]s (DBPs). The most common DBPs are [[trihalomethane]]s (THMs) and [[haloacetic acid]]s. Due to the [[carcinogenic]] potential of these compounds, federal regulations in the United States of America require regular monitoring of the concentration of these compounds in the distribution systems of municipal water systems. However, the [[World Health Organization]] has stated that the "Risks to health from DBPs are extremely small in comparison with inadequate disinfection." There are also other concerns regarding chlorine, including its volatile nature which causes it to disappear too quickly from the water system, and [[aesthetics|aesthetic]] concerns such as [[taste]] and odour. ==Alternatives== Chlorine in water is more than three times more effective as a disinfectant against ''[[Escherichia coli]]'' than an equivalent concentration of [[bromine]], and is more than six times more effective than an equivalent concentration of [[iodine]]<ref>{{cite journal | author=Koski TA, Stuart LS, Ortenzio LF | title=Comparison of chlorine, bromine, iodine as disinfectants for swimming pool water. | journal=APPLIED MICROBIOLOGY | volume=14 | issue=2 | year=1966 | pages=276-279 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=4959984 | id=PMID 4959984 }}</ref>. Several alternatives to traditional chlorination exist, and have been put into practice to varying extents. [[Ozone#Use in industry|Ozonation]] is used by some municipalities in the [[United States]]. Due to current regulations, systems employing ozonation in the United States still must maintain [[chlorine]] residuals comparable to systems without ozonation. Disinfection with [[chloramine]] is also becoming increasingly common. Unlike chlorine, chloramine has a longer half life in the distribution system and still maintains effective protection against pathogens. The reason chloramines persist in the distribution is due to the relatively lower redox potential in comparison to free chlorine. Chloramine is formed by the addition of ammonia into drinking water to form mono-, di-, and trichloramines. Whereas ''[[Helicobacter pylori]]'' can be many times more resistant to chlorine than ''Escherichia coli'', both organisms are about equally susceptible to the disinfecting effect of chloramine<ref>{{cite journal | author=Baker KH, Hegarty JP, Redmond B, Reed NA, Herson DS | title=Effect of oxidizing disinfectants (chlorine, monochloramine, and ozone) on Helicobacter pylori. | journal=APPLIED AND ENVIRONMENTAL MICROBIOLOGY | volume=68 | issue=2 | year=2002 | pages=981-984 |url=http://aem.asm.org/cgi/reprint/68/2/981 | format = [[PDF]] | id=PMID 11823249 }}</ref>. Water treated by filtration may not need further [[disinfection]]; a very high proportion of pathogens are removed by microorganisms in the filter bed. Filtered water must be used soon after it is filtered, as the low amount of remaining microbes may proliferate in time. The advantage of chlorine in comparison to ozone is that the residual persists in the water for an extended period of time. This feature allows the chlorine to travel through the water supply system, effectively controlling pathogenic backflow contamination. In a large system this may not be adequate, and so chlorine levels may be boosted at points in the distribution system, or [[chloramine]] may be used, which remains in the water for longer before reacting or dissipating. Another method which is gaining popularity is [[Ultraviolet#Disinfecting drinking water|UV disinfection]]. UV treatment leaves no residue in the water due to use of light as a microbial inactivation mechanism. However, this method alone will not remove bacterially produced toxins, pesticides, heavy metals, etc from water. Often, multiple steps are taken in commercially sold water. ==See also== *[[Water fluoridation]] *[[Water pollution]] *[[Sodium hypochlorite]] *[[Symclosene|Symclosene/trichloroisocyanuric acid]] is the chemical in chlorination tablets ==References== {{reflist}} ==External links== *[http://www.mpw.net/Pages/WaterWorks.html City of Milwaukee, Wisconsin Water Works] *[http://www.epa.gov/safewater/faq/emerg.html Emergency Disinfection of Drinking Water] ([[United States Environmental Protection Agency|US EPA]]) * [http://www.npi.gov.au/database/substance-info/profiles/20.html National Pollutant Inventory - Chlorine] * [http://monographs.iarc.fr/ENG/Monographs/vol52/volume52.pdf Chlorinated Drinking Water] ([[IARC]] Monograph) * [http://ntp.niehs.nih.gov/ntp/htdocs/LT_rpts/tr392.pdf NTP Study Report TR-392: Chlorinated & Chloraminated Water] (US [[NIH]]) * [http://www.americanchemistry.com/chlorine/ American Chemistry Council's Chlorine Chemistry Division] * [http://www.enceechlor.com/download/Drinking%20Water%20Chlorination%20-%20A%20Review%20of%20Disinfection%20Practices.pdf Disinfection Practices] [[Category:Water treatment]] [[Category:Chlorine]] [[de:Chlorierung]]