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'''Water chlorination''' is the process of adding [[chlorine]] ({{chem|Cl|2}}) to [[water]] as a method of [[water purification]] to make it fit for human consumption as [[drinking water]]. Water that has been treated with chlorine is effective in preventing the spread of [[waterborne diseases]].

==History==
[[John Snow (physician)|John Snow]] was the first to successfully use [[chlorine]] to disinfect the water supply in [[Soho]], London to stem the spread of the [[1854 Broad Street cholera outbreak]]. William Soper also used chlorinated lime to treat the sewage produced by [[typhoid]] patients in 1879.

In a paper published in 1894, [[Moritz Traube]] formally proposed the addition of chloride of lime ([[calcium hypochlorite]]) to water to render it “germ-free.”  Two other investigators confirmed Traube’s findings and published their papers in 1895.<ref>{{cite book | author1 = F.E. Turneaure |author2= and H.L. Russell | year = 1901 | title = Public Water-Supplies:  Requirements, Resources, and the Construction of Works | edition = 1st | location = New York | publisher = John Wiley & Sons | page = 493}}</ref> Early attempts at implementing water chlorination at a water treatment plant were made in 1893 in [[Hamburg, Germany]], and in 1897 the city of [[Maidstone, England]] was the first to have  entire water supply treated with chlorine.<ref>{{cite journal|title=Typhoid Epidemic at Maidstone|year=1897|journal=Journal of the Sanitary Institute|volume=18|month=October|pages=388}}</ref>

Permanent water chlorination began in 1905, when a faulty [[slow sand filter]] and a contaminated water supply led to a serious typhoid fever epidemic in [[Lincoln, England]].<ref>{{Cite web|url=http://www.icis.com/Articles/2008/08/25/9150211/chlorine-cleans-water-for-millions-but-comes-under-attack-after-health.html|title=A miracle for public health?|accessdate=2012-12-17}}</ref> Dr. Alexander Cruickshank Houston used chlorination of the water to stem the epidemic. His installation fed a concentrated solution of chloride of lime to the water being treated. The chlorination of the water supply helped stop the epidemic and as a precaution, the chlorination was continued until 1911 when a new water supply was instituted.<ref>Reece, R.J. (1907). “Report on the Epidemic of Enteric Fever in the City of Lincoln, 1904-5.” In Thirty-Fifth Annual Report of the Local Government Board, 1905-6:  Supplement Containing the Report of the Medical Officer for 1905-6. London: Local Government Board.</ref>

[[File:Manual Control Chlorinator.jpg|300px|thumb|Manual Control Chlorinator for the liquefaction of chlorine for water purification, early 20th century. From ''Chlorination of Water'' by Joseph Race, 1918.]]
The first continuous use of chlorine in the [[United States]] for disinfection took place in 1908 at Boonton Reservoir (on the [[Rockaway River]]), which served as the supply for [[Jersey City, New Jersey]].<ref>Leal, John L. (1909). “The Sterilization Plant of the Jersey City Water Supply Company at Boonton, N.J.” ''Proceedings'' American Water Works Association. pp. 100-9.</ref>   Chlorination was achieved by controlled additions of dilute solutions of chloride of lime ([[calcium hypochlorite]]) at doses of 0.2 to 0.35 ppm.  The treatment process was conceived by Dr. John L. Leal, and the chlorination plant was designed by [[George W. Fuller|George Warren Fuller.]]<ref>Fuller, George W. (1909). “Description of the Process and Plant of the Jersey City Water Supply Company for the Sterilization of the Water of the Boonton Reservoir.” ''Proceedings'' AWWA. pp. 110-34.</ref>  Over the next few years, chlorine disinfection using chloride of lime were rapidly installed in drinking water systems around the world.<ref>Hazen, Allen. (1916). ''Clean Water and How to Get It.'' New York: Wiley. p. 102.</ref>

The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed by a British officer in the [[Indian Medical Service]], Vincent B. Nesfield, in 1903. According to his own account, "It occurred to me that chlorine gas might be found satisfactory ... if suitable means could be found for using it.... The next important question was how to render the gas portable. This might be accomplished in two ways: By liquefying it, and storing it in lead-lined iron vessels, having a jet with a very fine capillary canal, and fitted with a tap or a screw cap. The tap is turned on, and the cylinder placed in the amount of water required. The chlorine bubbles out, and in ten to fifteen minutes the water is absolutely safe. This method would be of use on a large scale, as for service water carts.”<ref>{{cite journal|author= V. B. Nesfield|title=A Chemical Method of Sterilizing Water Without Affecting  Potability|url=http://www.publichealthjrnl.com/article/S0033-3506%2802%2980142-1/abstract|journal=Public Health|year=1902|pages=601-3}}</ref>

[[Major (United States)|Major]] [[Carl Rogers Darnall]], Professor of Chemistry at the [[Army Medical School]], gave the first practical demonstration of this in 1910. Shortly thereafter, Major [[William J. L. Lyster]] 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). This work became the basis for present day systems of municipal water ''purification''.

Chlorine gas was first used on a continuing basis to disinfect the water supply at the Belmont filter plant, [[Philadelphia]], [[Pennsylvania]] beginning in 1913. By 1941, disinfection of U.S. drinking water by chlorine gas had largely replaced the use of chloride of lime.<ref>{{cite book|last=Hodges|first=L.|title=Environmental Pollution|publisher=Rinehart and Winston|year=1977|edition=2nd|location=New York|page=189}}</ref><ref>Baker, Moses N. (1981). ''The Quest for Pure Water: the History of Water Purification from the Earliest Records to the Twentieth Century.'' 2nd Edition. Vol. 1. Denver: American Water Works Association. p. 341-342.</ref>

==Biochemistry==
As a [[halogen]], [[chlorine]] is a highly efficient disinfectant, and is added to public water supplies to kill disease-causing pathogens, such as [[bacteria]], [[viruses]], and [[protozoans]], that commonly grow in water supply reservoirs, on the walls of water mains and in storage tanks.<ref name=Calderon>{{cite journal | author = Calderon, R. L. | year = 2000 | title = The Epidemiology of Chemical Contaminants of Drinking Water | journal = Food and Chemical Toxicology | volume = 38 | pages = S13–S20 | doi = 10.1016/S0278-6915(99)00133-7 | pmid = 10717366 | issue = 1 Suppl}}</ref> The microscopic agents of many diseases such as [[cholera]], [[typhoid fever]], and [[dysentery]] killed countless people annually before disinfection methods were employed routinely.<ref name=Calderon/>

Chlorine is obtained from salt ([[NaCl]]).  It is a gas at atmospheric pressures but liquifies under pressure.  The liquified gas is transported and used as such.

As a strong [[oxidizing agent]], chlorine kills via the oxidation of organic molecules.<ref name=Calderon/> Chlorine and  hydrolysis product [[hypochlorous acid]] are neutrally charged and therefore easily penetrate the negatively charged surface of pathogens. It is able to disintegrate the lipids that compose the cell wall and react with intracellular [[enzymes]] and [[proteins]], making them nonfunctional. [[Microorganisms]] then either die or are no longer able to multiply.<ref>{{Cite book | author = Kleijnen, R.G. | date =  December 16, 2011 | title = The Chlorine Dilemma | publisher = Eindhoven University of Technology | pages = |url = http://students.chem.tue.nl/ifp42/Downloads/Final%20Report%20MDP%20Group%201%20-%20The%20Chlorine%20Dilemma.pdf | accessdate = January 18, 2014}}{{page needed|date=January 2014}}</ref> 

===Principles===
When dissolved in water, chlorine converts to an equilibrium mixture of chlorine, [[hypochlorous acid]] (HOCl), and [[hydrochloric acid]] (HCl):

:Cl<sub>2</sub>  +  H<sub>2</sub>O   <math>\overrightarrow{\leftarrow}</math>   HOCl +  HCl

In acidic solution, the major species are {{chem|Cl|2}} 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>{{cite journal | author = Shunji Nakagawara, Takeshi Goto, Masayuki Nara, Youichi Ozaqa, Kunimoto Hotta and Yoji Arata | title = Spectroscopic Characterization and the pH Dependence of Bactericidal Activity of the Aqueous Chlorine Solution | journal = Analytical Sciences | volume = 14 | pages = 691–698 | year = 1998 | doi = 10.2116/analsci.14.691 | issue = 4}}</ref>

===Shock chlorination===
Shock chlorination is a process used in many swimming pools, [[water well]]s, [[spring (hydrosphere)|springs]], and other water sources to reduce the [[bacteria]]l and [[algae|algal]] residue in the water. Shock chlorination is performed by mixing a large amount of [[sodium hypochlorite]], which can be in the form of a powder or a liquid such as chlorine bleach, into the water. Water that is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goes down to three [[parts per million]] (PPM) or less.

==Drawbacks to water chlorination==
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 compounds known as [[disinfection byproduct]]s (DBPs). The most common DBPs are [[trihalomethane]]s (THMs) and [[haloacetic acid]]s (HAAs). Trihalomethanes are the main disinfectant by-products created from chlorination with two different types, [[bromoform]] and [[dibromochloromethane]], which are mainly responsible for health hazards. Their effects depend strictly on the duration of their exposure to the chemicals and the amount ingested into the body. In high doses, bromoform mainly slows down regular brain activity, which is manifested by symptoms such as sleepiness or sedation. Chronic exposure of both bromoform and dibromochloromethane can cause liver and kidney cancer, as well as heart disease, unconsciousness, or death in high doses.<ref>{{Cite journal| publisher = ATSDR | year = 2011 | title = Public Health Statement: Bromoform & Dibromochloromethane}}{{full|date=July 2013}}</ref> Due to the potential [[carcinogenicity]] of these compounds, drinking water regulations across the developed world require regular monitoring of the concentration of these compounds in the distribution systems of municipal water systems.  The [[World Health Organization]] has stated that the "risks to health from these by-products are extremely small in comparison with the risks associated with inadequate disinfection" <ref>{{Cite journal| series = Guidelines for drinking-water quality Vol. 1, Recommendations. – 3rd ed. | year = 2006 | publisher = World Health Organization | url = http://www.who.int/water_sanitation_health/dwq/gdwq0506.pdf | postscript = <!-- Bot inserted parameter. Either remove it; or change  value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>

There are also other concerns regarding chlorine, including  volatile nature which causes it to disappear too quickly from the water system, and [[aesthetics|aesthetic]] concerns such as taste and odor.

==Alternative methods for water disinfection==
Chlorine in water is over three times more effective as a disinfectant against ''[[Escherichia coli]]'' than an equivalent concentration of [[bromine]], and over 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, and Iodine as Disinfectants for Swimming Pool Water | journal=Applied Microbiology | volume=14 | issue=2 | date=1 March 1966| pages=276–279 | pmc=546668 |  pmid=4959984 }}</ref>

===Ozone===
[[Ozone#Use in industry|Ozonation]] is used by many European countries and also in a few municipalities in the United States and Canada. This alternative is more costeffective and energy-intensive. It involves ozone being bubbled through the water, breaking down all parasites, bacteria, and all other harmful organic substances. However, this method leaves no residual ozone to control contamination of the water after the process has been completed.<ref name="Neumann">Neumann, H. (1981). "Bacteriological safety of hot tapwater in developing countries." ''Public Health'' Rep.84:812-814.</ref> Due to current regulations,{{year needed|date=January 2014}} systems employing ozonation in the United States still must maintain [[chlorine]] residuals comparable to systems without ozonation.{{citation needed|date=January 2014}}

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.

===Chloramine===
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]] | doi=10.1128/AEM.68.2.981-984.2002 | pmc=126689 |  pmid=11823249  }}</ref>

===Home filtration===
Water treated by filtration and home filtration may not need further [[disinfection]]; a very high proportion of pathogens are removed by materials in the filter bed. Filtered water must be used soon after it is filtered, as the low amount of remaining microbes may proliferate over time. In general, these home filters remove over 90% of the chlorine available to a glass of treated water. These filters must be periodically replaced otherwise the bacterial content of the water may actually increase due to the growth of bacteria within the filter unit.<ref name="Neumann" />

===UV radiation===
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 instead of chemical disinfectants.  However, this method alone (as well as chlorination alone) will not remove toxins from bacteria, pesticides, heavy metals, etc. from water.

==See also==
*[[chloramination]]
*[[disinfection]]
*[[drinking water]]
*[[Safe Drinking Water Act]]
*[[water fluoridation]]
*[[water industry]]
*[[water pollution]]
*[[water filters]]
*[[water supply network]]
*[[water treatment]]
*[[sodium hypochlorite]]
*[[symclosene|Symclosene/trichloroisocyanuric acid]] is the chemical in chlorination tablets

==References==
{{Reflist|2}}

==External links==
*[http://city.milwaukee.gov/water 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] ([[International Agency for Research on Cancer|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]{{Dead link|date=July 2010}}
*  http://ukwta.org/

[[Category:Water treatment]]
[[Category:Chlorine]]
[[Category:Scottish inventions]]