Chlorination



This article is about a water treatment process. For chlorination in organic chemistry, see Halogenation.

Chlorination is the process of adding the element 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 viruses, as well as amoeba.

Chlorination is also used to sterilize the water in swimming pools and as a disinfection stage in 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 Brig. Gen.) 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 hydrochloric acid:

Cl2 + H2O → HOCl + HCl

Depending on the pH, hypochlorous acid partly dissociates to hydrogen and hypochlorite ions:

HClO → H+ + ClO-

In acidic solution, the major species are Cl2 and HOCl while in alkaline solution effectively only ClO- is present. Very small concentrations of ClO2-, ClO3-, ClO4- are also found[1].

Drawbacks

Disinfection by chlorination can be problematic, in some circumstances. Chlorine can react with naturally occurring 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 aesthetic concerns such as taste and odour.

Alternatives

Several alternatives to traditional chlorination exist, and have been put into practice to varying extents. 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.

Water treated by filtration may not need further disinfection; a very high proportion of pathogens are removed by microorganisms in the filter bed.

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 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

References

  1. ^ .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, 14, 69, 1998.
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chlorination". A list of authors is available in Wikipedia.