Everything about Chlorination totally explained
» 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's 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
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
Brig. Gen.)
Carl Rogers Darnall (1867-1941), Professor of Chemistry at the
Army Medical School. Shortly thereafter, Major (later
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
2 + H
2O → 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 Cl
2 and HOCl while in alkaline solution effectively only ClO
- is present. Very small concentrations of ClO
2-, ClO
3-, ClO
4- are also found.
Drawbacks
Disinfection by chlorination can be problematic, in some circumstances. Chlorine can react with naturally occurring
organic compounds found in the water supply to produce dangerous compounds, known as
disinfection byproducts (DBPs). The most common DBPs are
trihalomethanes (THMs) and
haloacetic acids. 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
aesthetic concerns such as
taste and odour.
Alternatives
Several alternatives to traditional chlorination exist, and have been put into practice to varying extents.
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.
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's 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
UV disinfection. UV treatment leaves no residue in the water due to use of light as a microbial inactivation mechanism. However, this method alone won't remove bacterially produced toxins, pesticides, heavy metals, etc from water. Often, multiple steps are taken in commercially sold water.
Further Information
Get more info on 'Chlorination'.
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