Chlorination

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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 viruses.

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.

1 Chemistry in Water
2 Drawbacks
3 Alternatives
4 See also
5 External links

[edit] Chemistry in Water

When chlorine is added to water, hypochlorous and hydrochloric acids form:

Cl₂ + H₂O → HClO + HCl

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

HClO → H⁺ + ClO^-

The hypochlorite ion then most often degrades to a mixture of chloride and chlorate ions:

3 ClO^– → 2 Cl^– + ClO₃^–

[edit] 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."[1]

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.

[edit] 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 as 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 filtration. It leaves no residue in the water.

Many of the water treatment plants and their related piping infrastructure in the United States are outdated, some dating to the civil war era. Alternatives to chlorination are not feasible in most cases due to cost concerns. The cost of updating facilities to use more modern and safer disinfection methods is generally seen as too costly. Much of Europe, which has a more modern infrastructure, employs safer disinfection methods. Several European nations have banned the use of chlorine in drinking water due to health concerns. There is empirical evidence which suggests that communities which use chlorinated water have a higher incidence of cancer.