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, 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.
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.
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. 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.
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 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.