Chlorination
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.

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

# 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.
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.  It leaves no residue in the water.  However, this method alone will not remove bacterially produced toxins, pesticides, heavy metals, etc from water.  Some bacterial cell walls themselves are toxic to humans, whether dead or alive.  Often, multiple steps are taken in commercially sold water.