<P> Shock chlorination is a process used in many swimming pools, water wells, springs, and other water sources to reduce the bacterial and algal residue in the water . Shock chlorination is performed by mixing a large amount of hypochlorite into the water . The hypochlorite can be in the form of a powder or a liquid such as chlorine bleach (solution of sodium hypochlorite or calcium hypochlorite in water). Water that is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goes down to three parts per million (PPM) or until the calcium hypochlorite count goes down to 0.2 to 0.35 PPM . </P> <P> Disinfection by chlorination can be problematic, in some circumstances . Chlorine can react with naturally occurring organic compounds found in the water supply to produce compounds known as disinfection by - products (DBPs). The most common DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs). Trihalomethanes are the main disinfectant by - products created from chlorination with two different types, bromoform and dibromochloromethane, which are mainly responsible for health hazards . Their effects depend strictly on the duration of their exposure to the chemicals and the amount ingested into the body . In high doses, bromoform mainly slows down regular brain activity, which is manifested by symptoms such as sleepiness or sedation . Chronic exposure of both bromoform and dibromochloromethane can cause liver and kidney cancer, as well as heart disease, unconsciousness, or death in high doses . Due to the potential carcinogenicity of these compounds, drinking water regulations across the developed world require regular monitoring of the concentration of these compounds in the distribution systems of municipal water systems . The World Health Organization has stated that "the risks to health from these by - products are extremely small in comparison with the risks associated with inadequate disinfection". </P> <P> There are also other concerns regarding chlorine, including its volatile nature which causes it to disappear too quickly from the water system, and organoleptic concerns such as taste and odor . </P> <P> The use of chlorine dioxide as an alternative to chlorine is now widespread in both municipal and industrial applications . It eliminates potential of harmful DBPs and is a highly efficient disinfectant . </P>

Method of water purification include boiling chlorination and ionisation