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Chlorination is the process of adding chlorine to water as a method of water purification to make it fit for human consumption as drinking water or to act as a disinfectant in the treatment of sewage or other effluent waters . Water which has been treated with chlorine is effective in preventing the spread of disease. When chlorine is added to water, it reacts to form a pH dependent equilibrium mixture of chlorine, hypochlorous acid and hydrochloric acid: Cl2 + H2O → 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 Cl2 and HOCl while in alkaline solution effectively only ClO- is present. Very small concentrations of ClO2-, ClO3-, ClO4- are also found. 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, regulations require regular monitoring of the concentration of these compounds in the distribution systems of potable water systems. However, the World Health Organization has stated that the "Risks to health from DBPs are extremely small in comparison with inadequate disinfection."

20th ed. Standard Methods - For the Examination of Water and Wastewater



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