Drinking water treatment
The World Health Organization (WHO) has been concerned with health aspects of the management of water resources for many years and publishes various documents concerning the safety of the water environment and its importance for health. The message is clear: All water supplies should be disinfected.
Chlorine is generally the disinfectant of choice as it is reasonably efficient, cheap and easy to handle. In all but the smallest water treatment plants, chlorine is added to water as either in aqueous solution (usually sodium hypochlorite, NaOCl) or chlorine gas. The recommended concentration of NaOCl in water can vary between 0.5 mg/L and 2.0 mg/L.
Other disinfectants include ozone, ultraviolet light and iodine. These all have disadvantages. UV is not a particularly effective disinfectant and it is difficult to expose water for sufficient time for disinfection to be effective. Neither ozone nor UV provide a residual disinfectant and therefore offer no protection against recontamination in distribution. Both iodine and ozone are carcinogenic. There are also significant health and safety concerns, for operators, regarding the generation and application of ozone and chlorine (especially in the gaseous form). Iodine can also lead to thyroid problems with pregnant women and is generally more toxic than chlorine.