Chlorine gas is the most widely used reagent for disinfecting water. It possesses very considerable residual oxidizing capacity and is therefore useful for the destruction of organic matter.
Its lethal effect on bacteria is due to the destruction of the enzymes essential to the survival of pathogens. Water intended for human consumption must be free from putrid bacteria and pathogens. This result is achieved by disinfecting the water by the injection of chlorine water, either in the treated water tank or, if the plant is pressurized, in the rising main to the storage reservoir. In both cases, means must be provided for the efficient mixing together of the chlorine solution and the water being treated. A contact time of at least 30 minutes is required, at the end of which the residual chlorine concentration must still be between 0.1 and 0.2 mg/liter.