After sedimentation, the water is disinfected to eliminate any remaining pathogenic micro-organisms. The most commonly used disinfectant (the chemical used for disinfection) is chlorine, in the form of a liquid (such as sodium hypochlorite, NaOCl) or a gas. It is relatively cheap, and simple to use. When chlorine is added to water it reacts with any pollutants present, including micro-organisms, over a given period of time, referred to as the contact time. The amount of chlorine left after this is called residual chlorine. This stays in the water all the way through the distribution system, protecting it from any micro-organisms that might enter it, until the water reaches the consumers.
World Health Organization Guidelines (WHO, 2003) suggest a maximum residual chlorine of 5 mg l–1 of water. The minimum residual chlorine level should be 0.5 mg l–1 of water after 30 minutes’ contact time (WHO, n.d.). There are other ways of disinfecting water (e.g. by using the gas ozone, or ultraviolet radiation) but these do not protect it from microbial contamination after it has left the water treatment plant. Following disinfection the treated water is pumped into the distribution system.