Water can be disinfected in several ways. The most common and time-proven method is chlorination. Chlorine is not only a very effective disinfectant in the treatment process; the residual chlorine in the distribution system can provide continuous disinfecting effect in the process of distribution as well. The level of residual chlorine hence can be regarded as an excellent telltale for water quality monitoring. Ozone is a very effective disinfectant too. But it does not remain in the water for long after it is applied.