The rôle of formation and reduction of THM and chlorite concentrations in the disinfection of water with Cl2 and ClO2

Water disinfected by chlorine produces trihalomethanes (THM), while chlorine dioxide, an alternative to chlorine, is reduced mainly to undesirable chlorite. Mixtures of ClO₂ and Cl₂ may successfully reduce the formation of both THM and chlorite in the absence of bromide. When Lake Kinneret water in Israel was studied, however, it was found that such mixtures caused only a minor reduction in the formation of THM compared with chlorine alone. This is attributed to the effect of bromide present in the water of Lake Kinneret. The formation of THM and chlorite could, however, be satisfactorily reduced (by about 60% and 90%, respectively), if chlorine dioxide were allowed to react in water with the THM precursors before chlorine was introduced.