Low chlorine impurity might be beneficial in chlorine dioxide disinfection

Chlorine dioxide (ClO₂) is a prevalently used disinfectant alternative to chlorine, due to its effectiveness in pathogen inactivation and low yields of organic halogenated disinfection byproducts (DBPs). However, during ClO₂ generation, chlorine is inevitably introduced into the obtained ClO₂ solution as an "impurity", which could compromise the merits of ClO₂ disinfection. In this study, drinking water disinfection with ClO₂ containing 0‒25% chlorine impurity (i.e., at Cl₂ to ClO₂ mass ratios of 0‒25%) was simulated, and the effect of chlorine impurity on the DBP formation and developmental toxicity of the finished water was evaluated. With increasing the chlorine impurity in ClO₂, the chlorite level kept decreasing and the chlorate level gradually increased; meanwhile, an unexpected trend from decline to rise was observed for the total organic halogenated DBPs, with the minimum level appearing at 5% chlorine impurity. To unravel the mechanisms for the variations of organic halogenated DBPs with chlorine impurity, a quantitative kinetic model was developed to simulate the formation of chlorinated, brominated, and iodinated DBPs in the ClO₂-disinfected drinking water. The modeling results indicated that reactions involving iodide accounted for the decrease of organic halogenated DBPs at a relatively low chlorine impurity level. In accordance with DBP formation, ClO₂ with 5% chlorine impurity generated less toxic drinking water than pure ClO₂, while significantly higher developmental toxicity was induced until the chlorine impurity reached 25%. For E. coli inactivation, the presence of chlorine impurity enhanced the disinfection efficiency due to a synergistic effect of ClO₂ and chlorine. Therefore, disinfection practices with ClO₂ containing low chlorine impurity (e.g., <10%) might be favored (i.e., there is no need to eliminate low chlorine impurity in the ClO₂ solution), while those containing high chlorine impurity should be concerned.