Low chlorine impurity might be beneficial in chlorine dioxide disinfection.

Chlorine dioxide (ClO) 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 ClO generation, chlorine is inevitably introduced into the obtained ClO solution as an "impurity", which could compromise the merits of ClO disinfection. In this study, drinking water disinfection with ClO containing 0‒25% chlorine impurity (i.e., at Cl to ClO 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 ClO, 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 ClO-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, ClO with 5% chlorine impurity generated less toxic drinking water than pure ClO, 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 ClO and chlorine. Therefore, disinfection practices with ClO containing low chlorine impurity (e.g., <10%) might be favored (i.e., there is no need to eliminate low chlorine impurity in the ClO solution), while those containing high chlorine impurity should be concerned.