Inactivation of bacteriophage MS2 upon exposure to very low concentrations of chlorine dioxide

This study investigates the effects of very low concentrations of ClO₂ applied in drinking water practice on the inactivation of bacteriophage MS2. Concentrations of 0.5 mg/L, 0.1 mg/L and 0.02 mg/L ClO₂ inactivated at least 5 log units of MS2 after an exposure time of approximately 20, 50 and 300 min respectively. When the ClO₂ concentration was as low as 0.005 mg/L, inactivation of 1 log unit MS2 was observed after 300 min exposure. Increasing the contact time to 24 h did not increase the inactivation any further. Non-linear inactivation kinetics (tailing) were observed for all conditions tested. Repeated addition of MS2 to the reactor showed that tailing was not caused by a reduction of the biocidal effect of ClO₂ during disinfection. The Modified Chick-Watson, the Efficiency Factor Hom (EFH) model and the Modified Cerf model, a modification of the two-fraction Cerf model, were fitted to the non-linear inactivation curves. Both the EFH and the modified Cerf model did fit accurately to the inactivation data of all experiments. The good fit of the Modified Cerf model supports the hypothesis of the presence of two subpopulations. Our study showed that ClO₂ is an effective disinfectant against model organism MS2, also at the low concentrations applied in water treatment practice. The inactivation kinetics followed a biphasic pattern due to the presence of a more ClO₂-resistant subpopulation of MS2 phages, either caused by population heterogeneity or aggregation/adhesion of MS2.