Quantitative evaluation of the synergistic sequential inactivation of Bacillus subtilis spores with ozone followed by chlorine

This investigation of sequential disinfection, with ozone followed by free chlorine, was carried out using Bacillus subtilis spores, to make a quantitative evaluation and to improve the mechanistic understanding of their synergistic effect. This study shows that the extent of the synergistic effect in the inactivation of Bacillus subtilis spores appears to be dependent upon the level of preozonation. However, when the ozone pretreatment level exceeded the lag phase of the ozone inactivation curve, the chlorine inactivation curves were almost identical regardless of the level of preozonation. When this sequential disinfection was performed in the reverse order, no enhanced disinfection was observed. This difference, depending on the order of disinfectant application in sequential disinfection, was explained in terms of the enhanced disinfection being the result of the greater intracellular diffusion of free chlorine, caused by the cell surface disruption induced by ozone. The practical implications of this synergistic sequential inactivation with ozone followed by free chlorine were discussed, along with the issue of selecting the amount of each oxidant to use in water treatment plants, to achieve a specific level of microorganism inactivation.