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Research reveals that hydroxyl radicals derived from the ozone chain reaction are effective for the inactivation of Bacillus subtilis endospore in water containing natural organic matter. Data indicate that the type of contamination and pH influence disinfection rate by hydroxyl radicals, whose disinfection CT value is several orders of magnitude smaller than that of ozone.

Despite a wealth of experimental evidence concerning the efficacy of the biocidal action associated with the TiO2 photocatalytic reaction, our understanding of the photochemical mechanism of this particular biocidal action remains largely unclear. It is generally accepted that the hydroxyl radical (·OH), which is generated on the surface of UV-illuminated TiO2, plays the main role. However, our understanding of the exact mode of action of the hydroxyl radical in killing microorganisms is far from complete, and some studies report that other reactive oxygen species (ROS) (H2O2 and O2·-, etc.) also play significant roles. In particular, whether hydroxyl radicals remain bound to the surface or diffuse into the solution bulk is under active debate. In order to examine the exact mode of action of ROS in inactivating the microorganism, we tested and compared the levels of photocatalytic inactivation of MS-2 phage and Escherichia coli as representative species of viruses and bacteria, respectively. To compare photocatalytic microbial inactivation with the photocatalytic chemical degradation reaction, para-chiorobenzoic acid, which rapidly reacts with a hydroxyl radical with a diffusionlimited rate, was used as a probe compound. Two different hydroxyl radical scavengers, tert-butanol and methanol, and an activator of the bulk phase hydroxyl radical generation, Fe2+, were used to investigate their effects on the photocatalytic mode of action of the hydroxyl radical in inactivating the microorganism. The results show that the biocidal modes of action of ROS are very different depending on the specific microorganism involved, although the reason for this is not clear. It seems that MS-2 phage is inactivated mainly by the free hydroxyl radical in the solution bulk but that E. coli is inactivated by both the free and the surface-bound hydroxyl radicals. E. coli might also be inactivated by other ROS, such as O2·- and H2O2, according to the present results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Studies the effect of hydrogen-ion concentration (pH) on the inactivation of Bacillus subtilis spores with ozone in a batch reactor. Effect of radicals on ozone inactivation of microorganism; Dependence of the ozone decay rate on pH; Use of the modified Chick-Watson model.