Inactivation of Escherichia coli with ozone: chemical and inactivation kinetics

The apparent chemical and inactivation reactions taking place during the disinfection of Escherichia coli with ozone in the presence of humic acid were investigated with continuous-flow tubular reactors. The apparent decomposition of dissolved ozone in the presence of humic acid and E. coli cells was modeled successfully with mixed second-order rate expressions within a time scale relevant to E. coli inactivation with ozone. The rate for the ozone inactivation of E. coli in the presence of humic acid was slower than that in the absence of natural organic matter due to the faster decomposition of dissolved ozone and thus the lower exposure of E. coli cells to the disinfectant in the former case. However, the second-order inactivation rate constant was approximately the same in the presence and absence of humic acid confirming that molecular ozone rather than radicals was the species generally responsible for inactivation. The overall rate of reaction between ozone and organic matter associated with E. coli cells was slower that the rate of E. coli inactivation by ozone. Only 25% of the initial relatively fast ozone demand were satisfied by the time that practically all micro-organisms present in solution were inactivated. TEM micrographs revealed that noticeable changes in the interior of E. coli cells did not take place until most of the cells present in the sample were non-viable.