The inactivation of C. parvum and spores of C. perfringens by ozone treatment in natural water was compared in a lab-scale continuous-flow system. In addition the inactivation of the natural occurring spores of sulphite-reducing clostridia (SSRC) in this water was monitored in one of the lab-scale systems as well as in a full-scale ozonation process. The survival ratio of C. parvum oocysts was determined using the CD-1 neonatal mouse infectivity test and for C. perfringens and SSRC the survival ratio was assessed with the standard anaerobic colony count on the iron-sulphite medium. The results of the lab-scale experiments revealed an inactivation rate constant k (Chick-Watson modelling) at 10oC for C. parvum of 0.14 (SD = 0.014; P<0.001) and for C. perfringens of 0.25 (SD = 0.01; P<0.001). Moreover, first results of monitoring the SSRC inactivation in full-scale ozonation processes indicated that the inactivation rate constant for these wild strains was in the same order of magnitude as determined for C. perfringens. Further research is needed to compare inactivation ozone kinetics for C. perfringens D10 and SSRC at different temperatures and in other natural waters. Results of additional lab-scale experiments with C. perfringens strain D10 indicated that the Ct of the gas-feed chamber should be incorporated in the design of a full-scale ozonation. Moreover, setting the Ct with the contact time was not as effective for the inactivation capacity as setting the Ct with the ozone concentration
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