Ozone inactivation kinetics of Cryptosporidium in phosphate buffer

The rate of inactivation of \ICryptospridium parvum\N oocysts by ozone in 0.05 M phosphate buffer at pH 6–8 was studied at 22 ± 1°C in batch reactors. Infectivity in neonatal CD-1 mice was used as the criterion for oocyst viability. Ozone inactivation data were fitted to the Incomplete gamma Hom (I.g.H.) and Chick-Watson (\in = 1) model, both of which incorporate a first-order rate constant for the disappearance of aqueous ozone during the contact time. For a 0.05 M phosphate buffer ranging in pH from 6 to 8, a single I.g.H. model was found to adequately describe the kinetics of \ICryptosporidium\N inactivation by ozone at 22°C. The I.g.H. model for pH 6–8 was found to provide a significantly better fit to the ozone inactivation data when compared with the Chick-Watson model. The effect of pH on ozone inactivation kinetics was associated with ozone residual stability over the pH range of 6–8. The sensitivity of \ICryptosporidium\N to ozone at 22°C was therefore not statistically different at pH 6 when compared with pH 8. The inactivation behavior of \ICryptosporidium\N by ozone was characterized by a tailing-off effect, with approximately equal importance of ozone concentration and contact time. The I.g.H. model for pH 6–8 can be used as an aid in the design of ozone disinfection systems, and this robust fitted model was used to formulate ozone design criteria—the initial ozone residual required for a given contact time for 1–3 log units inactivation of \ICryptosporidium\N at 22°C. Uncertainty associated with the ozone design criteria for 2 log units inactivation was quantified using inverse prediction intervals. An ozone design criterion was established that gives a 95% probability of achieving 2 log units inactivation of \ICryptosporidium\N at 22°C, corresponding to an approximate safety factor of 0.7 log units.