Development of a Ct equation taking into consideration the effect of lot variability on the inactivation of Cryptosporidium parvum oocysts with ozone

Cryptosporidium parvum oocysts are prevalent in surface water and ground water under the influence of surface water, and are difficult to inactivate using free chlorine, the most common disinfectant currently used for treating drinking water. In contrast, it has been shown that ozone is a more effective disinfectant than chlorine. US EPA is currently evaluating a treatment rule, which addresses the control of C. parvum oocysts in drinking water. The use of Ct   (average disinfectant concentration multiplied by characteristic contact time) values is being considered as one of the options for demonstrating adequate control of this microbial contaminant. The purpose of this study is to incorporate the variability in inactivation kinetics among different lots of oocysts and to develop a statistical model for CtCt based on first-order delayed Chick–Watson inactivation kinetics. A Bayesian approach is used to estimate the delayed Chick–Watson kinetic parameters. A log-linear regression analysis is then used to represent the effect of temperature on the resulting kinetic parameters. The overall model developed in this study provides an approach for water utilities and regulatory agencies to decide on the level of safety needed when developing treatment requirements for the inactivation of C. parvum oocysts with ozone as part of broader risk assessment considerations.