Development of a Ct equation for the inactivation of Cryptosporidium oocysts with ozone

Cryptosporidium parvum, a protozoan parasite, has been implicated in a number of waterborne disease outbreaks. It is difficult to inactivate using free chlorine, but appears to be easily inactivated by ozone. Therefore, the US EPA has promulgated the Interim Enhanced Surface Water Treatment Rule, which for the first time, addresses the control of C. parvum in drinking water. The use of Ct (concentration of disinfectant in mg/L times, time in minutes) values is being considered as one of the options for controlling this organism. This paper proposes a Ct equation, based on first order kinetics, to provide guidance to drinking water utilities for the application of ozone for controlling C parvum oocysts in drinking water. The equation, which provides mean estimates of inactivation, was developed using standard statistical techniques and currently available field and bench scale data. In addition, the possibility of using a statistically conservative upper bound Ct value in order to insure an appropriate safety factor is explored.