Development of a Ct equation for the inactivation of Cryptosporidium oocysts with chlorine dioxide

Cryptosporidium parvum, a protozoan parasite, has been implicated in a number of waterborne disease outbreaks and is difficult to inactivate using free chlorine. It appears, however, to be inactivated more easily by other oxidants such as chlorine dioxide or ozone. A major element of the EPA (US EPA) strategy for controlling C. parvum (oocysts) in drinking water is the possible use of Ct (concentration of disinfectant in mg/L times time in minutes) values. To support this strategy a Ct equation, based on first-order kinetics, is proposed to provide guidance to drinking water utilities for the application of chlorine dioxide for controlling C. parvum oocysts. The equation is based on standard statistical techniques using available bench scale data. It can predict mean inactivation levels as well as a statistically conservative upper bound Ct value. This upper bound could be used to insure an appropriate safety factor for the protection of public health.