Effect of micro-mixing conditions on predictions of Cryptosporidium inactivation in an ozone contactor

The U.S. EPA is considering segregated flow analysis as an alternative calculation method to determine Cryptosporidium, parvum inactivation credit in continuous-flow ozone contactors in drinking water treatment facilities. A computer method is presented in which C. parvum inactivation in the reactive flow segment of a hypothetical ozone contactor with a pre-determined residence time distribution is calculated based on the assumption of either completely segregated or completely micro-mixed flow. In a series of computer simulation, using typical ozonation conditions in a water treatment facility, inactivation predicted assuming complete segregation was 0.3 to more than 2.0 log greater than that predicted assuming complete micro-mixing, on depending on the level of back-mixing, ozone decomposition rate and inactivation level. CFSTR-in-series model predictions of inactivation were between those of segregated flow analysis and micro-mixed analysis. It was concluded that segregated flow analysis calculations may result in significant over-prediction of C. parvum inactivation credit in ozone contactors and should be used with caution.