Sequential treatment schemes using ozone or chlorine dioxide as primary disinfectant and free chlorine or monochloramine as secondary disinfectant were investigated for potential synergistic effects in the inactivation of C. parvum oocysts. Viability was determined by a modified in vitro technique based on sporozoite counts that was consistent with animal infectivity data. Primary disinfection with ozone resulted in the disappearance of a predominant shoulder in the secondary inactivation curve for both free and combined chlorine. The rate of secondary inactivation with monochloramine increased by approximately 250 percent compared to the post-shoulder rate with monochloramine alone.The level of synergy from using chlorine dioxide as the primary disinfectant was lower compared to that for ozone.

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