Synergy in sequential inactivation of Cryptosporidium parvum with ozone/free chlorine and ozone/monochloramine

The main objective of this study was to investigate the inactivation kinetics of Cryptosporidium parvum oocysts with sequential disinfection schemes involving ozone as a primary disinfectant, and free chlorine or monochloramine as a secondary disinfectant. Two types of synergistic effects were observed. Ozone pre-treatment resulted in the removal of the relatively more pronounced initial lag phases observed for monochloramine and hypochlorous acid. An additional and more important synergistic effect was an enhancement in the rate of secondary inactivation with both hypochlorous acid and monochloramine after complete removal of the lag phase by ozone pre-treatment. A stronger synergy was observed at a lower temperature. The secondary inactivation rate was 1.1–2.8 (hypochlorous acid) and 2.4–9.2 (monochloramine) times faster than the corresponding post lag-phase primary inactivation rate at respective temperatures of 30–10°C. Consistency between the two viability assessment methods, modified in-vitro excystation and animal infectivity, was demonstrated or shown for both primary inactivation with ozone and secondary inactivation with ozone/monochloramine.