Few data are available concerning Cryptosporidium parvum inactivation by chlorine dioxide in natural waters. This study used a cell culture method and in vitro excystation to assess the inactivation kinetics of Cryptosporidium oocysts in three different natural waters (collected postfiltration) and in deionized-distilled water. No significant (p<0.05) differences in inactivation kinetics were observed with the various water matrices tested. In addition, the study examined the relationship between easily measured microbial indicators, such as Bacillus subtilis (aerobic) spores and Clostridium sporogenes (anaerobic) spores, and Cryptosporidium parvum oocysts. The bacterial spores were found to be more sensitive (by a factor of 5 to 10) than C. parvum oocysts to chlorine dioxide inactivation, and, therefore, would not be useful indicators. Finally, oocysts from two different suppliers were evaluated and showed marked differences with respect to resistance to chlorine dioxide inactivation.
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