AWWA WQTC55182

Cryptosporidium parvum oocysts have a relatively strong resistance to inactivation with chemical disinfectants commonly used in drinking water treatment. As a result, exposure to significantly higher CT (i.e., product of disinfectant concentration and contact time) values is required compared to those needed to inactivate other waterborne pathogens. Of particular concern is that the CT values for the two most common disinfectants free and combined chlorine are so high that these chemicals are considered practically ineffective when used as primary disinfectants under conditions typically used in full-scale drinking water treatment plants. In contrast, recent studies have shown that one to two logs of C. parvum oocyst inactivation can be achieved at lower CT values by secondary treatment with free or combined chlorine when these chemicals are used after primary disinfection with ozone. In contrast no such synergy is observed with sequential disinfection schemes using chlorine dioxide as the primary disinfectant followed by secondary treatment with free or combined chlorine. The use of Bacillus subtilis spores as surrogates for C. parvum oocysts has been proposed. The primary inactivation kinetics of B. subtilis spores with ozone and monochloramine have been shown to agree reasonably well with the kinetics of C. parvum oocyst inactivation with these two disinfectants at the temperature of 20 degrees C. However, no information is currently available about how well the inactivation kinetics with these two disinfectants compare at temperatures other than 20 degrees C. Furthermore, no comparative study has been reported for the inactivation kinetics of C. parvum oocysts and B. subtilis spores with chlorine dioxide or free chlorine when these disinfectants are used alone, or with sequential applications of ozone or chlorine dioxide as primary disinfectants and free or combined chlorine as secondary disinfectants. Based on the needs identified in the preceding paragraph, the overall objective of this project was to perform a comprehensive comparative study of the inactivation kinetics of C. parvum oocysts and B. subtilis spores with ozone, chlorine dioxide, free chlorine, and monochloramine when these disinfectants are used singly or applied sequentially. Includes 12 references, figures.