AWWA ACE58265

Cryptosporidium parvum oocysts have emerged as the microbial water contaminant with greatest resistance to chemical disinfectants. There is particular concern because both the free and combined forms of chlorine, the most common disinfectant in the United States, when used as primary inactivation agents are considered practically ineffective in controlling C. parvum oocysts under typical drinking water conditions (Gyurek et al., 1997; Rennecker et al., 2000, 2001; Driedger et al., 2000, 2001; Corona-Vasquez et al., 2002a,b). In contrast, ozone and chlorine dioxide are both considered viable chemical disinfectants (Rennecker et al., 1999, 2000, 2001; Gyurek et al., 1999; Ruffell et al., 2000; Driedger et al., 2000, 2001; Li et al., 2001; Corona-Vasquez et al., 2002a,b) but there is concern about potentially high disinfection requirements (Rosen et al., 2002). The resistance of C. parvum oocysts to ozone and chlorine dioxide, though known to vary with various factors (Clark et al., 2002; Sivaganesan et al., submitted), can be 20 to 50 times higher than that of Giardia lamblia cysts (Corona-Vasquez et al., 2002a), the microbial contaminant with highest resistance to chemical disinfection regulated under the current version of the Enhanced Surface Water Treatment Rule. A promising alternative for more efficient control of C. parvum oocysts is the sequential application of certain combinations of disinfectants. Recent studies have shown that the efficiency of C. parvum oocyst inactivation by both free and combined chlorine can be increased significantly after limited exposure to ozone (Li et al., 1999, 2001; Finch et al., 2000; Rennecker et al., 2000, 2001; Driedger et al., 2000, 2001; Corona-Vasquez et al., 2002b; Rochelle et al., 2002). In contrast, pretreatment with chlorine dioxide does not appear to enhance as much the rate of secondary inactivation by free or combined chlorine (Corona-Vasquez et al., 2002a) The goal of this paper is to review the current status of synergistic sequential disinfection technologies for the control of C. parvum oocysts and to provide recommendations for additional research that might be needed to complete the development and possible implementation of sequential CT requirements. Includes 20 references.