AWWA WQTC63973

The objective of this experimental laboratory study was to test the hypothesis that Cryptosporidium parvum oocysts in conventionally filtered drinking water are aggregated and are, therefore, more resistant to ultraviolet light. Two independent filtration experiments were carried out in which the alum dose was intentionally maintained at sub-optimal levels in order to simulate poorly filtered water conditions. Samples of the filtered water were exposed to an ultraviolet (UV) fluence of 5 mJ/cm² using a collimated beam apparatus. A tissue homogenizer was used to disrupt the aggregates and disperse the oocysts before or after UV exposure. Oocyst inactivation was determined using a cell culture - foci detection method. The mean log inactivation of samples (n = 6) homogenized before and after UV exposure was 2.4 log and 1.6, respectively. The difference was statistically significant at the 95% confidence level (p-value = 0.02). In comparison, the mean inactivation of oocysts seeded into phosphate buffered de-ionized water was 3.0 log at the same UV dose. These findings were interpreted as indirect evidence that oocysts present in the filtered water samples were aggregated. Direct evidence of oocyst aggregation was also observed by examination and enumeration of particulate matter captured on a series of PCTE membrane filters. The results of this study suggest that if C. parvum oocysts are present in poorly filtered drinking water, they are likely to be aggregated to some extent and will exhibit a greater resistance to UV than oocysts seeded directly into clean water. Although UV disinfection will provide an additional protective barrier against oocysts that breakthrough a filtration process, the degree of protection may be less than predicted based on seeding studies carried out using clean water. Includes 12 references, tables, figures.