Student Research: Lesley A. Leggett
UV disinfection is a promising treatment technology to comply with recent US EPA rules (Long Term 2 Enhanced Surface Water Treatment Rule and Stage 2 Disinfectants and Disinfection Byproducts Rule). UV is highly effective against chlorine-resistant microorganisms Crytosproidium and Giardia and has low potential to form disinfection byproducts. However, there are some microorganisms that are still highly resistant to UV irradiation such as adenoviruses. In fact, the UV doses necessary to achieve a significant inactivation of adenovirus are beyond practical dose ranges used in water treatment processes. Therefore, we determined the effectiveness of sequential disinfection using UV irradiation and free chlorine at low doses against a human adenovirus. Adenovirus type 2 (Ad2) was suspended in phosphate buffered saline (PBS), 5 mL was aliquoted to a 60X15 mm cell culture dish and irradiated with a specific dose of either low pressure (LP) or medium pressure (MP) UV irradiation to give an approximately 1 log10 inactivation of Ad2. The UV-irradiated Ad2 was then exposed to ~0.3 mg/L of a free chlorine solution at pH 8.0 and 4 C in PBS for 10 minutes. A small volume of chlorinated sample was removed at various time points, serially diluted 10-fold, and assayed in A549 cells using the 50% tissue culture infectious dose method (TCID50). A high level of inactivation was achieved within 30 seconds by free chlorine following UV exposure (>2.0 log10 after LP UV and >2.5 log10 after MP UV). Inactivation with free chlorine only was slightly greater, with >2.5 log10 inactivation achieved at 30 seconds, but results were not significantly different. Inactivation by sequential disinfection was additive rather than synergistic. The results of this study suggest that sequential disinfection with UV irradiation and free chlorine should provide highly effective inactivation of human adenoviruses within practical UV and free chlorine dose ranges.