Mofidi et al., Bacterial Survival after UV - 1 - Bacterial Survival After Ultraviolet Light Disinfection: Resistance, Regrowth and Repair Alexander A. Mofidi*, Paul A. Rochelle, Connie I. Chou, Himansu M. Mehta METROPOLITAN WATER DISTRICT OF SOUTHERN CALIFORNIA La Verne, California 91750-3399 Karl G. Linden James P. Malley DUKE UNIVERSITY UNIVERSITY OF NEW HAMPSHIRE Durham, North Carolina 27708-0287 Durham, New Hampshire 03824-3534 ABSTRACT Bench-scale experiments were conducted with low- and medium-pressure ultraviolet (UV) lamps to inactivate pathogenic and nonpathogenic strains of Escherichia coli and mixtures of naturally occurring heterotrophic bacteria suspended in granular-media filtered water. Prior to UV exposure, E. coli life cycle characteristics were measured to understand the effects of cell dormancy—a characteristic that may be similar to E. coli present in the environment—on inactivation kinetics. Following UV exposure, bacteria were incubated at 20 o C in lighted and dark environments to simulate subsequent treated water storage in open-air and enclosed reservoirs, respectively. Approximately 9 mJ/cm 2 of UV light provided 2-log inactivation of heterotrophic bacteria. However, suspensions of heterotrophic bacteria treated with 140 mJ/cm 2 were able to regrow to pre-treatment levels during light or dark incubation. Incubation of UV- treated bacteria with a 2.5 mg/L chloramine residual prevented regrowth. For E. coli, resistance to UV varied significantly. To provide 2-log inactivation of all the strains studied, a dose between 0.2 and 7 mJ/cm 2 was required. E. coli resistance to UV was observed to increase when DNA replication was dormant. UV-inactivated, light-incubated E. coli were able to repair within 2 hours of treatment. To provide a more complete barrier to pathogens and sustain public health protection, UV treatment should be followed with a residual disinfectant (e.g., chloramines). Corresponding author. Tel.: +1-909-392-5463; fax: +1-909-392-5166. E-mail address: amofidi@mwdh2o.com © American Water Works Association, 2002 American Water Works Association Annual Conference and Exibition