Ultraviolet irradiation (UV) is widely used for drinking water disinfection. The majority of the knowledge about UV disinfection is derived from pure-culture, lab-scale studies. Results from such studies are then extrapolated to determine the UV doses necessary to attain the desired microorganism reductions in full-scale systems. This project will investigate the effects of UV treatment within a full-scale water treatment plant on microbial communities in the distribution system, with a focus on opportunistic pathogens and nitrifying bacteria. Project results will provide guidance to utilities on (1) the benefits and costs of continuous operation of UV disinfection, and (2) whether management of microbial communities within the water treatment plant should be considered to help address microbial concerns within the distribution system. Research partner: Ann Arbor Water Utilities Department.