AWWA WQTC56989

To investigate the impact of ultraviolet (UV) disinfection on water distribution system biofilms, two model distribution systems were operated in parallel: one received a simulated potable water disinfected with UV light, whereas the control system received the same influent without treatment. In each of five experiments, the bulk and biofilm populations were monitored until the density of heterotrophic bacteria reached steady-state, at which time an opportunistic pathogen was added to the systems to simulate an intrusion event. In addition, the microbial composition of the biofilm and bulk fluid communities of each system were investigated using intergenic transcribed spacer (ITS) analysis, a method of DNA fingerprinting. UV disinfection did not consistently decrease the density of heterotrophic bacteria in the biofilm; densities were consistently greater in the UV system in 2-hour HRT experiments, but greater in the control system in 12-hour HRT experiments. The DNA fingerprints from the UV system varied greatly among the experiments. The variety of strains surviving UV irradiation to form a part of the biofilm in the UV system, despite the high UV dose applied, may be due to particle shielding of susceptible strains of bacteria. Collimated beam tests of isolates from the UV system confirmed that bacteria that were susceptible to UV irradiation in planktonic form were present in the biofilm of the UV system. Includes 2 references.