Impact of ultraviolet disinfection on biofilm growth in a model distribution system

To investigate the impact of 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 biofilm in the UV system, as compared to the control, was more responsive to the increased nutrient availability afforded by a decrease in HRT, plausibly because the biofilm consisted mainly of secondary cells. The steady-state density of the opportunistic pathogen during the simulated intrusion events was not consistently lower in the biofilm of the UV system, as compared to the control, but was instead significantly correlated to the density of heterotrophic bacteria. However, the relationship between densities of the opportunistic pathogen and heterotrophic bacteria was not established as being causal. The DNA fingerprints of the biofilm 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.