Challenge Organisms for Inactivation of Viruses by Ultraviolet Treatment

BACKGROUND: Ultraviolet reactors that are used for drinking water disinfection need to be tested to ensure they are delivering the required doses and achieving the desired level of pathogen inactivation to protect public health. Testing methods can use biological, chemical, or particle surrogates, although not all approaches have been approved for reactor validation. The non-pathogenic bacteriophage MS2 and Bacillus subtilis spores are currently recognized as the standard challenge organisms for reactor validation because they are more UV resistant than most waterborne pathogens. However, an important group of potentially waterborne viruses, the adenoviruses, is considerably more UV resistant than other pathogens and the standard MS2 and B. subtilis biodosimeters. Based on the high UV resistance of adenoviruses, the USEPA’s Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and Ground Water Rule (GWR) require a UV dose of 186 mJ/cm for 4-log inactivation of viruses. This level of MS2 inactivation is achieved with a dose of approximately 85 mJ/cm, thereby limiting the utility of MS2 for reactor validation at the high doses required for virus inactivation credit, or other high-dose applications. An alternative challenge organism is required that has similar or higher UV resistance compared to adenoviruses. The ideal challenge organism would be non-pathogenic, easily grown to high concentrations, easily enumerated in a growth or infectivity assay, display minimal shoulder (lag) or tailing of the dose response curve, stable for relatively long periods with minimal die-off, robust under typical testing conditions, and effectively model the reactor’s disinfection efficiency toward the target pathogen.