Comparative inactivation of bacillus subtilis spores and MS-2 coliphage in a UV reactor: Implications for validation

Biodosimetry is the currently accepted method for validation of fluence delivery in ultraviolet (UV) reactors for water disinfection. This method utilizes the inactivation of a surrogate microorganism to predict the reduction equivalent fluence and subsequent inactivation of a target pathogen. Two common surrogates - Bacillus subtilis spores and MS-2 coliphage-were used to examine the relationship between surrogate type and biodosimetry results. A pilot-scale LP UV reactor was investigated at two flow conditions (7.5 and 15 gpm) and four different UV 253.7 nm water transmittance (UVT, 1 cm) values between 82 and 91%. The calculated reduction equivalent fluence differed from a maximum of 30% at 7.5 gal./min and 15% at 15 gal./min between the surrogates tested, depending on the UVT. These differences were attributed to the sensitivity of organisms used, hydraulic inefficiences, and UV fluence distribution in the reactor, thus the choice of validation microbe may impact the determination of reduction equivalent fluence in UV reactors.