Comparing Cryptosporidium AND MS2 Bioassays—implications for UV Reactor validation

The use and selection of microbial surrogates to validate ultraviolet (UV) reactor performance remain a challenge. Proper interpretation of bioassay results requires an appreciation of the relative UV dose-response of the surrogate and target organisms and the limitations surrogates impose. The aim of this study was to improve the understanding of MS2 coliphage's use as a bioassay surrogate for the waterborne pathogen Cryptosporidium parvum. Side-by-side MS2 and Cryptosporidium bioassays were performed to directly link MS2 and Cryptosporidium inactivation. At a flow rate of 757 L/min (200 gpm), the low-pressure, high-output UV reactor provided >4.7-log Cryptosporidium inactivation at an MS2 equivalent dose of 45 mJ/cm 2 . Although these data support the use of MS2 as a test organism to validate UV reactor performance, MS2 is a very conservative surrogate for Cryptosporidium. Use of a test organism with a more comparable UV dose-response would allow validation of much higher levels of log inactivation.