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/cm2. 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.
