Ultraviolet (UV) light reactors used for disinfection of drinking water require validation toensure they are delivering UV doses sufficient to protect public health. Validation methodsinvolve inactivation of biological surrogates (challenge organisms) or non-biological approachessuch as chemical or particle actinometry. The bacteriophage MS2 has typically been the mostfrequently used challenge organism for reactor validation; it is non-pathogenic and considerablymore UV resistant than many bacterial, viral, or protozoan pathogens. However,Cryptosporidium oocysts are significantly more sensitive and adenoviruses display much higherlevels of UV-resistance, thereby limiting the utility of MS2 as an accurate validation surrogate.Surrogates that more closely model the response to UV of target pathogens are required. Theprimary objective of this study was to evaluate the response to UV of a variety of potentialchallenge organisms that might be used for reactor validation. Inactivation of MS2 was withinthe 90% prediction limits specified in the US Environmental Protection Agency (USEPA) Disinfection Guidance Manual and theresponse of PP7, PM2, and B. pumilus overlapped MS2 within the range of doses investigatedwith 80 mJ/cm<sup>2</sup> achieving 4 - 4.5 log inactivation. The T phages were very sensitive to UV; T4,T7 and Phi29 were the organisms that most closely modeled Cryptosporidium inactivation.Bam35 and PRD1 were similar with a response mid-way between the T phages and the MS2group. For many bacteriophages, their response to UV light was independent of their respectivehost's response. The only organisms with UV responses falling within the adenovirusinactivation zone were Deinococcus spp. A UV dose of 120 mJ/cm<sup>2</sup> resulted in 4.6 log, 4.0 log,and 4.1 log inactivation of D. radiodurans, D. radiopugnans, and adenovirus 2, respectively.Therefore, Deinococcus spp. may be good surrogates for adenoviruses in bench level testing.Further work is necessary to determine whether they are also suitable surrogates in larger scalereactors. Includes 32 references, tables, figures.
