The inactivation rate constants (k-value) of pathogenic viruses, bacteria and protozoa and indicator or model microorganisms for monochromatic UV253.7 radiation have been calculated from literature studies describing experiments of seeded microorganisms in collimated beam apparatus with low-pressure lamps. Results show that data of most studies fitted well in the first-order Chick-Watson disinfection kinetics. Viruses require the highest UV fluence and the double-stranded DNA adenovirus is by far the most persistent species with an UV fluence requirement of 125 mJ/cm2 for 3 log inactivation. Translation of these k-values to full-scale UV systems requires accurate fluence assessment in these continuous flow systems with fluence distribution, variations in water quality (UV transmittance) and operational aspects like lamp age and fouling. Determination of the reduction equivalent fluence or REF with seeded model organisms is an accepted methodology to validate UV systems. Frequently used model organisms F-specific RNA phage MS2 and spores of Bacillus subtilis have low k-values. For validation of systems with low fluence requirement to inactivate UV sensitive organisms, alternative model organisms are suggested. Further research is needed to verify the potential of REF verification of full-scale systems by monitoring inactivation of indigenous spores (anaerobic or aerobic) with large volume sampling.