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Water Company Europoort started a pilot plant (MP)UV study to determine the UV-fluence to meet the Dutch drinking water standards. The results of large volume sampling of this pilot plant demonstrated that environmental spores of sulphite-reducing clostridia (SSRC) were highly resistant against UV. With the pilot plant at a flow of 180 m³/h a challenge test was conducted to compare the susceptibility of environmental SSRC and lab-cultured spores of C. perfringens, MS2 bacteriophages and Bacillus subtilis. The latter was dosed as a biodosimeter with a calibrated UV253.7nm sensitivity to confirm the germicidal fluence. This test demonstrated that environmental SSRC were a factor of 1.6 more resistant against UV than the lab-cultured spores of C. perfringens. Furthermore, the results of these environmental SSRC indicated that they are more resistant to UV than other relevant micro-organisms for drinking water safety. Environmental SSRC calibrated with biodosimetry and determined with large volumes sampling is a potential parameter for on-site verification of the efficacy of UV-systems. Further research is necessary to determine to what extent the UV susceptibility of SSRC is consistent in the water and to find the cause of the increased resistance of environmental micro-organisms. [ABSTRACT FROM AUTHOR]