Quantitative microbiological risk assessment requires quantitative data to assess consumer exposure to pathogens and the resulting health risk. The aim of this study was to evaluate data sets on the occurrence of Cryptosporidium oocysts in raw water and on the removal of model organisms (anaerobic spores, bacteriophages) to perform such a risk assessment. A tiered approach was used by first calculating approximate point estimates and when the point estimate was close to the required safety level (10(4) annual risk of infection), fitting the data to probability distributions and Monte Carlo analysis to calculate the distribution of the risk of infection. Sensitivity analysis showed that the variability in the Cryptosporidium data in raw water (largely introduced by the variability of the recovery efficiency of the detection method) determined most of the variance in the risk estimate.

The inactivation of C. parvum and spores of Cl. perfringens by ozone treatment in natural water in a lab-scale continuous-flow system was compared. In addition the inactivation of the natural occurring spores of sulphite-reducing clostridia (SSRC) in this water was monitored in one of the lab-scale systems as well as in a full-scale ozonation process. The survival ratio of C. parvum oocysts was determined using the CD-1 neonatal mouse infectivity test and for Cl. perfringens and SSRC the survival ratio was assessed with the standard anaerobic colony count on the iron-sulphite medium. The results of the lab-scale experiments revealed an inactivation rate constant k (Chick-Watson modelling) at 10°C for C. parvum of 0.14 (SD=0.014; P<0.001) and for Cl. perfringens of 0.25 (SD=0.01; P<0.001). Moreover, first results of monitoring the SSRC inactivation in full-scale ozonation processes indicated that the inactivation rate constant for these wild strains was in the same order of magnitude as determined for Cl. perfringens. Further research is needed to compare inactivation ozone kinetics for Cl. perfringens D10 and SSRC at different temperatures and in other natural waters. Results of additional lab-scale experiments with Cl. perfringens strain D10 indicated that the CT of the gas-feed chamber should be incorporated in the design of a full-scale ozonation. Moreover, setting theCT with the contact time was not as effective for the inactivation capacity as setting the CT with the ozone dosage.

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.

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A low-pressure (LP-UV) and medium-pressure UV lamp (MP-UV) were compared for the formation of nitrite, assimilable organic carbon (AOC), and genotoxicity using pretreated surface water in a bench UV unit. The results have been compared with the by-products formed in pretreated surface water using a pilot unit equipped with four mediumpressure lamps identical to the one used in the bench unit. It was found that although significant nitrite formation occurred in the MP-UV bench tests, it remained well below the European standard (0.1 mg/L NO − 2 ) in the pilot unit. The LP-UV gave no or very low amounts of nitrite (bench unit). The AOC and genotoxicity increase with LP-UV was negligible. The AOC content and genotoxicity after MP-UV irradiation in the bench unit was significant as well, whereas AOC was formed limitedly in the pilot unit. Genotoxicity was elevated but not significant. Résumé :Une lampe UV à basse pression (LP-UV) et une lampe UV à pression moyenne (MP-UV) ont été comparées quant à la formation de nitrite, de COA (carbone organique assimilable) et pour la génotoxicité en utilisant de l'eau de surface prétraitée dans une unité UV de laboratoire. Les résultats ont été comparés aux sous-produits formés dans l'eau de surface prétraitée en utilisant une unité pilote équipée de 4 lampes à pression moyenne, identiques à celle utilisée dans l'unité de laboratoire. Bien que des quantités importantes de nitrite aient été formées dans les essais MP-UV en laboratoire, il a été trouvé que le niveau demeurait bien en deçà de la norme européenne (0,1 mg/L NO − 2 ) dans l'unité pilote. La lampe LP-UV ne donnait pas de nitrite ou seulement en très faibles quantités (unité de laboratoire). L'augmentation de la teneur en COA et de la génotoxicité avec la lampe LP-UV était négligeable. Après une irradiation MP-UV dans l'unité de laboratoire, la teneur en COA et la génotoxicité étaient aussi significatives, alors que la formation de COA était limitée dans l'unité pilote. La génotoxicité était élevée mais pas significative.