Data on E. coli incidence in drinking water samples have been evaluated for 4 European countries. Within the EC project MicroRisk, large volume sampling was done in the United Kingdom (with disinfectant residual), the Netherlands (mainly without disinfectant residual) and Germany (without disinfectant residual). No E. coli were found and very low background concentrations (< 10−4 per L) were calculated. Furthermore, data of 280,000 water samples collected in France (with disinfectant residual), the Netherlands and Germany (both with and without disinfectant residual) were evaluated for E. coli incidence. In total, similar results were obtained for Germany and the Netherlands. In France, significantly higher incidences occurred as more small rural supply systems were included. The detailed data evaluation revealed a slight increase of mean E. coli concentrations during distribution in Germany and the Netherlands, for both disinfected and non-disinfected supply zones. This suggests that, if technical measures are taken to avoid contamination during distribution, non-disinfected supply zones can be regarded as being as safe as disinfected supply zones. Furthermore, the indicator principle of E. coli for faecal contaminations is valid in non-disinfected supply zones. In chlorinated systems, on-line-monitoring of chlorine residuals represents a good means to detect ingress of organic material.

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