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The occurrence of coliforms in drinking water distribution systems may be explained by (i) inadequate water treatment, (ii) post‐treatment contamination, or (iii) coliform growth in the network. In order to confirm the third hypothesis, a 24 h‐starved dense suspension of Escherichia coli was injected into an experimental water distribution system. Results from this experiment clearly indicate that E. coli may find ecological conditions in drinking water distribution systems which will allow growth, particularly in the biofilm phase. Chlorination is an appropriate tool to limit coliform much more easily than the total heterotrophic bacterial biomass. However, in all cases, biofilm associated bacteria are more difficult to kill than suspended bacteria, because of chlorine consumption by the pipe material, and because of a diffusion‐limited reaction between chlorine and the biofilm.

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