Contamination of drinking water by coliforms from biofilms grown on rubber-coated valves.

In water samples from drinking water distribution systems, coliform bacteria (predominantly Citrobacter species) were repeatedly detected. Disinfection and flushing of the systems did not erase the problem. The pattern of the coliform occurrences indicated contamination originating from biofilms. After inspection of internal surfaces of the systems, no significant biofilm growth was observed on pipe surfaces, but in a number of cases, visible biofilms were detected on rubber-coated valves which harboured the same coliform species as those found in the drinking water samples. In these cases, the rubber-coated valves seemed to act as point sources for the contamination of water.