A biofilm is a microbial layer that is formed on the inside of water distribution pipes. The source of the bacteria is the water itself. Even treated water is not 100% free of bacteria. These bacteria remain after surviving the treatment process or ingress into the pipes by other means and thereby escape treatment.
Biofilms are clusters of long strings of bacterial cells rangeing in length from 1 - 20 micrometers, and 0.1 - 10 millimeters thick.
Biofilms have been implicated in water discolouration and unpleasant odours.