Biofilms in Drinking Water Distribution Systems Information

Biofilms in Drinking Water Distribution Systems: Significance and Control
Mark W. LeChevallier

A biofilm is a collection of organic and inorganic, living and dead material collected on a surface. It may be a complete film or, more commonly in water systems, small patches on pipe surfaces. Biofilms in drinking water pipe networks can be responsible for a wide range of water quality and operational problems. Biofilms can be responsible for loss of distribution system disinfectant residuals, increased bacterial levels, reduction of dissolved oxygen, taste and odor changes, red or black water problems due to iron or sulfate-reducing bacteria, microbial-influenced corrosion, hydraulic roughness, and reduced materials life (Characklis and Marshal, 1990).

Microorganisms in biofilms can include bacteria (including coccoid round, rod-shaped, filamentous, and appendaged bacteria), fungi, and higher organisms like nematodes, larvae, and Crustacea. Recently, researchers have shown that viruses and parasites like Cryptosporidium can be trapped in biofilms. Although viruses and Cryptosporidium do not grow in a biofilm, they can attach to biofilms after a contamination event. Therefore, it is important to thoroughly flush the distribution system to remove these organisms following a contamination event.

A primary reason that many water utilities become concerned with biofilms in drinking water systems is due to growth of coliform bacteria in the pipe network. In 1993 in the United States alone, nearly 4,400 water systems affecting 21 million people violated drinking water standards for total coliform bacteria (Pontius, 1995). Similar trends were noted for 1994 and 1995, with over 12,000 systems exceeding accepted coliform levels. Of concern are the nearly 2,000 systems every quarter that are significant noncompliers and repeatedly detect coliform bacteria in finished drinking water. Although some of these systems experience coliform occurrences due to cross connections and other operational defects, a large proportion of the systems can trace their problems to regrowth of the bacteria in distribution system biofilms.
National Academies of Sciences, Engineering, and Medicine. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. https://doi.org/10.17226/9595.