Water companies are concerned about biofilms in drinking water systems, particularly the growth of coliform bacteria in the pipe network.
The occurrence of coliform bacteria is related to: filtration methods; temperature; disinfectant type; residual, assimilable organic carbon (AOC) levels; corrosion control; and pipe material selection.
The development of biofilms in drinking water distribution systems can cause pipe degradation and changes in the water clarity, taste and smell but the main problem is related to the public health. Biofilms are the main responsible for the microbial presence in drinking water and can be reservoirs for pathogens. Understanding of the mechanisms underlying biofilm formation and behaviour is of utmost importance to create effective control strategies.
Historically, in the United States alone in 1993, nearly 4,400 water systems affecting 21 million people violated drinking water standards for total numbers of coliform bacteria. Similar trends were noted in 1994 and 1995, with over 12,000 systems exceeding accepted levels. Of concern are the nearly 8,000 systems every year that are significant non-compliers and where coliform bacteria are repeatedly detected in finished drinking water. Although some of this contamination is due to cross connections and other operational defects, a large proportion can trace their problems to growth of bacteria as biofilms in the distribution system.
The numbers of coliform bacteria present in water has traditionally been used as an indicator of the adequacy of drinking water treatment. A newer interpretation of this concept implies that drinking water is not adequately treated if coliform bacteria can grow and spread in distribution system biofilms. One concern is whether opportunistic pathogens such as , Legionella pneunophila, Mycobacterium avium complex (MAC), or other microbes can also grow and spread.
Treatments to reduce biofilms in water systems include flushing and shock chlorination. On average, flushing removes microorganisms from the water, but does not normally dislodge biofilms. Shock chlorination (10 mg/L, 1–24 h) is very effective against E. Coli and but C. perfringens spores are partly resistant (Van Bel et al., 2019).
Further reading on biofilms in drinking water systems
Identifying Future Drinking Water Contaminants (1999). Chapter: 10 Biofilms in Drinking Water Distribution Systems: Significance and Control. https://www.nap.edu/read/9595/chapter/12.
Van Bel, Nikki, Hornstra, Luc M., Van der Veen, Anita, Medema, G.J.. Efficacy of flushing and chlorination in removing microorganisms from a pilot drinking water distribution system. MDPI, Water, 2019, 11, 903. https://doi.org/10.3390/w11050903.
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