Biofilm in a water pipe
Photo courtesy of Utility Service Group
Microbes’ propensity to attach to both living and inanimate surfaces improves their likelihood of survival and proliferation. They produce a sticky material that binds them together and anchors them to the surface forming dense, complex colonies of microorganisms known as biofilm. Biofilms are very common in nature1, especially on water- and food-contact surfaces. The interiors of almost all water distribution systems eventually develop biofilms that may harbor pathogenic microbes and promote metal pipe corrosion, scaling and sediment buildup. Biofilms can discolor water or cause it to take on disagreeable tastes or odors. As the US drinking water infrastructure ages, biofilm control has become a more pressing public health concern.
A recent article in the American Water Works Association journal, Opflow2, notes that monitoring and controlling biofilms is a sustainable practice that helps water treatment plant operators lower both capital and operational costs, maintain aesthetic water quality, and ensure regulatory compliance. The article describes several strategies used to keep biofilms in check. These consist of chemical and physical methods briefly summarized below.
Battling Biofilms with Chemistry
You Can’t Put Lipstick on This Pig
The term “pig” may have initially been used in reference to the filthy condition of the emerging swab. Wikipedia informs that it may have originated from the squealing noise made by early “pigs”—bundles of straw wrapped in wire—as they were forced through pipe. Wikipedia also notes that “pig” has become a “backronym” for “Pipeline Inspection Gauge” or “Pipeline Intervention Gadget.”
A residual level of disinfectant is essential to help control biofilms, but disinfectant alone cannot penetrate or dissolve a biofilm mass. Reducing levels of nutrients in water can help reduce biofilm growth. “Booster disinfection” could help maintain disinfectant levels throughout the water system and help remove ammonia, which supports the growth of certain bacteria. Changing the disinfection method also could be helpful in controlling biofilms. Chloramines are commonly added to drinking water as a secondary disinfectant to protect water as it journeys from the treatment plant to the tap. More research is needed to ascertain the effectiveness of chloramines on biofilms3.
Physical Options for Addressing Biofilms
Physical methods can be effective, especially in addressing “mature,” well-developed biofilms. Flushing water through pipes at high velocities, for example, can help remove biofilms from smooth pipe interiors, such as PVC. Biomass may be more difficult to address in pipes with rough interior surfaces or tuberculation. In those cases more aggressive physical methods are used to scour pipe interiors. Pushing a flexible swab with an abrasive outer coating (commonly referred to as a pig) through a pipe by means of hydraulic pressure is one of the most common methods employed to physically remove scale and biofilm. Pigs are currently available in various forms to serve specific purposes including, but not limited to, cleaning, scraping, wiping, brushing, DVD inspection, gauging, and batching. “Ice pigging” is a recently developed technique that utilizes plugs of ice to scour biofilm, scale and sediment from water main interiors. An advantage of ice over conventional pigs is that ice cannot become stuck in a restriction in the pipe bore; it simply melts in a relatively short period of time.
Tackling Biofilm in Aging Infrastructure
Drinking water quality is influenced by many factors including the characteristics of the source water, treatment methods and distribution system characteristics. Older water infrastructure is usually more vulnerable to contamination through leaks and breaks, which increases the possibility of pathogenic microbes such as Legionella, Giardia and certain amoeba—including possibly Naegleria–entering and becoming established in biofilms and being released into the water supply on an intermittent or ongoing basis.
Controlling biofilms is an important step in delivering high quality water. Given the advanced age of much of our nation’s water infrastructure, it is becoming ever more important.
Fred Reiff, P.E., is a retired official from both the U.S. Public Health Service and the Pan American Health Organization, and lives in the Reno, Nevada area.
1Biofilms even exist in the human body in the intestinal tract and as dental plaque.
2Miller, R., Friedman, M., Koci, D. and Moore, R., “Biofilm Control: compliance,” Opflow, v. 39, no. 9, September, 2013.
3Biofilm research is a priority at Montana State University’s Center for Biofilm Engineering where biofilm control measures are being studied as well as beneficial applications of biofilms in industry, medicine and other areas.