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Review of Nitrification and Distribution System Water Quality Frederick Bloetscher and Daniel E. Meeroff
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he goal of water utilities is to provide highquality drinking water that does not pose a public health concern, and to provide sufficient quantities of that water when needed to their customers (Bloetscher, 2011). An adequate and safe potable water supply is a key requirement for a stable society; however, the provision of safe drinking water is a public trust issue—the public trusts the utility system’s ability to deliver adequate supplies of safe drinking water. The expectation of water systems in the United States and Canada is that they will operate 24 hours a day, uninterrupted, and provide safe, clean drinking water for all who need it (Bloetscher, 2008). When these expectations are not met, problems with public trust will likely occur; Flint, Mich., is a prime example. Rarely does the public understand what is required to meet these expectations, which is a credit to the effectiveness to which these systems have been designed and operated for well over 100 years. The proof is that, in the U.S., for the most part, publicly owned water and sewer systems comply with all regulations on a consistent basis, but the backlog of infrastructure needs means that longer-term compliance may become an issue. Biofilms are one of the issues that can complicate regulatory compliance.
Biofilms Defined Biofilms result from microorganisms, which are the most widely distributed life forms on the planet (Chapelle, 1993). They are known to inhabit and thrive in the presence of moisture and nutrients, both of which exist in plentiful supplies in water distribution networks, where microorganisms can grow in the form of biofilms (Videla, 1996). Biofilms are complex aggregates of microorganisms embedded in a highly hydrated extracellular matrix that show structural heterogeneity resulting from a diverse and complex microcosm. Biofilms are often observed as an unwanted accumulation attached to a surface, such as the inner wall of a water distribution pipeline. During biofilm growth, microorganisms excrete extracellular polymeric substances, which lead to the formation of a slime layer that connects cells and anchors them to the surface and to each other. From the microbial
perspective, biofilms provide an ideal habitat as a source of nutrients, oxygen stratification, resistance to velocity currents, and protection from grazers and biocides (Videla, 1996). From the utility perspective, the undesirable accumulation of biofilms with actively growing slime layers can lead to biofouling. If left uncontrolled, biofilms can catalyze the formation of calcium carbonate deposits that restrict the effective pipe diameter and become a considerable issue for water distribution systems, particularly with regard to hygienic, operational, and economic consequences. Pathogenic microorganisms have been isolated from biofilms, including viruses, fungi, yeast, protozoa (such as amoebae and ciliates), invertebrates, and microbial toxins (Eboigbodin et al., 2008; Bachmann and Edyvean, 2006; Meeroff et al., 2019; and references therein). The presence of these microorganisms in potable water distribution systems represents a potential public health threat. Specifically, release of pathogens harbored in biofilms can lead to an increase in the incidence of gastrointestinal symptoms from waterborne infections caused by bacterial, viral, and parasitic microorganisms. The Centers for Disease Control and Prevention identified biofilms as the source for 65 percent of human bacterial infections from community water supply-associated outbreaks (EPA, 2008c). Gofti et al. (1999) reported epidemiological evidence that children showed close to four digestive problems per person per year and one episode of diarrhea per person per year, attributable to pathogens that developed in the water transmission network after centralized disinfection. According to the World Health Organization, diseases associated with unsafe water distribution, sanitation, and hygiene cause approximately 1.7 million deaths per year (Prentice, 2002). Mature biofilms in drinking water distribution systems are a highly diverse potential source of human pathogens. A wide range of primary pathogens (i.e., that cause disease in healthy individuals) and opportunistic pathogens (i.e., that cause disease in individuals with underlying conditions that may facilitate infection) have demonstrated the ability to survive and thrive in biofilms. Primary pathogens, opportunistic pathogens and indicator organisms, including
10 August 2022 • Florida Water Resources Journal
Frederick Bloetscher, Ph.D., P.E., and Daniel E. Meeroff, Ph.D., E.I., are professors at Florida Atlantic University in Boca Raton.
Clostridium (Emde et al., 1992), E. coli (Emde et al., 1992; Geldreich, 1996; Sartory and Holmes, 1997), Enterobacter (LeChevallier et al., 1987; Emde et al., 1992; Geldreich, 1996; Sartory and Holmes, 1997; Lee and Kim, 2003), Legionella (Murga et al., 2001), Pseudomonas (LeChevallier et al., 1987; Emde et al., 1992; Geldreich, 1996; Norton and LeChevallier, 2000; Lee and Kim, 2003), and Staphylococcus (Gelreich, 1996; Lee and Kim, 2003), among others (see Bloetscher et al., 2010; Meeroff et al., 2019), have been reported in biofilms collected from water distribution networks. An important point to consider is that only coliforms are routinely analyzed for in drinking water, as mandated under the Total Coliform Rule (TCR) and the Groundwater Rule of the Safe Drinking Water Act (SDWA); however, the pathogens and opportunistic pathogens are not. Table 1 summarizes some of the bacteria typically found in a biofilm. In addition to health effects resulting from pathogens, biofilms can also contribute to taste, odor, and color issues, which may lead to operational changes at the treatment plant or for the transmission network. Biofilms may also compromise the proper enumeration of indicator organisms and weaken pipe integrity by microbially influenced corrosion (MIC), which is defined as a microbially mediated electrochemical process that permits the onset and acceleration of corrosion (Videla, 1996). Once mature colonies are established, the effects of MIC are often seen. Microbes cause corrosion directly through metabolic processes that form corrosive chemical species, such as ammonia, hydrogen sulfide, sulfate, ferric, or manganic chlorides (Dillon, 1995). Within the community structure of the biofilm, sulfur can be reduced by anaerobic bacteria to release hydrogen sulfide, which can significantly increase the susceptibility of the pipe to pitting. At the same time, any aerobic bacteria present in the biofilm can corrode metals directly via oxidation. The heterotrophic biomass typically found in a biofilm is supported by the
