Monochloramine Cometabolism by Nitrifying Biofilm Relevant to Drinking Water

2016 © American Water Works Association JOURNAL AWWA JULY 2016 | 108:7 Chloramine use is widespread in US drinking water distribution systems as a secondary disinfectant, and its use is predicted to increase with the final implementation of the Stage 2 Disinfectants and Disinfection Byproducts Rule (USEPA 2005). Chloramination is beneficial from the perspective of controlling formation of regulated disinfectant by-products. However, chloramination may promote the growth of nitrifying bacteria—i.e., ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB)—because of naturally occurring ammonia; residual ammonia remaining from initial chloramine formation; and ammonia released from chloramine decay, oxidation of natural organic matter, corrosion, pipe surface reactions, and nitrite oxidation under various conditions in chloraminated water systems (AWWA 2013, Wilczak et al. 1996). Nitrification (i.e., microbially mediated ammonia and nitrite oxidation) is a significant problem in many distribution systems that use chloramines as the secondary disinfectant. For example, Wilczak and colleagues (1996) surveyed 67 medium and large utilities practicing chloramination and found that 63% of them experienced nitrification to some degree and about 25% had moderate to severe nitrification problems. A variety of factors may influence the likelihood of nitrification episodes, including disinfectant concentration, chlorine to nitrogen (Cl2-to-N) mass ratio, free ammonia concentration, temperature, pH, presence of sediments, dead ends, older unlined water mains, and detention time in distribution systems. In addition to ammonia metabolism, AOB also can cometabolize a variety of chemicals via the nonspecific enzyme ammonia monooxygenase (AMO). Cometabolism can be defined as the fortuitous biodegradation of a target chemical through reactions catalyzed by nonspecific microbial enzymes (Horvath 1972). For example, Hooper and colleagues (1997) provided a summary detailing that the AMO enzyme of Nitrosomonas europaea (an AOB pure culture) can cometabolize more than 35 halogenated chemicals, using ammonia as the growth substrate. Recent research has shown that N. europaea, as well as mixed cultures of nitrifiers present in natural waters, treatment plants, and distribution systems, can cometabolize the four regulated trihalomethanes at rates that are relevant in drinking water treatment applications (Wahman et al. 2006, 2005). Monochloramine decay/demand and nitrification in chloraminated drinking water systems have received considerable study. Only recently, however, has the significance of monochloramine cometabolism in monochloramine demand been quantified with a suspended AOB pure culture, N. europaea (Maestre et al. 2013), and a nitrifying mixed culture (Speitel et al. 2014). AOB monochloramine cometabolism likely results from monochloramine’s structural similarity to ammonia and Biological monochloramine removal (i.e., cometabolism) by a pure culture ammonia-oxidizing bacteria—Nitrosomonas europaea—and a nitrifying mixed culture have recently been shown to increase monochloramine demand. Although important, these previous suspended-culture batch kinetic experiments were not representative of drinking water distribution systems, where bacteria grow predominantly as biofilm attached to pipe walls or sediments, and physiological differences may exist between suspension and biofilm growth. The current research made an important next step in extending the previous results by investigating monochloramine cometabolism by biofilm grown in annular reactors under conditions relevant to drinking water. Estimated monochloramine cometabolism kinetics were similar to those of ammonia metabolism, and monochloramine cometabolism was a significant loss mechanism (30–40% of the observed monochloramine loss). These results demonstrate that monochloramine cometabolism occurred in nitrifying biofilm relevant to drinking water; thus, cometabolism may be a significant contribution to monochloramine loss during nitrification episodes in distribution systems. Monochloramine Cometabolism by Nitrifying Biofilm Relevant to Drinking Water