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Monochloramine Cometabolism by Ammonia-Oxidizing Bacteria. Report #4341
Citation:
Speitel, G., J. Maestre, AND D. Wahman. Monochloramine Cometabolism by Ammonia-Oxidizing Bacteria. Report #4341. Water Research Foundation, Denver, CO, 2014.
Impact/Purpose:
The hypothesis of this research was that under some conditions monochloramine cometabolism plays a significant role in causing disinfectant loss in distribution systems and as a defense mechanism that allows nitrifiers to persist in pipe-wall biofilm. The research objectives were to (1) define the relative significance of monochloramine cometabolism in causing disinfectant loss over the typical range of conditions encountered in drinking water distribution systems, (2) provide a mathematical description of monochloramine cometabolism for inclusion in distribution system water quality models (e.g., EPANET-MSX).
Description:
Chloramine use is widespread in United States (US) drinking water distribution systems as a secondary disinfectant. In a recent survey of water utilities, 30% of the respondents used chloramines to maintain distribution system residual (AWWA Water Quality and Technology Division Disinfection Systems Committee 2008), while other recent surveys suggest that between 8 and 12% of drinking water utilities are contemplating a future switch to chloramination (Seidel et al. 2005,AWWA Water Quality and Technology Division Disinfection Systems Committee 2008). Upon implementation of the Stage 2 Disinfectants and Disinfection By-products Rule (i.e., ~2013-2015), chloramination for secondary disinfection in the US is predicted to increase to 57% of all surface and 7% of all ground water treatment systems (USEPA 2005). While beneficial from the perspective of controlling regulated disinfectant by-product formation, 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 (NOM), corrosion, pipe surface reactions, and nitrite (NO2-) oxidation under various conditions in chloraminated water systems (Wilczak et al. 1996,American Water Works Association 2006). Nitrification (i.e., microbially-mediated ammonia and nitrite oxidation) is a significant problem in many distribution systems where chloramines are used as the secondary disinfectant. For example, Wilczak et al. (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. Thus, nitrification control is a major issue in practice and likely will become an issue of growing importance as the prevalence of chloramination increases. A variety of factors may influence the likelihood of nitrification episodes, including disinfectant concentration, chlorine to nitrogen (Cl2:N) mass ratio, free ammonia concentration, temperature, and detention time in distribution systems. In addition to ammonia metabolism, AOB also can cometabolize a wide variety of chemicals via the non-specific enzyme, ammonia monooxygenase (AMO). Cometabolism can be defined as the fortuitous biodegradation of a target chemical through reactions catalyzed by non-specific microbial enzymes. For example, Hooper et al. (1997) provided a summary detailing that the AMO enzyme of Nitrosomonas europaea can cometabolize over 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 (THMs) at rates that are relevant in drinking water treatment applications (Wahman et al. 2005,Wahman et al. 2006). Based on its structural similarity to ammonia and other chemicals that AOB can cometabolize, it is highly likely that monochloramine is similarly cometabolized. As proposed by Woolschlager (2000), AMO would convert monochloramine to chlorohydroxylamine, which would be converted by hydroxylamine oxidoreductase (HAO) to nitrite and chloride in an analogous fashion to ammonia metabolism. Abiotically, chlorohydroxylamine may also rapidly disassociate into nitroxyl (HNO) and chloride with HNO subsequently reacting by a series of competing reactions, resulting in a mixture of nitrate, nitrite, nitrous oxide, and nitrogen gas (Giles 1999, Wahman and Speitel 2014). To date, the role that monochloramine cometabolism plays in determining monochloramine residual concentrations in drinking water distribution systems subject to nitrification has not been considered in practice.
