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Cometabolism of Monochloramine by Distribution System Relevant Mixed Culture Nitrifiers
Citation:
Maestre, J., D. Wahman, AND G. Speitel. Cometabolism of Monochloramine by Distribution System Relevant Mixed Culture Nitrifiers. Presented at 2014 AWWA Water Quality & Technology Conference, New Orleans, LA, November 16 - 20, 2014.
Impact/Purpose:
The kinetic parameter estimates provide the necessary information for incorporation of NH2Cl cometabolism into water quality models (e.g., EPANET-MSX), improving our ability to simulate nitrification occurrence. At low NH2Cl concentrations, 30% of the NH2Cl loss in the annular reactors was attributable to AOB cometabolism, further highlighting the potential significance of this NH2Cl decay mechanism.
Description:
Monochloramine (NH2Cl) is increasingly used as a residual disinfectant. A major problem related to NH2Cl is nitrification in distribution systems, leading to rapid NH2Cl residual loss. Ammonia-oxidizing bacteria (AOB), which oxidize ammonia (NH3) to nitrite, can cometabolize chemicals with similar structures to NH3. Although NH2Cl decay and nitrification onset conditions are widely studied, we only recently quantified the significance of NH2Cl cometabolism in NH2Cl loss with the AOB pure culture, Nitrosomonas europaea. This previous research demonstrated the possible importance of NH2Cl cometabolism during nitrification and provided an approach for including NH2Cl cometabolism in water quality models. Although an important first step, further study was required to investigate NH2Cl cometabolism by distribution system relevant AOB. Two culture sources were used in batch kinetic experiments: (1) suspended mixed culture and (2) annular reactor biofilm grown with and without NH2Cl. These experiments characterized NH2Cl cometabolism kinetics for representative drinking water AOB cultures. Experiments were performed at a variety of conditions relevant to distribution system nitrification. Typically, each experiment consisted of a cometabolic (CM) experiment (active biomass, NH2Cl, NH3), positive control (PC, active biomass, NH3), and negative control (NC, inactive biomass, NH2Cl, NH3). The PC confirmed active NH3 degradation and estimated NH3 kinetic coefficients. The NC used chlorobenzene-inactivated biomass to estimate the abiotic NH2Cl reaction rate constant with biomass. The CM experiment and the kinetic parameters determined from the controls were used to estimate the NH2Cl cometabolism rate constant. The kinetic and equilibrium expressions for NH2Cl autodecomposition and nitrite oxidation were implemented in AQUASIM along with the rate expressions for the biotic model (i.e., NH3 metabolism, NH2Cl cometabolism, and NH2Cl inactivation) to estimate kinetic parameters. The CM experiments had more rapid NH2Cl decay than the NC, providing cometabolism evidence. Ammonia initially decayed rapidly, but then NH2Cl inactivation of NH3 metabolism caused a significant slowing of both NH3 and NH2Cl decay, illustrating the need for a metabolism inactivation term in the model. Excellent fits to NH3 and NH2Cl concentrations were observed over a broad range. The magnitude of the NH2Cl cometabolism rate constant was comparable to the NH3 first order rate constant, indicating the potential importance of NH2Cl cometabolism as a decay mechanism in distribution systems. The kinetic parameter estimates provide the necessary information for incorporation of NH2Cl cometabolism into water quality models (e.g., EPANET-MSX), improving our ability to simulate nitrification occurrence. At low NH2Cl concentrations, 30% of the NH2Cl loss in the annular reactors was attributable to AOB cometabolism, further highlighting the potential significance of this NH2Cl decay mechanism.
URLs/Downloads:
http://www.awwa.org/Portals/0/files/education/conferences/wqtc/wqtc14/pdfs/WQTC14FinalProgramWeb.pdf