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Cometabolism of Monochloramine by Nitrosomonas europaea under Distribution System Conditions
Citation:
Maestre, J., D. Wahman, AND G. Speitel. Cometabolism of Monochloramine by Nitrosomonas europaea under Distribution System Conditions. In Proceedings, 2012 AWWA Water Quality Technology Conference (CD ROM), Toronto, ON, CANADA, November 04 - 08, 2012. AWWA Research Foundation, Denver, CO, 1-100, (2012).
Impact/Purpose:
An increasing number of water utilities are using monochloramine (NH2Cl) as a residual disinfectant. A major problem related to chloramination is the occurrence of nitrification in distribution systems. A rapid decay in the NH2Cl residual has been associated with the onset of nitrification. Ammonia-oxidizing bacteria, which oxidize ammonia to nitrite, are able to cometabolize other chemicals with similar structures to ammonia. Although much research has been dedicated to NH2Cl decay and nitrification onset conditions, the significance of ammonia oxidizers and NH2Cl cometabolism in NH2Cl loss has not been studied. This research not only demonstrates the importance of NH2Cl cometabolism during nitrification episodes, but also provides an approach for including NH2Cl cometabolism in water quality models.
Description:
Batch kinetic experiments were carried out with a pure culture of N. europaea to characterize the kinetics of NH2Cl cometabolism. Nitrite, nitrate, NH2Cl, ammonia and pH were measured. The experiments were performed at a variety of conditions relevant to distribution system nitrification. Each experiment consisted of a cometabolic (CM) experiment (active biomass, NH2Cl, ammonia), positive control (active biomass, ammonia), and negative control (inactive biomass, NH2Cl, ammonia). The positive control confirmed active ammonia degradation and was used to estimate the ammonia kinetic coefficients. The negative control, consisting of a batch experiment with inactive biomass (chlorobenzene-inactivated), was used to estimate the abiotic NH2Cl reaction rate constant with biomass. The CM experiments 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 part of the model (i.e., ammonia metabolism, NH2Cl cometabolism, and NH2Cl inactivation of metabolic activity). Kinetic parameters were estimated by simultaneously fitting data from ten different experimental conditions, using the data fitting routine in AQUASIM. The CM experiments were characterized by more rapid decay of NH2Cl relative to the negative control,providing evidence of cometabolism. Ammonia initially decayed rapidly, but then NH2Cl inactivation of ammonia metabolism caused a significant slowing of both the ammonia and NH2Cl decay, thus illustratingthe need for a metabolism inactivation term in the modeling. Experiments at different ammonia and NH2Cl concentrations showed no evidence of enzyme competition. An experiment performed at a very low ammonia concentration showed no NH2Cl cometabolism, presumably because of a shortage of reducing power for the ammonia monooxygenase AMO) enzyme. As i
