AWWA JAW62008

Chloramination has replaced free chlorine as a secondary disinfectant at many utilities because of regulations limiting trihalomethanes (THMs) and haloacetic acids (HAAs) in the distribution system. Chloramine use is advantageous because it forms substantially lower concentrations of THMs and HAAs than free chlorine. However, the potential for nitrification, which is the bacterially mediated conversion of ammonia to nitrite and nitrate, is a trade-off that utilities must consider in their monitoring and maintenance practices. Although many theoretical aspects of nitrification are understood, practical management of the chloraminated distribution system still poses a challenge to utility managers. The absence of adequate data collection to assess nitrifying conditions and the absence of a comprehensive framework for analyzing this data contribute to the difficulties in effectively managing a chloraminated distribution system to eliminate or minimize nitrification. In this article, nitrification potential curves are introduced as a possible operational strategy to prevent nitrification in chloraminated distribution systems. Nitrification potential curves are constructed based on the parameters that define growth and inactivation kinetics of ammonia-oxidizing bacteria (AOB) in environments that contain chloramine and ammonia. To demonstrate the applicability of nitrification potential curves, the authors conducted long-term, pilot-scale studies that covered a range of chlorine-to-ammonia application ratios and chloramine residual concentrations that are typical of full-scale chloraminated systems. Results indicate that the total chlorine and free ammonia concentrations associated with the presence or absence of nitrification are closely linked to the growth and inactivation kinetics of AOB in the distribution system. These growth and inactivation kinetics can be estimated to produce a nitrification potential curve that predicts whether the system's operating conditions are likely to lead to nitrification. The straightforward methodology outlined may be useful for full-scale system assessment to explain the presence and absence of nitrification. Includes 33 references, tables, figures.