Haloacetic acid formation during chloramination: role of environmental conditions, kinetics, and haloamine chemistry
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This dissertation addresses the development of strategies to limit haloacetic acid (HAA) formation resulting from chloramination in drinking water treatment. The impact of several variables that influence HAA formation, such as natural organic matter (NOM), pH, Cl2:N ratio, disinfectant residual concentration, and bromide ion concentration were studied. A multi-factor, two-level, factorial experimental design and statistical analysis of the collected data determined pH and bromide concentration to be the most significant factors contributing to HAA formation. In addition to these variables, the rate of HAA formation during chloramination is a key consideration in determining strategies for minimizing formation. The kinetics of HAA formation during chloramination are characterized by an initial rapid period of formation followed by a period of slower formation. However, many plants now have a significant period of free chlorination prior to ammonia addition for purposes of meeting disinfection requirements. During short periods of prechlorination (5 or 20 minutes), significantly more HAA formation occurred relative to chloramination alone. This research was not only focused on explaining DXAA formation in terms of NOM characteristics and basic water quality and operating parameters, but also expands upon our knowledge of haloamine chemistry with a particular focus on DBP formation. In the presence of bromide, bromine-substituted haloacetic acids, as well as bromine-substituted haloamines, are formed, greatly increasing the number of chemical species that may be relevant in controlling DXAA formation. These bromine-substituted haloamines decay more rapidly and are significantly more reactive than their chlorine-substituted counterparts in forming HAAs. This research provides the fundamental underpinning for new strategies that will help water utilities select operating conditions that minimize the formation of the most reactive haloamine species, thereby leading to decreased HAA formation during chloramination.