Using a comprehensive model to identify the major mechanisms of chloramine decay in distribution systems

The principle mechanisms of chloramine residual decay in drinking water distribution systems is examined using a comprehensive model of chloramine reactions calibrated to distribution system data. The results reveal that four principle chloramine decay mechanisms must be considered, including : reactions with hypochlorous acid/ion (HOCl/OCl - ) ; an auto-catalytic reaction in which chloramines spontaneously decay in the absence of other reactants ; oxidation reactions with reduced forms of organics and iron ; and biologically-catalyzed reactions, such as the reactions with nitrite produced by nitrifiers as well as the direct cometabolism of chloramines by nitrifiers. The chloramine reaction model fits the distribution system data best when all of these reactions, including cometabolism of chloramines by nitrifiers, are included in the model.