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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.

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A pilot-scale study was initiated to examine the behavior of viruses pulse injected into a distribution system. The influence of a free-chlorine residual and that of virus preadsorption to clay particles was evaluated by tracing the viruses both in the water flow and after elution from the biofilm. These experiments demonstrated, first, that virus preadsorption on 40 mg of Na-montmorillonite per liter increased the residence time of the viruses within the pilot plant by roughly three times and, second, that preadsorption to clay did not prevent viruses from being inactivated by chlorine. Moreover, with no clay added, a greater amount of viruses was recovered from the biofilm than from the water flow (by a factor of 2 or 10 in the absence or presence of chlorine, respectively), indicating a tendency for virus accumulation within biofilms.

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