The decay of chlorine in drinking water involves a complex set of reactions that is usually simplified to first order kinetics in models of water quality in distribution systems. However, to be useful in optimising chlorine dosing regimes, the kinetics expression should accurately describe the shape of the chlorine decay curve for different chlorine doses and be able to simulate re-chlorination. After considering the nature of the reactions involved in chlorine decay, five simplified reaction schemes were evaluated for their suitability to describe chlorine concentration in bulk water. Each scheme was fitted to a sample of experimental data of chlorine decay in raw water obtained from Warragamba Dam (the major source of water supplied to Sydney, Australia). A scheme involving two parallel reactions of organic carbon compounds with chlorine is both necessary and sufficient to satisfy the requirements of modelling chlorine decay accurately.

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