Kinetics of chlorine decay

Proper understanding, characterization, and prediction of water quality behavior in drinking water distribution systems are critical to ensure meeting regulatory requirements and customeroriented expectations. This article investigates the factors leading to loss of chlorine residual in water distribution systems. Kinetic rate equations describing the decay of chlorine were developed, tested, and evaluated using data collected in field-sampling studies conducted at several water utility sites. Results indicated that chlorine decay in distribution systems can be characterized as a combination of first-order reactions in the bulk liquid and first-order or zero-order mass transfer-limited reactions at the pipe wall. Wall reaction kinetic constants were inversely proportional to pipe roughness coefficients. Wide variations in both bulk reaction constants and wall reaction constants were observed among the sites.