Chlorinated effluents from cooling water condensers and wastewater treatment plants enter receiving streams and can affect aquatic organisms. The kinetics for dissipation of chlorine residuals (primarily monochloramine) in water under different light conditions (dark, sunlight, and UV light) were investigated by conducting a series of closed batch studies and found to be first-order with respect to monochloramine concentration. This kinetic information was then incorporated into a two-dimensional transport model to describe concentration profiles in a river by considering the decay reaction, mixing, and dispersion, and by approximating the river as a straight rectangular channel. According to predictions of chlorine residual persistence, chlorination at night could have more impact on aquatic life than that during the day.

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