Evaluation of Two-Species Model for Chlorine Decay and THM Formation under Rechlorination Conditions

by Dominic Boccelli,
R. Summers,
James Uber,

Document Type: Proceeding Paper

Part of: WRPMD'99: Preparing for the 21st Century


(No paper) Typically, chlorine decay has been described by a first-order decay model. The first-order model, however, has two drawbacks: 1) the reactive species within the water is assumed in excess, and 2) the inability of the model to describe chlorine decay from time t = 0. A second-order decay model with fictitious reactive compound, while still a simplification, is investigated here as an alternative to the first-order approach. By accounting for the reactive material, the second-order model is found to describe chlorine decay more accurately, particularly under rechlorination scenarios. Rechlorination occurs during booster disinfection and, given distribution systems are looped networks, whenever two, or more, influent links in a distribution system connect. Therefore, the second-order model has the potential to more accurately describe chlorine decay in a distribution system water quality model. An additional model for describing trihalomethane (THM) formation, which assumes formation is linear with respect to chlorine demand, is also tested. Chlorine decay, with and without rechlorination, was observed using five different treated source waters: four surface waters and one ground water. The finished dissolved organic carbon of the source waters ranged from 0.65 mg/L to 3.50 mg/L. Initial chlorine doses produced initial chlorine concentrations ranging from 0.71 to 8.75 mg/L. Rechlorination consisted of a single rechlorination dose applied some time after the initial dose. The resulting chlorine concentrations after rechlorination dose ranged between 0.37 and 8.75 mg/L and were applied from 5 to 430 hours after the initial chlorine dose. For describing the chlorine decay of the initial dose, the second-order model always performed as well as, or better than, the first-order model.

Subject Headings: Chlorine | Decomposition | Trihalomethanes | Model accuracy | Water supply systems | Water treatment | Water quality | Surface water

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