Measurement of THM and Precursor Concentrations Revisited: The Effect of Bromide Ion

In the absence of bromide ion (Br), method 5710B from Standard Methods is adequate for measuring trihalomethane (THM) precursor concentrations, provided a free available chlorine (FAC) residual of 3 mg/L is maintained at the end of incubation, and method 5710E from Standard Methods is appropriate for predicting THM concentrations at the consumer's tap. In the presence of Br⁻, method 5710B is adequate for measuring THM precursor concentrations provided only total precursor is desired. Because the initial Br⁻/average FAC dosage molar ratio influences bromine substitution, THM species concentrations cannot be predicted using method 5710B. Instead, method 57Í0E must be used. This is particularly important when evaluating THM precursor removal unit processes when the use of method 571 OB may indicate greater bromine substitution in the unit process effluent than in the influent.