Modelling the effect of bromide on chlorine decay in raw and coagulated surface waters

Abstract

A novel and simple semi-mechanistic model describing the effect of bromide on chlorine decay was developed. A well validated two-reactant parallel second order (2R) chlorine decay model is available for planning and operation. Four parameters - reaction rate coefficients and initial concentrations of fast and slow reacting compounds - define the model. Bromide concentration varies in water bodies but considering the variability requires conducting individual tests and parameter estimation for each bromide concentration. The proposed model was developed and tested using data obtained from six water sources from the US and Australia. The samples included a mix of raw and coagulated waters with different pH (7.5–8.0), DOC (1.43–5.54 mg-C/L), SUVA (1.73–2.90), chlorine dose (2.32–8.29 mg/L), bromide (14–844 μg/L), chlorine to bromide ratios (4.7–230.4), and temperatures (20–23 °C). when it was assumed that the initial concentrations of fast and slow reacting compounds would not be affected by the addition of bromide, a linear relationship was established between fast and slow decay rate coefficients and the initial bromide concentration in each water source. The linearity coefficient (β) ranged from 1.73 ± 7.24E-04 to 3.56 ± 1.79E-03 L/μg-Br, but a common value of 3.00E-03 ± 5.79E-04 described the effect of bromide in all four tested sources (DOC 1.43–5.54 mg/L) from the US at pH 8 units and 20 °C. This model simplifies the consideration of the effect of bromide in chlorine decay and reduces the number of tests and parameters required to simulate water supply systems that incorporate the effect of bromide.