Global NEST Journal, Vol 7, No 1, pp 1-16, 2005 Copyright© 2005 Global NEST Printed in Greece. All rights reserved MODELING BROMIDE EFFECTS ON THE SPECIATION OF TRIHALOMETHANES FORMATION IN CHLORINATED DRINKING WATER K.M. MOK 1 * H. WONG 2 X.J. FAN 2 1 Department of Civil and Environmental Engineering University of Macau, Av. Padre Tomás Pereira S.J., Taipa Macau 2 The Macau Water Supply Co. Ltd. 718, Avenida do Conselheir Borja, Macau Selected from papers presented in 9 th International Conference on Environmental Science and Technology (9CEST2005) 1-3 September 2005, Rhodes island, Greece * to whom all correspondence should be addressed: tel: (853) 3974354, fax: (853) 838314 e-mail: kmmok@umac.mo ) ABSTRACT Speciation behavior of trihalomethanes formation in chlorinated waters due to the effects of bromide is mathematically modeled by a three-tier scheme. Equations for the mole fractions of the mono-, di-, and trihalo-intermediates at each tier level in terms of the initial bromide to chlorine molar ratio are derived and verified by comparison with results of other theoretical studies based on an assumption of constant bromination to chlorination reaction reactivity ratio. It is identified that the usage of applied chlorine concentration or available chlorine concentration for calculation of the relative initial bromide to chlorine concentration would yield different reactivity ratio and should be used consistently. The model equations are also tested by laboratory experiments done with source water at a local plant. It is found that equations with an overall bromination to chlorination rate constant ratio of 18.66 can capture the THM speciation behavior of all four tested incubation periods well. KEYWORDS: Trihalomethanes; Speciation; Bromide effects; Kinetic model; Drinking water 1. INTRODUCTION Chlorination is a common process used for disinfection of drinking water in the world. It is a process using chlorine to destroy pathogenic bacteria and algae in raw water. This process has only been considered as a life-saver until Rook (1974) and Bellar et al. (1974) showed that potent trihalomethanes (THMs) of four species can be formed as disinfection by-products in chlorination. They are Chloroform (CHCl 3 ), Bromodichloromethane (CHBrCl 2 ), Dibromochloromethane (CHBr 2 Cl) and Bromoform (CHBr 3 ). Since then THMs have been researched intensively. It is known that the theoretical risk to humans varies among individual THM species, but the brominated ones are more toxic (U.S. EPA, 1992). Hence, the formation of brominated THMs during the disinfection process is of great concern. It is known that when there is bromide existing in raw water, the active halogen shifts from chlorine to bromine (Rook and Gras, 1978), which means that the reactions incorporating bromine into natural organic matters (NOM) are faster than those incorporating chlorine (Symons et al., 1993). Hence the potential for forming Br-THMs is higher if bromide exists. Studies indicate that Br-THMs could be produced even at a considerably low concentration of bromide relative to chloride concentration (Minear and