Modelling the formation of brominated trihalomethanes in chlorinated drinking waters

The chlorination of water containing bromide and natural organic matter (NOM) leads to the formation of brominated trihalomethanes (THMs). The extent of brominated THM formation depends, inter alia, on the bromide:chlorine concentration ratio ([Br−]:[chlorine]). A reaction scheme is proposed from which a simple kinetic model is developed that mathematically relates the extent of bromination, and the relative abundances of the four THMs, to the [Br−]:[chlorine] ratio. In the scheme, the trihalogenated precursors to THMs are formed by three steps each of which substitutes either bromine or chlorine into an activated carbon site in the NOM. This leads to six pairs of competing bromination:chlorination reactions, whose rate constant ratios (kb:kc) have been estimated by using the model to fit THM data obtained from the chlorination of 17 waters from New Zealand. The individual kb:kc ratios range from 4 to 15. From a plot of the ratio of total bromine to total chlorine present in the THMs as a function of the [Br−]:[chlorine] ratio, an apparent overall kb:kc ratio of 9.1 is obtained. Using USEPA cancer potency factors, the model is used to predict the relative cancer risk associated with THMs as a function of the [Br−]:[chlorine] ratio. This risk increases steeply to a peak at a [Br−]:[chlorine] ratio of approximately 0.15, then gradually decreases to the value associated with bromoform alone. The risk associated with THMs may vary considerably through changes in the [Br−]:[chlorine] ratio as the result of natural variation in the [Br−], or through poor control of chlorine dosing.