Raw water bromide : relationship to distribution and extent of formation of trihalomethanes in finished drinking water

A study was conducted to examine the effect of raw water bromide on the formation and distribution of trihalomethanes (THMs) in finished Twenty major water supplies in East Tennessee were drinking water. selected for their significant levels of bromine-containing THMs, as demonstrated in previous studies. These cities were sampled quarterly for raw water pH, temperature, NVTOC, and bromide content, as well as finished water pH and NVTOC, and applied chlorine dose, coupled with conjunctive measurement for 7-day THMs.

Few data regarding bromide levels in natural waters are currently available since most conventional colorimetric applications lack desired sensitivity, as does direct ion chromatography (IC). Although Fishman's kinetic permanganate method is adequate in the 10 100 µg/l range, it is time-consuming, and prone to certain interferences. An IC method using a sample preconcentration column has been evaluated. The method produced a 1 µg/l minimum detection level using deionized water based standards, with 3% relative precision at 30 µg/1. Concentrator capacity varied and the highest-capacity concentrator was selected, followed by examination of raw water samples regarding ranges and types of other ions present, such that the maximum concentration volume selected (50 ml) resulted in 100% retention of bromide. Bromide levels in the raw waters sampled were found to range from 10 to 255 µg/1.

Concurrent with the field sampling, laboratory chlorination experiments were made using Tennessee River Water, under controlled laboratory conditions of bromide level, chlorine dose, pH, ionic strength, temperature, and organic precursor concentration. Resultant THM formations were monitored over a 96 hour reaction period. The results concurred with observations made in previous work by such researchers as Bird, and Rook.

Nonlinear regression models for THM formation were generated using the laboratory chlorination data with respect to pH, temperature, chlorine dose, and bromide and NVTOC level. Actual values for these variables were substituted into the regression models, using the seasonal field data. Resultant predictive THM values were then compared to actual THM values for those dates sampled. In general, these models were found to give acceptable fits, with an overall index of agreement of 74.1 for one of the models (equation IV-35, Dud method).