Effects of Nanofiltration on Trihalomethane and Haloacetic Acid Precursor Removal and Speciation in Waters Containing Low Concentrations of Bromide Ion
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A study was undertaken to determine trihalomethane (THM) and haloacetic acid (HAA) precursor removal by nanofiltration (NF). Cross-flow experiments were conducted with three NF membranes and six natural waters spanning total organic carbon (TOC) concentrations in the range 3.3-13.1 mg/L and bromide ion concentrations in the range 0.5-2.3 μM. Permeate and feedwaters were analyzed for all nine HAA species containing bromine and chlorine. The rejection of THM and HAA precursors decreased with increasing feedwater recovery suggesting that the transport of these materials is controlled by molecular diffusion across the polymeric membranes rather than by physical sieving at membrane surfaces. Therefore, the maximum feedwater recovery in NF systems designed for disinfection byproduct control may be limited by deteriorations in permeate water quality in addition to fouling caused by precipitation of sparingly soluble salts. High rejections of TOC coupled to low removals of bromide ion by some NF membranes resulted in a shift toward more brominated THM and HAA species in permeate waters upon chlorination. Concentrations of selected highly brominated species are shown to increase upon NF under certain conditions. Empirical equations relating THM and HAA bromine incorporation factors in feed and permeate waters to pH, Br-/TOC, and Cl2/TOC ratios are also presented.
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