Membrane-based desalting processes including reverse osmosis (RO), forward osmosis (FO), and membrane distillation (MD) were systematically evaluated for concentrating RO brine. Basic characteristics of membrane processes were first examined. Commercial polyamide RO exhibited higher water and lower salt permeability coefficients than cellulose FO membrane. However, salt rejection by FO seemed to be higher than RO primarily due to the hindrance of reverse draw solute flux. The water flux of MD comparable to RO was obtained when temperature gradient was more than 20-30 degrees C. The applicability of RO, FO, and MD was further tested with real brine obtained from full-scale RO plant processing brackish water. Results demonstrated that water flux was not significantly reduced in MD, while severe flux decline was observed in both RO and FO at high recovery. To elucidate major causes of different flux behaviors, the fouled membrane surfaces were analyzed by scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction. Foulant analysis suggested that CaCO3 scaling occurred particularly at high water recovery, which was in good agreement with water quality simulation. CaCO3 scaling, however, had only small impact on flux behavior in MD. From these findings, MD could be suggested as the best option for concentrating industrial RO brine if low-grade heat (below 50-70 degrees C) is available.
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