Performance of reverse osmosis membranes in the treatment of flue-gas desulfurization (FGD) wastewaters

Reverse osmosis (RO) was studied to reduce salinity of flue gas desulfurization (FGD) wastewaters after softening with Na2CO3·H2O and ultrafiltration (UF). Two commercial thin film composite polyamide RO membranes (SWC-2540 and ESPA-2540, from Hydranautics) in spiral-wound configuration were tested and their performance in terms of salinity reduction as well as permeate flux, fouling index and water recovery was evaluated. Experimental runs were performed according to the feed and bleed configuration in selected operating conditions. For the SWC-2540 membrane experiments were also performed in total recycle configuration in order to evaluate the effect of operating pressure on permeate flux and quality. Experimental results indicated that the SWC-2540 membrane showed a better performance in the rejection of ions: Mg2+ ions were completely rejected, while the rejection towards monovalent ions such as Na+ was of about 95.5%. The ESPA-2540 membrane showed rejections towards Ca2+ and Mg2+ higher than 86.5% whilst the observed rejection towards Na+ was of 80%. For the SWC-2540 membrane an increased rejection for Ca2+ and Na+ ions was observed by increasing the operating pressure in the range 16-50 bar. Mg2+ ions were totally rejected independently by the operating pressure.