Exploring high charge of phosphate as new draw solute in a forward osmosis–membrane distillation hybrid system for concentrating high-nutrient sludge

Abstract

For the first time, a high charge of phosphate was used as the draw solute in a forward osmosis–membrane distillation (FO–MD) hybrid system for concentrating high-nutrient sludge. A high water flux (12.5L/m2h) and a low reverse salt flux (0.84g/m2) were simultaneously achieved at pH9 by using 0.1M Na3PO4 as the draw solute and deionized water as the feed solution in the FO process. The specific reverse salt flux of 0.1M Na3PO4 (Js/Jw=0.07g/L) was considerably less than that of 0.1M NaCl (Js/Jw=0.37g/L) because the complexion between Na+ and HPO42− at pH9 led to the reduction of free Na+ ions, which subsequently reduced the reverse salt diffusion substantially. Moreover, for a feed solution with an initial sludge concentration of 3500mg/L, the sludge concentration could be concentrated to 19,800 and 22,000mg/L in the pressure-retarded osmosis (PRO) and FO membrane orientations, respectively, after 15h of operation. Four types of MD membranes were selected for draw solution recovery; of these, a polytetrafluoroethylene membrane with a pore size of 0.45μm was the most effective in achieving a high water flux (10.28L/m2h) and high salt rejection (approximately 100%) in a diluted Na3PO4 draw solution.