Draw solution solute selection for a hybrid forward osmosis-membrane distillation module: Effects on trace organic compound rejection, water flux and polarization

Abstract

The integrated forward osmosis membrane bioreactor and membrane distillation (OMBR-MD) is an emerging technique for wastewater reclamation and trace organic compound (TrOC) removal. The selection of draw solution (DS) for this system should consider the particularities of both forward osmosis and membrane distillation. The effect of five different draw solutions on forward osmosis and TrOC rejection in a hybrid submerged forward osmosis – membrane distillation (FO-MD) module was investigated in this study. The roles of each solution on FO and MD water flux, FO reverse salt flux, temperature, concentration polarization and rejection of 7 TrOCs were explored. The salt used in the draw solution influenced both FO water flux (NaCl = MgOAc2 (4.8 L.m−2.h−1) > NaOAc (4.6 L.m−2.h−1) > MgCl2 (3.6 L.m−2.h−1) > EDTA-Na2 (2.8 L.m−2.h−1)) and MD water flux (NaOAc (4.0 L.m−2.h−1) > NaCl (3.8 L.m−2.h−1) > MgCl2 (3.3 L.m−2.h−1) > MgOAc2 (3.1 L.m−2.h−1 > EDTA-Na2 (3.0 L.m−2.h−1)). MgCl2 draw solution produced the lowest FO reverse salt flux, demonstrating its potential to prevent salinity build-up when applied in OMBR-MD. MgCl2 and NaCl were the only salts completely rejected by the distillation membrane, which salt losses during draw solution reconcentration. Regardless of the draw solution salt, the integrated FO-MD module could effectively remove 6 of the 7 TrOCs. MgCl2 and NaOAc afforded the highest TrOC rejections, while the lowest rejections were observed when MgAOc2 was used. MgCl2 was selected as the best DS salt for OMBR-MD by using the technique for order preference by similarity to ideal solution (TOPSIS) tool.