Recovery of the Diluted Draw Solution of Os-MBR and Production of Purified Water by Membrane Distillation

Abstract

This study investigates the application of direct contact membrane distillation (DCMD) for concentrating diluted draw solution (DS) from osmotic membrane bioreactor (Os-MBR). A flat sheet PTFE hydrophobic membrane with a pore size of 0.2 μm was used to perform the experiments. The effect of circulation velocity, temperature, and channel spacer effect on permeate flux were investigated. Results indicated that the permeate flux increased with feed temperature and velocity. Spacers enhanced permeate water flux up to 35% and the sequence of spacer design in terms of flux was: diamond design > ladder design > without spacer. DCMD process was also applied during long period for reconcentration of synthetic aqueous draw solutions of two organic (sodium acetate & magnesium acetate) and three inorganic salts (sodium chloride, magnesium chloride, & calcium chloride) under the optimal operating conditions. The optimal DCMD condition was achieved using aqueous NaCl solution. Organic salts Mg(CH3COO)2 gave prolong operational period in OsMBR-MD and required more MD membrane flushing. MD membrane flushing followed the order as Mg(CH3COO)2 > CH3COONa > CaCl2 > MgCl2 > NaCl. NaCl gave better performance in terms of flux and recovery as compared to all other salts. For all organic and inorganic DS, the DCMD system was able to achieve DS recovery as per Os-MBR requirement and also produced good quality product water due to high TDS and contaminant rejection (>99%).