Optimising the recovery of EDTA-2Na draw solution in forward osmosis through direct contact membrane distillation

Ethylenediaminetetraacetic acid disodium (EDTA-2Na) has been demonstrated as an excellent draw solution in the forward osmosis (FO) process because of its high osmotic pressure together with low reverse salt flux but its application is hindered by difficulties in the recovery of draw solution. Hence, in this study, microporous hydrophobic membranes were used in direct contact membrane distillation (DCMD) to concentrate the diluted EDTA-2Na draw solution. The MD was found to require lower operating pressures than do all other widely applied pressure-driven membrane processes, particularly in RO. This study systematically investigated the effect of different polytetrafluoroethylene membranes under various cross flow velocities of 2.67–14.67 cm/s, feed temperatures of 35–60 °C, and distillate temperatures of 10–20 °C in DCMD process for regeneration of diluted EDTA-2Na. The results revealed that DCMD system could achieve a salinity rejection rate exceeding 99.99%; furthermore, the conductivity of the permeate distillate was consistently below 6.4 µS/cm for all of the EDTA-2Na feed concentrations. More importantly, the water flux slightly decreased from 8.27 to 7.04 L/m² h when the concentration of the EDTA-2Na feed increased from 0.1 to 0.5 M, corresponding to increased osmolality from 300 to 1411 mOsm/kg, indicating that water flux in DCMD is not significantly influenced by the osmotic pressure gradient across the membrane. This study demonstrated that MD could be an effective method for EDTA-2Na recovery in FO–MD systems and could economically utilize the wasted heat from industrial sources.