Fractionation of monovalent ions from seawater brine via softening nanofiltration and selective electrodialysis: Which is better?

Abstract

In this study, a direct and systematic comparison of softening nanofiltration (SNF) and selective electrodialysis (SED) processes was made for the first time. First, the effects of key operating parameters of the two processes were investigated regarding monovalent-ion separation performance. The optimal operating pressure and feed flow rate for SNF were 2.4 MPa and 1000 L/h, while the optimal cell-pair voltage and replenishment flow rate for SED were 0.8 V and 6 L/h, respectively. Subsequently, two-stage SNF and two-stage SED were conducted based on their optimization. The concentration and purity of NaCl in two-stage SNF permeate were 32.03 g/L and 93.38%, while those in two-stage SED mixed concentrate were 183.04 g/L and 95.20%, respectively. Finally, a valid and fair metric was established to make a thorough comparison of the two membrane processes for the seawater brine reclamation. The results indicated that the SNF process was more suitable for only NaCl fractionation from seawater brine due to its relatively low specific energy consumption of NaCl in the purified brine and its high monovalent-ion separation capacity. However, SED should be recommended preferentially if the NaCl concentration effect is considered simultaneously. This investigation could provide insight and guidance for applications of each technology.