Developing a reclamation strategy for softening nanofiltration brine: A scaling-free conversion approach via...

A significant amount of concentrated, scaling-prone brine can be generated during the conversion of unconventional water resources to freshwater, thus necessitating the zero discharge of concentrated brine to meet environmental and resource requirements. In this study, a two-stage feed-and-bleed electrodialysis metathesis (FB-EDM) process was implemented to reclaim softening nanofiltration (SNF) brine. To determine the optimized process parameters, experiments were conducted with various initial diluate to concentrate volume ratios (V_D:V_C), applied voltages, replenishment flow rates (Q_rp), and initial diluate compartment concentration ratios (C_D1:C_D2). The results indicated that these parameters (except for the initial volume ratio) significantly influenced the FB-EDM process. The optimized conditions included a V_D:V_C of 2:1, voltage of 1.5 V per repeating unit, Q_rp of 4 L/h, and C_D1:C_D2 of 1.5:1. The two-stage FB-EDM process operating under the optimized conditions achieved an energy consumption of <0.9 kWh/kg salt, and the total dissolved solids (TDS) in terms of Cl-type and Na-type salts reached 199.1 and 224.4 g/L, respectively; the corresponding overflow rates were 1.17 and 1.14 L/h, respectively. The developed system thus demonstrated approximately 85% TDS removal and ionic conversion of the brine; additionally, the self-crystallization of CaSO₄·2H₂O was realized by blending the Cl-type and Na-type salts. This process therefore represents a suitable method for converting SNF brine into highly-concentrated liquid salts, and provides a reclamation strategy for miscellaneous salts.