Developing a reclamation strategy for softening nanofiltration brine: A scaling-free conversion approach via continuous two-stage electrodialysis metathesis.

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 (VD:VC), applied voltages, replenishment flow rates (Qrp), and initial diluate compartment concentration ratios (CD1:CD2). The results indicated that these parameters (except for the initial volume ratio) significantly influenced the FB-EDM process. The optimized conditions included a VD:VC of 2:1, voltage of 1.5 V per repeating unit, Qrp of 4 L/h, and CD1:CD2 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.

Sustainable disposal of seawater brine by novel hybrid electrodialysis system: Fine utilization of mixed salts.

Sustainable seawater brine treatment demands an essential paradigm shift for effective recovery of resources and high value utilization of mixed-salts. Here, a novel hybrid electrodialysis (ED) system was proposed that integrated an innovative hybrid selective ED (HSED) and a developed selective bipolar membrane ED (SBMED). The HSED process allowed simultaneous recovery of major divalent cations and anions from seawater brine when NaCl was selectively enriched. Then, the impure NaCl-rich stream was fed directly into the SBMED process for acid/base preparation without any purification pretreatment. Detailed analysis of the HSED process showed that increasing unit voltage from 2.33 V to 2.67 V would improve the removal ratio of Ca2+, Mg2+ and SO42- from 54.7%, 41.4% and 13.3% to 78.9%, 76.6% and 32.1%, respectively. In addition, the increment of initial concentration of product streams promoted the transport of various ions from the feed and middle compartments. The fine utilization performance, in terms of ionic removal ratio and fractionation ratio of divalent ions in the HSED process, was more limited by the initial concentration of product streams. Furthermore, the SBMED stack was found to have nearly identical performance over five cycles, indicating that the presence of a trace amount of hardness cations did not induce scaling. The current study thus provided a novel suitable strategy with a perspective of fine utilization for practical applications in sustainable disposal of seawater brine.

Novel energy-efficient electrodialysis system for continuous brackish water desalination: Innovative stack configurations and optimal inflow modes.

Electrodialysis (ED) is a well-established brackish water (BW) desalination technology that has been commercially applied for decades. However, the energy efficiency of BWED cannot approach optimization because of the low salt concentration of BW. In this study, a novel two hydraulic-stage ED desalination system was presented to enhance mass transfer and reduce energy consumption. In terms of energy-efficient strategies, it involved not only innovative membrane stack configurations (resin-filled electrode cells and asymmetric cell pairs design) but also optimizing inflow modes (electrolytes parallel flow and dilute/concentrate counter flow). Results showed that thin resin-filled (1 mm) electrode cells, asymmetric cell pair design (cell pairs ratio of 1st and 2nd-hydraulic stages, 1.2), and optimizations of general inflow mode were beneficial for savings 10-30% of energy consumption at the same salt removal ratio (SR). The synergistic effects of these strategies indicated that this novel ED system could save ∼40% of the energy consumption at the same SR, compared with conventional two hydraulic-stage ED system (CED). Three stage continuous BWED performance tests, compared with a CED, showed that a 36.9% total energy saving could be achieved using the novel ED system when the BW concentration decreased from 3500 mg/L to the quality requirement of drinking water (∼450 mg/L). It was therefore possible to open the way for saving energy in BWED systems.