High-recovery and chemical-free desalination of sodium chloride-containing waters with modified electrodialysis metathesis

Summary

Reverse osmosis (RO) is a dominant desalination technology, but it faces scaling challenges, particularly with 2: 2 salts like calcium sulfate (CaSO₄). This study introduces a modified electrodialysis metathesis (mEDM) process, incorporating monovalent-selective ion exchange membranes (mIEMs), to prevent scaling and increase water recovery in brackish water desalination without the need for chemical additives. The mEDM process effectively separates divalent cations and anions, preventing oversaturation that leads to scaling. This process relies on feedwater containing sufficient sodium chloride, as NaCl plays a critical role in the ion-recombination mechanism of mEDM. Laboratory experiments conducted using NaCl-MgSO₄ solutions (4:1 molar ratio, TDS 3.5 g/L) and brackish groundwater (TDS 3 g/L) demonstrated three key outcomes: (i) water recoveries up to 95 %, (ii) scaling prevention at levels outperforming both RO and traditional electrodialysis (ED), and (iii) specific energy consumption (SEC) ranging from 1.1 to 1.7 kWh/m³. Importantly, the study revealed that mEDM outperformed other technologies in scaling prevention, particularly in managing calcium sulfate scaling. At 90 % gross water recovery (GWR), the mEDM system remained below critical scaling thresholds, showing superior performance over RO and ED, which experienced scaling risks at 55 % and 72 % net water recovery (NWR), respectively. The mEDM system's ability to tolerate higher water recovery rates before scaling occurred highlights its potential for high-recovery desalination applications. Additionally, mEDM can be integrated with RO systems, which typically operate at 80 % water recovery, allowing for a safe extension to 92 % by effectively treating the RO retentate and mitigating scaling risks. These findings position mEDM as a scalable, chemical-free solution for efficient desalination, suitable for brackish groundwater treatment and zero-liquid discharge applications. Future studies should investigate mEDM's performance under more complex water matrices and evaluate its economic and environmental viability.