Hypersaline Electrodialysis Desalination: Intrinsic Membrane and Module Performance Trade-offs

Abstract

This study assesses the potential of electrodialysis (ED), traditionally applied to demineralize brackish waters, for the emergent challenge of hypersaline desalination. The analysis reveals that the desalination performance of hypersaline ED is determined by two intrinsic membrane trade-offs─ion conductivity–charge selectivity and ion conductivity–water resistivity─and a process trade-off between energy consumption and concentrate volume reduction. The charge selectivity and ion–water selectivity of ion-exchange membranes (IEMs), which are both influenced by the structural property of water uptake, are principal factors affecting membrane-level performance, whereas the operating current density simultaneously impacts the module-level metrics of specific energy consumption and water recovery yield. With current commercial IEMs, the energy costs of ED can be competitive with prevailing thermally driven evaporative processes for the desalination of hypersaline streams < ≈100,000 ppm TDS (equivalent to ≈1.5 M NaCl). To enable energy-efficient ED for higher salinities, membranes capable of suppressing the detrimental effect of water permeation need to be developed. This can be attained by polymeric IEMs with low water per fixed charge site or through material innovation beyond the charged polymers of conventional IEMs.