Energy savings by implementing a serpentine flow path in electrodialysis

Abstract

Electrodialysis (ED) can achieve a high product recovery and operate with low energy consumption for brackish water desalination. Therefore, it is receiving significant attention as a potential competitor to household RO systems. Commonly, the incoming feed water is split into multiple parts and fed between a series of alternating anion and cation exchange membranes in parallel flow, and a constant electric potential is applied at the end electrodes. As the diluate stream salinity decreases, its electrical resistance increases and correspondingly, the current density decreases along the flow direction. This study compares this conventional parallel flowpath against a serpentine flow arrangement which allows flow of both diluate and concentrate streams to adjacent channel multiple times before leaving the module. The serpentine arrangement results in nearly constant local electrical resistance since various salinity levels are arranged in series between the electrodes, and therefore a nearly uniform current density over the entire electrode area. Desalinating a feed stream at 3 g/kg salinity down to 0.5 g/kg with a serpentine flow path having two channels results in around 9% savings in stack energy. Since the serpentine design consumes higher pumping power, a net energy saving of 7.7% is predicted. This design can enable a high salt reduction in a single pass with a small footprint for household desalination applications. However, overall current density is constrained by the limiting current density at the diluate exit.