Design of spiral-wound electrodialysis modules

Spiral-wound electrodialysis (ED) modules are of interest because, in a parallel flow configuration where both the diluate and concentrate streams flow from the inner electrode to the outer electrode along a spiral path, the applied current density decreases as the concentration in the diluate stream and associated limiting current density (LCD) decrease. By matching the applied current density as closely as possible to the LCD at any given location in a stack, the required amount of membrane area is minimized, reducing capital cost. This work presents an analytical model for a spiral-wound ED module and experimental validation of that model using a prototype stack with two cell pairs and four revolutions. A constant voltage was applied and the total current and stream conductivities at mid-stack and the output were recorded. Experimental results agreed with the model for all parameters to within 15%. The model was used to explore the most cost-effective spiral stack designs for desalting brackish groundwater, examining both a standard Archimedean spiral (as is common for spiral-wound RO modules), and a novel ideal spiral. The ideal spiral shape was found to reduce total cost by 21% and capital cost by 39% with respect to an Archimedean spiral.