Two ion selective semipermeable membranes, such as are used in electrodialysis cell assemblies, are joined to one another at their edges so as to form a pouch with ion permeable walls. The membranes are provided with end flow ports into the pouch region. The pouch is wound into a spiral so as to resemble a spiral wound reverse osmosis module. A central hub electrode and a peripheral cylindrical electrode are provided. With suitable electrolyte streams the system operates as an electrodialysis cell assembly.