An optimization study was carried out for an electrodialysis system (ED) operating continuously in stages, based on material balances for the species in each single cell of a generic stage, applied to the separation of NaCl, KCl or Na2SO4 from water solutions. The effect of process variables affecting the investment and operating costs, expressed as membrane and electric energy costs, respectively, is considered. The results indicate that Na2SO4 removal is more expensive than NaCl or KCl due to the divalent ion effect. It is possible to find optimal solutions for minimizing total costs under different scenarios by changing design and operating variables such as cell thickness and recycle ratio.