In a process for treating an effluent from a flue-gas desulfurization system wherein flue gas containing sulfur oxides and chlorine values is contacted with limestone or a hydrated lime slurry, the sulfur oxide gas is absorbed and fixed in the slurry as gypsum and the gypsum is separated leaving a liquid containing chloride ions, the improvement which comprises filtering the liquid containing the chloride ions from gypsum/coal fly ash containing therein by filtration, subjecting the filtrate to electrodialysis in an electrodialyzer comprising concentration compartments and dilution compartments formed by cation-exchange membranes and univalent anion-exchange membrane to produce a concentrated chloride ion containing solution in the dilution compartments, wherein at least two of either of said cation-exchange membranes or anion-exchange membrane are positioned side-by-side and directly parallel to each other, subjecting the concentrated chloride ion containing solution to further concentration by evaporation in an evaporator, kneading the thus further concentrated solution with cement or a mixture of cement and coal ash to form a solid, thereafter disposing of the solid, and recycling the dilute chloride ion containing solution as make up water for the slurry.