PURPOSE: To deposit electrolyte salts efficiently from a soln. of electrolyte salts by maintaining the degree of supersaturation necessary for depositing the electrolyte salts from said soln. continuously with an electrodialysis device.

CONSTITUTION: The waste washing water 1' which is passed through a filter A so as to be removed of suspended solids is mixed with the desalted water 5 from an electrodialysis device D and the mixture is fed to a reverse osmosis device B. The nonpermeated liquid 4 wherein electrolyte salts are concentrated to a certain extent is fed into an intermediate vessel C. The liquid 4 in the vessel C is desalted in the device D and the treated water 5 is circulated to the inlet of the device B. On the other hand, the concentrated soln. 6 of the device D is circulated through a concentrate vessel E to the device D. The concentrated soln. 7 in the vessel E is fed to a crystallization vessel F, where the soln. 7 is cooled indirectly with the cooling water 8 produced in a refrigerator I to supersaturate the dissolved salts and the salts corresponding to the supersaturation in the soln. 7 are deposited by crystal growth.