A single column contains a moving ion-exchange bed in which the treatments of regeneration, washing and exhaustion are effected simultaneously in three different vertical sections of the column. The successive batches of exchange resins are fed into the bottom of the column so as to bring about the intermittent progression of the bed, and each successive batch of resins is recovered at the top of the column. The rates of flow of the purified liquid, the regenerant flow and the discharged effluent are controlled so that the raw liquid inflow rate automatically adjusts itself to assure that an adequate flow distribution will be maintained in the different sections of the column.