The waste aq. solns. of Na2SO4, were processed by electro-electrodialysis under constant current d. of 7.5 A/dm(2). The SO42- ions concns. in chambers of the apparatus were detd. by conductometry on the samples. The decreasing of the SO42-. concn. in the dialyzate down to below 20 g/dm(3) resulted in an adverse large increase of the cell voltage and unit power consumption. A nanofiltration of dialyzate and its recycling was redommended.