Abstract The cost of membrane replacement is a considerable part of expenses at brackish water desalination by electrodialysis. That's why to lessen the membrane surface it is desirable to carry out the process of electrodialysis desalination with high current density. But if one uses high current density a danger arises for membrane hard polarization. To prevent the membrane polarization one should increase the velocity of saline water flow in the compartments of electrodialysis unit which require some additional electric energy. To decrease the critical velocity of the polarization the authors studied the hydraulic peculiarities of the water passage through the compartments of the electrodialysis unit. The zig-zag type compartments and the compartments with spacers were studied. Corrugated and perforated PVC sheets, expanded PVC sheets and several types of the nylon network were used as spacer materials. Dependence of hydraulic resistance coefficient of zig-zag type compartments and compartments with various types of turbulising spacer on Reynolds number was established as well as dependence of critical current density on saline water velocity and its salt content. Taking these dependences into account optimal conditions of brackish water desalination in various types of electrodialysis unit construction were found.