PURPOSE: To permit efficient sepn. of coloring components in biologically treated water by housing a pair of electrodes having an intermittent polarity change mechanism to change anode and cathode to cathode and anode in an electrolytic cell and introducing a feed water supply conduit and pressurize air supply conduit into the electrolytic cell.

CONSTITUTION: A pair of the electrodes 4a having the intermittent polarity change mechanism to change the anode and cathode to the cathode and anode are housed into the electrolytic cell 4. The feed water supply conduit 1 and pressurized air supply conduit 2 are introduced into the cell 4. More specifically, the electrolytic treatment and pressure flotation treatment are combined, by which the coloring components incorporated into the biologically treated water are efficiently separated and the scum sticking to the cathode surface is automatically striped. The deterioration of the electrolytic sepn. effect is thereby prevented and further the scum is forcibly floated, by which the effect advantageous for the through removal of scum is obtd.