peroxide added into the reaction vessel is appropriately selected as a function of the cod of the wastewater preferably the molar ratio of hydrogen peroxide to cod is within the range from 0 3 to 1 2 as previously indicated the mass transfer rate at the cathode 15 is increased by the fluidized particulate carrier 11 and a high proportion of iron iii to iron ii is sustained by continuous addition of hydrogen peroxide as a consequence the current efficiency of electrolytic reduction is steadily sustained at above 70 during the reaction when the reaction is completed alkali is added into the vessel 10 to adjust the ph value to 6 9 and polymers are added thereto to flocculate the iron hydroxide particles into flocs then the cod reduced waste water is removed as effluent from the reaction vessel to thereby complete the treatment according to this invention the electrolytic reduction can be performed at a current efficiency of above 70 as long as the concentration of iron iii is greater than 1 000 mg l furthermore this method is particularly suited for treating high cod concentrated wastewater where a cod removal of 85 98 or even higher can be achieved the method for treating wastewater according to this invention can be applied in various fields as follows 1 removal of copper or nickel ions in wastewater for example wastewater from electroplating printed circuit board manufacturing and metal finishing 2 treatment of the high cod concentrated wastewater for example wastewater from chemical engineering factories synthetic fiber factories and paper making factories or.