A method and an apparatus for wastewater treatment using a nitrogen/phosphorus removal process combined with a floatation separation of sludge that is replaced by the gravity settling secondary clarifier are provided for effectively removing organic substances, nitrogen, phosphorus, granular substances and the like from wastewater. Al or Fe electrode plate is used for generating the micro bubbles so that phosphorus is removed by Al or Fe ions dissolved from the anode. With the method and the apparatus, a concentration of the microorganisms in the bioreactor is maintained high so that the volume and area of the reactor are reduced, that removal efficiency for organic substances, nitrogen, phosphorus is increased, that deterioration of settlability due to sludge bulking or micro floc and deterioration of out-flowing water quality are prevented, that the time required for settling is reduced to increase the processing quantity of wastewater, and that removal efficiency for phosphorus is maximized by simultaneous performances of biological phosphorus removal and chemical phosphorus removal by chemical agglomeration in the solid-liquid floatation separation vessel.