To provide an apparatus for treating waste water capable of enhancing throughput and a treatment speed even if a grease trap has low throughput, also having a small size, a simple structure and a low cost, without requiring an installation space, and easily maintained and managed; and a method for treating the waste water capable of preventing the accumulation of floating scum and settled sludge by a synergistic effect of treatment by the grease trap and the apparatus for treating the waste water.

The apparatus for treating the waste water 1 is configured to include an inflow pipe 4 on which a suction port of primary treatment water treated by the grease trap 50 is formed, an inflow separation tank 2 having an inflow port 3, an electrolytic treatment tank 7 communicating with the inflow separation tank 2, one to a plurality of floatation separation tanks 8 to 10 communicating with the electrolytic treatment tank 7, an outflow separation tank 11 communicating with the floatation separation tanks 8 to 10, a floating scum discharge port 18 formed roughly similar height to an outflow port 12 at any one side wall or more of tanks 7 to 10, and a floating scum recirculation pipe 19 for recirculating the floating scum to the grease trap 50.