To provide a waste water treatment apparatus capable of accurately controlling the return flow rate of a nitration soln. and capable of keeping the return flow rate constant regardless of the fluctuations of the water level in a nitration tank.

In a waste water treatment apparatus equipped with a nitration soln. return means returning the nitration soln. in a nitration tank 2 to a denitrifying tank 1, the nitration soln. return means is equipped with a water wheel type storage pump 3 pumping up the treated water in the nitration tank 2, a water receiving tank receiving the treated water pumped up by the water wheel type storage pump 3, a nitration soln. return passage 4 and treated water sending-out passage 5 connected to the water receiving tank and a flow rate control means regulating the ratio of the flow rates from the water receiving tank to the nitration soln. return passage 4 and the treated water sending-out passage 5.