In the field of wastewater or effluent treatment, a method for treating wastewater includes a first step of treating carbon contained in the wastewater by means of oxygen, a second step of treating the nitrogen contained in the treated water at the end of the first step, a third step of determining a ratio between a concentration of carbon and a concentration of nitrogen in the water at the end of the first step, a fourth step of controlling the supply of oxygen necessary for the first step as a function of a difference between the determination carried out in the third step and a strictly positive setpoint of concentration ratio. A device implementing the method is also provided.