Process for treating effluents containing nitrogen in the form of ammonium, using chemical oxidation and reduction in a sequenced batch reactor, the method comprising a sequence of four steps which is carried out cyclically in this order: - a combined feeding and emptying stage, comprising a feeding stage, during which a volume of effluent to be treated is introduced into the reactor, and an emptying stage, during which a clarified fraction of the reactor contents is discharged, said emptying and feeding stages taking place simultaneously, - at least one first aeration stage, during which oxygen or air is injected into the reactor for partial oxidation of the ammonium to nitrite, - a second stage, in anoxia, during which, by interrupting the injection of oxygen or air, gaseous nitrogen is produced by reacting the nitrites produced and the remaining ammonium, - a settling stage, during which sludge settles to the bottom of the reactor and the contents of the reactor are clarified near its surface, in which, during the feed stage, the volume of effluent to be treated is introduced near the bottom of the reactor, in which, during the emptying step, the clarified fraction of the reactor contents is evacuated to a predefined evacuation level close to the surface of the reactor contents, in which the level of the contents of the reactor is kept substantially constant during the emptying and feeding stages by regulating the emptying flow rate and/or the feeding flow rate by detecting the surface of the contents of the reactor using a surface sensor, in which the feeding and emptying steps are stopped when the sludge bed expands above a reference level in the reactor, the reference level being chosen at a predetermined distance below a discharge level from which the contents are taken during the emptying step. - a combined feeding and emptying stage, comprising a feeding stage, during which a volume of effluent to be treated is introduced into the reactor, and an emptying stage, during which a clarified fraction of the reactor contents is discharged, said emptying and feeding stages taking place simultaneously, - at least one first aeration stage, during which oxygen or air is injected into the reactor for partial oxidation of the ammonium to nitrite, - a second stage, in anoxia, during which, by interrupting the injection of oxygen or air, gaseous nitrogen is produced by reacting the nitrites produced and the remaining ammonium, - a settling stage, during which sludge settles to the bottom of the reactor and the contents of the reactor are clarified near its surface, in which, during the feed stage, the volume of effluent to be treated is introduced near the bottom of the reactor, in which, during the emptying step, the clarified fraction of the reactor contents is evacuated to a predefined evacuation level close to the surface of the reactor contents, in which the level of the contents of the reactor is kept substantially constant during the emptying and feeding stages by regulating the emptying flow rate and/or the feeding flow rate by detecting the surface of the contents of the reactor using a surface sensor, in which the feeding and emptying steps are stopped when the sludge bed expands above a reference level in the reactor, the reference level being chosen at a predetermined distance below a discharge level from which the contents are taken during the emptying step.