A method for the biological denitrification of wastewater that comprises a nitrification-denitrification sequence, said nitrification-denitrification sequence allowing ammonia nitrogen to be removed by production of nitrates (nitrification) followed by production of gaseous nitrogen N 2 by denitrification (removal of nitrates), which consists of reduction of the nitrates to nitrites and then to nitrogen, said nitrification-denitrification sequence being followed, for a first fraction of the water, by a post-denitrification step during which an electron donor is injected into this first fraction, whereas a second fraction of the water passes through a bypass, and then is mixed with the first fraction downstream of the post-denitrification step, characterized in that: - the first fraction of the wastewater is submitted, during post-denitrification, to almost complete denitrification in order to exit the step with a concentration of nitrates [N-NO 3] below 4 mg/L, to minimize the production of nitrous oxide N 2 O, - and the bypass rate is determined from: - measurement of the concentration of nitrates [N-NO 3] in the water upstream of post-denitrification, - the desired concentration of nitrates [N-NO 3] for the mixture of the two fractions downstream of the post-denitrification treatment, - and the concentration of nitrates [N-NO 3] in the first fraction at outlet from post-denitrification, before mixing the two fractions. - and the injection of electron donor used is controlled: - on the one hand, by measuring (9, 27b) the concentration of nitrates [N-NO 3] in the water upstream of post-denitrification, and by measuring the flow rate of the wastewater, - and on the other hand, by estimating the nitrogen-containing burden ([N-NO 3] concentration) of the first fraction at outlet from post-denitrification, before mixing the two fractions, said estimate being calculated as a function of measurement of the concentration of nitrates [N-NO 3] at treatment outlet after mixing the two fractions, and of the bypass rate used, - the estimated or measured value of the electron donor flow rate is compared with a desired set value, and the flow rate of electron donor injected is altered to reduce the difference between estimated value and desired value. - the first fraction of the wastewater is submitted, during post-denitrification, to almost complete denitrification in order to exit the step with a concentration of nitrates [N-NO 3] below 4 mg/L, to minimize the production of nitrous oxide N 2 O, - and the bypass rate is determined from: - measurement of the concentration of nitrates [N-NO 3] in the water upstream of post-denitrification, - the desired concentration of nitrates [N-NO 3] for the mixture of the two fractions downstream of the post-denitrification treatment, - and the concentration of nitrates [N-NO 3] in the first fraction at outlet from post-denitrification, before mixing the two fractions. - measurement of the concentration of nitrates [N-NO 3] in the water upstream of post-denitrification, - the desired concentration of nitrates [N-NO 3] for the mixture of the two fractions downstream of the post-denitrification treatment, - and the concentration of nitrates [N-NO 3] in the first fraction at outlet from post-denitrification, before mixing the two fractions. - and the injection of electron donor used is controlled: - on the one hand, by measuring (9, 27b) the concentration of nitrates [N-NO 3] in the water upstream of post-denitrification, and by measuring the flow rate of the wastewater, - and on the other hand, by estimating the nitrogen-containing burden ([N-NO 3] concentration) of the first fraction at outlet from post-denitrification, before mixing the two fractions, said estimate being calculated as a function of measurement of the concentration of nitrates [N-NO 3] at treatment outlet after mixing the two fractions, and of the bypass rate used, - on the one hand, by measuring (9, 27b) the concentration of nitrates [N-NO 3] in the water upstream of post-denitrification, and by measuring the flow rate of the wastewater, - and on the other hand, by estimating the nitrogen-containing burden ([N-NO 3] concentration) of the first fraction at outlet from post-denitrification, before mixing the two fractions, said estimate being calculated as a function of measurement of the concentration of nitrates [N-NO 3] at treatment outlet after mixing the two fractions, and of the bypass rate used, - the estimated or measured value of the electron donor flow rate is compared with a desired set value, and the flow rate of electron donor injected is altered to reduce the difference between estimated value and desired value.