The present study aims at understanding and optimizing the efficiency of denitrification in biological nutrient removal activated sludge systems treating municipal wastewater. The nitrogen removal performance recorded at 22 intermittently aerated BNR WWTPs was compared to results from ASM1 simulations. The reduction of the heterotrophic yield in anoxic conditions (YH,anox) to a value of 0.54 gCODproduced/gCODremoved was confirmed, while simulations with the default ASM1 value of 0.67 gCODproduced/gCODremoved for any electron acceptor over-predicted effluent nitrate. Simulations with the modified ASM1 (dual YH) was then used to determine the daily aerobic time limits to meet a nitrogen discharge objective of 10 gN.m-3 to support operational optimization. Finally, the relevance of pre-denitrification in front of an intermittently aerated tank was studied to support design decisions. The data recorded from full-scale investigations at WWTPs operated at F/M ratio lower than the French conventional design value of 0.10 kgBOD5.(kgMLVSS.d)-1 indicate a contribution of the anoxic zone to nitrogen removal for over-aerated plants (aerobic time over 18 h.d-1). For higher F/M ratios, the simulations suggest that the anoxic zone has a higher contribution to nitrogen removal whatever the aerobic time.