The objective of this paper is to present the simulation results of implementing and intelligent environmental decision support system (IEDSS) to overcome the limitations of the manual control at the wastewater treatment plant (WWTP) of Vall del Ges (North- East, Spain). The proposed IEDSS integrates online information with knowledge management techniques and aims to reduce the pollution load leaving the plant, at the same time that keeps the operational costs at minimum. To this end, a fuzzy logic engine (FLE) controls the effluent nitrate and ammonium concentration manipulating both aeration and internal recycle flow respectively. The results show significant savings in terms of operational costs and a slightly enhancement of the nitrification/denitrification (N/D) efficiency improving the overall nitrogen removal.