In this paper the following new control objectives for biological wastewater treatment plants (WWTPs) have been established: to eliminate violations of total nitrogen in the effluent (N-tot,N-e,) or ammonium and ammonia nitrogen concentration (NH) in the effluent (NHe) and at the same time handle the customary requirements of improving effluent quality and reducing operational costs: The Benchmark Simulation Model No. 1 (BSM1) is used for evaluation, and the control is based on Model Predicitive Control (MPC) and fuzzy logic. To improve effluent quality and to reduce operational costs, a hierarchical control structure is implemented to regulate the dissolved oxygen (DO) on the three aerated tanks. The high level of this hierarchical structure is developed with a fuzzy controller that adapts the DO set points of the low level based on the NH concentration in the fifth tank (NHS). The low level is composed of three MPG Controllers with feedforward control (MPG + FF). For avoiding violations of N-tot,N-e, a second fuzzy controller is used to manipulate the external carbon flow rate in the first tank (q(EC1)) based on nitrate nitrogen in the fifth tank (NOS) plus NHS. For avoiding violations of NHe, a third fuzzy controller is applied to manipulate the internal recirculation flow rate (Q(rin)) based on NH5 and NH in the influent. Simulation results show the benefit of the proposed approach.