This paper presents different control strategies for biological wastewater treatment plants, with the goal of avoiding violations of effluent pollution limits while, at the same time, improving effluent quality and decreasing operational costs. The control strategies are based on Model Predictive Control (MPC) and functions that relate the input and manipulated variables. The Benchmark Simulation Model No.1 (BSM1) is used for evaluation. A hierarchical structure regulates the dissolved oxygen (DO) of the three aerated tanks based on the ammonium and ammonia nitrogen concentration (NH) in the fifth tank (NH5). An MPC with feedforward compensation is proposed for the lower level and an affine function is selected for the higher level. A tuning region is determined modifying the tuning parameters of the higher level, in which the effluent quality and operational costs are simultaneously improved in comparison with the default control strategy of BSM1. To eliminate violations of total nitrogen in the effluent (N-tot,N-e), an affine function, implemented with a sliding window, adds external carbon flow rate in the first tank based on nitrate nitrogen in the fifth tank (NO5) plus NH5. To avoid violations of NH in the effluent (NHe), a combination of a linear function and an exponential function that manipulates the internal recirculation flow rate based on NH5 and NH in the influent is proposed. As a result, N-tot,N-e violations and NHe violations are avoided for dry, rain and storm weather conditions. In addition, an improvement of effluent quality and a reduction of operational costs are achieved at the same time, except in the cases of rain and storm weathers for NHe violations removal, in which the costs increase. (C) 2015 Elsevier B.V. All rights reserved.