Abstract: This paper reports on a new approach in modelling and controlling an alternating aerobic-anoxic wastewater treatment plant. The plant is controlled using a model predictive approach adapted for the specific problem. The model embedded in the control structure is a linear time-variant approximation to ASM No.1. The predictive structure only considers the zero frequency component of controlled variables for the optimal duty cycle value calculation. Simulations of the controller behaviour are presented considering no plant-model mismatch.