Model predictive control and fuzzy control in a hierarchical structure for wastewater treatment plants

This paper presents a hierarchical two-level control structure for a biological wastewater treatment plant (WWTP), with the goal of simultaneously improving the effluent quality and reducing operational costs. The Benchmark Simulation Model No.1 is used as working scenario. The idea is to adjust the dissolved oxygen in the fifth tank (DO5) according with the present working conditions, instead of keeping it in a fixed value. Thus is concreted by a hierarchical structure. MPC with inlet flow rate feedforward control (MPC + FF) is proposed for the lower level to control nitrate nitrogen concentration of the second tank (NO2) and DO5. The high level is the responsible for adjusting the DO setpoint in the fifth tank (DO5 setpoint) of the controller of the low level, according with the ammonium and ammonia nitrogen concentration in the fifth tank (NH5). The controller for the high level is based on a Fuzzy control. A tuning region has been determined for the high level controller, in which the results show a simultaneously improvement of the effluent quality and operational cost.