Process based control architecture for avoiding effluent pollutants quality limits violations in wastewater treatment plants

This paper presents a hierarchical control architecture for operation of biological wastewater treatment plants. The main goal of this control architecture is conceived being that of avoiding violations of effluent pollution limits while, at the same time, keeping reasonable values for the quality of the effluent as well as for the costs of operating the plant. Regarding the working scenario, the benchmark established within the BSM1 platform is adopted as working scenario to simulate a wastewater treatment plant. 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). The proposed architecture is a two layer cascaded control where the lower level deals with the goal of keeping the process variables to the desired set-points that are determined by the upper layer. The main contribution of the paper is to operate the upper layer under the idea of not keeping a fixed set-point but following simple rules dictated by the process reaction equations. While an Model Predictive Control with feedforward compensation is used for the lower level, an affine function is selected for the higher level. To avoid the total nitrogen in the effluent (N_tot,e) to go over the establish limits, an affine function, implemented with a sliding window, that determines a supplementary external carbon to be added in the first tank. Also effluent violations of NH, (NH_e), limits is tackled by means of a combination of a linear function and an exponential is proposed. As a result, N_tot,e violations and NH_e violations are avoided.