Improving the performance of a WWTP control system by model-based setpoint optimisation

The aim of this work was the improvement of a WWTP control system using a model-based setpoint optimisation. For this purpose, an anaerobic/anoxic/aerobic (A2/O) pilot WWTP was simulated using the IWA ASM2d model under different influent conditions. Several control strategies for an efficient biological C/N/P removal were evaluated in this WWTP: i) open loop; ii) dissolved oxygen control in the aerated reactors; iii) maximum performance of nutrient removal; iv) optimised fixed setpoints for the controlled variables; v) daily optimised setpoints; vi) two different sets of optimised setpoints for weekdays and weekends and vii) hourly optimised setpoints. A single cost function based on the operating costs by converting the effluent quality into monetary units was chosen for evaluating the plant performance (i.e. the control loops setpoints were optimised to obtain low effluent N and P discharges with the minimum costs). Setpoint optimisation was shown as a good tool to improve the performance of the system. In this case study, control strategy (vi) was selected as the best choice considering the trade-off cost-benefit. The optimised control system resulted in around a 45% decrease of operational costs with respect to the open loop scenario, a significant improvement of the effluent quality and a drastic decrease of the time above discharge limits.