A hierachical Plant wide operation in wastewater treatment plants: overall efficiency index control and event-based reference management

In this paper, a hierarchical control strategy is proposed to improve the WWTP performance from a plantwide perspective. An integral performance index (N/E index) is defined to measure the removal efficiency as the ratio between the nitrogen eliminated in the biological rectors (KgrN) and the energy (KWh) required to remove that amount of nitro genated compounds. That index is controlled by an upper level PI controller that changes the dissolved oxygen (DO) set point of the lower level PI control loop. When the DO concentration is driven to excessively low limits, the N/E index set point can be unreachable. Under this situation, the nitrogen elimination can be negatively affected. Therefore, it is not desirable to keep the plant in this situation for a long time. Therefore, an event-based controller is implemented to adapt the N/E index set-point to realistic values preventing long operation periods with the lowest DO concentration.