Interaction Analysis and Control of Bioreactors for Nitrogen Removal

Efficient control of wastewater treatment processes are of great importance. The requirements on the treated water (effluent standards) have to be met at a feasible cost. This motivates the use of advanced control strategies. In this thesis the activated sludge process, commonly found in the biological wastewater treatment step for nitrogen removal, was considered. Multivariable interactions present in this process were analysed. Furthermore, control strategies were suggested and tested in simulation studies. The relative gain array (RGA), Gramian based interaction measures and an interaction measure based on theH2 norm were considered and compared. Properties of the H2 norm based measure were derived. It was found that the Gramian based measures, and particularly the H2 norm based measure, in most of the considered cases were able to properly indicate the interactions. The information was used in the design of multivariable controllers. These were found to be less sensitive to disturbances compared to controllers designed on the basis of information from the RGA. The conditions for cost-efficient operation of the activated sludge process were investigated. Different fee functions for the effluent discharges were considered. It was found that the economic difference between operation in optimal and non-optimal set points may be significant even though the treatment performance was the same. This was illustrated graphically in operational maps. Strategies for efficient control were also discussed. Finally, the importance of proper aeration in the activated sludge process was illustrated. Strategies for control of a variable aeration volume were compared. These performed overall well in terms of treatment efficiency, disturbance rejection and process economy.