Sensitivity of optimal operation of an activated sludge process model

This paper describes a systematic sensitivity analysis of optimal operation conducted on an activated sludge process model based on the test-bed Benchmark Simulation Model No. 1 (BSM1). The objective is to search for an operational structure that leads to optimal economic operation, while promptly rejecting disturbances at lower layers in the control hierarchy avoiding thus violation of the more important regulation constraints on effluent discharge. We start by optimizing a steady-state nonlinear model of the process. The resulting active constraints must be chosen as economic controlled variables. These are the effluent ammonia from the bioreactors and the final effluent total suspended solids at their respective upper limits, as well as the internal recycle flow rate at its lower bound. The remaining degrees of freedom need to be fulfilled, and we use several local (linear) sensitivity methods to find a set of unconstrained controlled variables that minimizes the loss between actual and optimal operation; particularly we choose to control linear combinations of readily available measurements so to minimize the effect of disturbances and implementation errors.