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) and the activated sludge model no. 1 (ASM1). The objective is to search for a control 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. Here, a new steady-state secondary settler mathematical model is developed based on the theory of partial differential equations applied to the conservation law with discontinuous fluxes. The resulting active constraints must be chosen as economic controlled variables. These are the effluent ammonia from the bioreaction section and the final effluent total suspended solids at their respective upper limits, in addition to 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 on the optimal static performance of the plant. It is expected that the proposed methodology and results obtained therein can be used in practice as general rules-of-thumb to be tested in actual wastewater treatment plants of the kind discussed in this paper.

This article describes the systematic design of a control structure for a biological wastewater treatment process as given by the test-bed Benchmark Simulation Model No. 1 (BSM1) and Activated Sludge Model No. 1 (ASM1). The objective of this work was to formalize and implement a systematic and yet simple procedure for the selection of control structures in wastewater treatment plants (WWTPs) and to show that the application of the proposed methodology agrees with the "empirical"' findings regarding the operation of this process. The motivation underlying this endeavor was to search for a control configuration that leads to optimal economic operation while promptly rejecting disturbances at lower layers in the control hierarchy, thus avoiding violation of the more important regulatory constraints on effluent discharge. We started by optimizing a steady-state nonlinear model of the process for various important disturbances. The results confirmed that it is economically optimal to control the oxygen concentration in the aerobic basins and the nitrate in the second anoxic tank at their respective lower bounds, whereas the effluent ammonia from the bioreactors should be controlled at its upper limit. In addition, because it is good practice to operate with minimal manipulation, the wastage flow rate should be fixed at its nominal optimal set point. The proposed decentralized control configuration, consisting of simple PI controllers, is capable of maintaining the process well within the regulatory limits at a small cost when dynamic disturbances represented by three weather files affect the process, therefore suggesting that, according to the applied systematic methodology, more complex (multivariable) regulators are not necessary for the ASM1 process.

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