This paper proposes a new cost minimisation control scheme for a predenitrification type of biological wastewater treatment process and evaluates the validity of the control scheme based on the benchmark process presented by Copp. The control scheme adopting a hierarchical control structure incorporates lower-level controllers that consist of a set of local dynamic controllers and a higher-level static optimiser that provides the set points of the lower-level controllers based on the total cost index of Vanrolleghem and Gillot. Prior to benchmarking, this paper derives a simplified process model used for the optimiser, which is able to approximate the benchmark process model effectively as well as is simplified sufficiently for faster set point optimisation for on-line purposes. Numerical experiments evaluate the effectiveness of the proposed control scheme from various viewpoints including process operational and optimisation viewpoints.
This paper presents a real-time optimal control scheme based on a total cost index (TCI) by using the so-called extremum seeking control (ESC). The ESC searches the minimum value of a given performance index where the TCI consisting of the operational cost and the effluent quality cost is used as the index. An improved ESC is proposed where a transformed TCI by monotonically increasing function is utilized to improve the convergence property of the ESC while keeping the argument of the optimal point invariant. The feasibility of the ESC is tested for two types of pseudo-anaerobic-oxic process control: one is the return sludge recycle ratio control, and the other is the aeration control for an alternate zone which can be anaerobic or aerobic depending on the influent condition. Simulation study illustrates that the proposed ESC is able to find a near optimal point successfully and the TCI can be reduced by about 2.7 to 3.8% compared with that of a typical operating condition.
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