Control of the dissolved oxygen concentration (DO) in an activated sludge process is of considerable importance because of economy and process efficiency. A strategy for designing a nonlinear DO controller was developed. The basic idea was to take the nonlinear oxygen transfer function into account in the controller design. Simulations illustrated that a nonlinear DO controller outperforms a standard PI controller. This was also confirmed in a pilot-scale plant experiment. A supervision (set-point) controller for the DO is also outlined. Initial simulations showed promising results where the aeration system is used more efficiently and the effluent ammonia concentration could be maintained low.

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