In this paper, possible choices of optimal set-points and cost minimizing control strategies for the denitrification process in an activated sludge process are discussed. In order to compare different criterion functions, simulations utilizing the European COST benchmark are considered. By means of operational maps the results are visualized. It is found that there is a clear set-point area where the process can be said to be efficiently controlled in an economic sense. For most reasonable operating points this optimal area corresponds to a nitrate concentration in the anoxic compartment in the interval 1-3 mg(N)/l. Furthermore, the location of this optimum does not seem to be very sensitive to changes in the ASM1 parameters. With an appropriate nitrate cost function, the legislatory authorities can place this economic optimum in an area where also the effluent regulations are met. It is also discussed how this efficient control can be accomplished.
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