In this paper three different control strategies for energy saving were applied to modified Ludzak-Ettinger process. In first strategy the internal recycling flow rate was maintained constant and a fuzzy logic controller for aeration system was used that manipulates the air flow rate
of blowers in order to maintain the dissolved oxygen concentration in the aerobic zone at a desired set point (2 mg/l). In the second strategy, a fuzzy logic controller for internal recycling was used. This controller maintains nitrate concentration in the anoxic zone at a desired set
point (1 mg N-NO3/l). Finally, in the third strategy, a supervisory control layer was applied to the fuzzy logic controllers. In the supervisory control layer, the optimum set points for DO concentration (aerobic zone) and nitrate concentration (anoxic zone) are determined. Through a simulation
study it was shown that important energy consumption saving for internal recycling is obtained when the control strategies are applied. Moreover, the application of different control strategies improved the process performance. In general, the best results were obtained using the third strategy.
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