Biological nitrogen removal in an activated sludge process is obtained by two biological processes: nitrification and denitrification. Denitrifying bacteria need sufficient amounts of readily metabolized carbon. The objective of this paper is to develop an automatic control strategy for adjusting the external carbon flow rate so that the nitrate concentration in the last anoxic compartment is kept at a low pre-specified level. A simple model based feedforward control combined with a standard feedback PI controller is suggested. Simulation results show that the suggested controller, despite being simple, effectively attenuates process disturbances.

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