Comparison between model- and pH/ORP-based process control for an AAA system

Model-based and on-line sensor control schemes in an alternating aerobic/anoxic system for nitrogen removal were applied and compared for system performance. A linear model was developed by simplifying the well known Activated Sludge Model No. 1 and incorporated into an optimization scheme. A program was developed to detect control points on the pH and ORP profiles to control aeration duration in an alternating system. Both systems demonstrate good COD (85%) and nitrogen (76-80%) removal efficiencies, with a range of the aeration ratio (aeration cycle time/total cycle time) 0.23 to 0.3. Both systems could save significant aeration energy, in addition to handling the variation of influent nitrogen loading. The model-based optimization approach has some advantages over the pH and ORP sensor control scheme, since the effluent ammonia level can be set by an operator according to the effluent regulation. Conversely, the on-line sensor based control scheme can be easily applied into a real system, since it does not require calibration or measurements of influent compositions.