Evaluation of Different Nitrogen Control Strategies for a Combined Pre and Post-Denitrification Plant

In the paper different nitrogen control strategies are proposed and tested on a simulation model of a combined preand post-denitrification plant. The plant configuration corresponds to the DomžaleKamnik wastewater treatment plant that will be upgraded for nitrogen removal using MBBR (moving bed biofilm reactor) technology. The aim of the study is to find an optimal control strategy in terms of required effluent quality and operating (i.e. carbon and aeration) costs. The tested control strategies address aeration control, internal recirculation control, and external carbon dosage control, and are based on PI and feedforward control algorithms. Simulation results indicate that the nitrate PI controller that manipulates external carbon flow-rate and the ammonia PI controller that manipulates oxygen concentration in the aerobic reactors give the best performance with respect to the effluent quality and operating costs. In addition, it was shown that the control authority of the internal recirculation flow-rate was rather limited as the internal recycle flow-rate on the real plant can be increased only up to 200% of the average influent flow-rate. Hence, no improvement of effluent quality could be achieved with internal recycle control. While the improvement of effluent quality with the proposed overall control scheme is small if compared to the basic control scheme with optimal set-points, the energy savings are quite significant reaching up to 40%.