This paper presents the comparison of four control strategies for the A 2 /O WWTP configuration for simultaneous C, N and P removal. The control strategies: i) external COD-P control; ii) external recycle flow-P control; iii) nitrate control in the last anoxic reactor; iv) ammonia control in the last aerobic reactor, were combined with other common control loops to build different control structures and were simulated in Matlab/Simulink under different influent conditions. A systematic approach was conducted with all the strategies to assess their potential effectiveness, according to the following steps: theoretical design, setpoint optimization and, finally, a detailed comparison of the control results against a reference operation and an optimized reference scenario. The optimization of the reference operation presented a 7 % reduction of the total operational cost. The simulation results showed that some control strategies further reduced 3-7.5% the WWTP operational costs while the effluent quality is greatly improved.
The first part of this work presents the development and implementation in the Benchmark Simulation Model No 1 (BSM1) of a modified Activated Sludge Model No 3 (ASM3). The enhanced ASM3 presented in this study has three modifications compared to the original ASM3. The first modification is the representation of the simultaneous heterotrophic biomass growth on the primary substrate and on the internal storage products. Second, nitrification is modeled as a two-step process with nitrite as an intermediate product, bringing an increased degree of complexity to the mathematical model. Third, the denitrification process is modeled as a three step process with nitrite and nitric oxide as intermediates. The nitric oxide is introduced in the model to account for the inhibition of some enzymes that are responsible for the growth of the heterotrophic bacteria under aerobic conditions. For a better representation of the real plant behavior, the secondary settler is modeled to be reactive. The built reactive settler model is the combination of the settler model described by Takács in 1991 and the enhanced ASM3. The second part of this research consists of the investigation of five control strategies applied to the waste water treatment plant (WWTP). The control architectures studied in this research are multi-input-multi-output (MIMO) Model Predictive Controllers (MPC). The assessment of these strategies is made from three points of view: control performance, cost evaluation and quality of the effluent. The simulation results show that operational costs can be reduced using automatic control.
The present paper presents an enhanced Activated Sludge Model No. 1 in which nitrification and denitrification are modelled as two-step processes. The nitrification process is considered as a sequence of two steps carried out by two distinct genera of bacteria, with nitrite as an intermediate product. For the denitrification process a parallel approach was used that considers nitrate and nitrite to be directly reduced to molecular nitrogen. The new model is compared to the original Activated Sludge Model No. 1 using the Benchmark Simulation Model No. 1. In order to make the simulation model more realistic the secondary settler was considered to be reactive. The new modelling approach showed different dynamics for the autotrophic biomass and the growth substrate of the heterotrophic biomass, enhancing the agreement with the real process behaviour. The last part of the paper presents the investigation of finding the optimal set-point for a cascade control scheme based on the Model Predictive Controller at the outer control level and three PI controllers at the inner level. The control architecture is designed to keep the nitrate nitrogen level at the end of the aerated zone at predefined set-points. The wastewater treatment plant performance is evaluated using the operational costs criteria. The simulation results show that noteworthy costs can be saved using this control strategy.
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