Hierarchical nitrite control for greenhouse gas emissions reduction in wastewater treatment plants

Substantial amounts of greenhouse gases (GHG) have been demonstrated to be emitted in wastewater treatment plants (WWTP). One of the GHG with a great influence is nitrous oxide (N
₂
O), which is emitted during the nitrification and denitrification processes in a biological wastewater treatment. This paper proposes the implementation of a control strategy in order to reduce N
₂
O emissions in the nitrification process. Due to the fact that N
₂
O emissions are produced as an intermediate in the nitrification process, the idea of the present work is based on the implementation of a nitrite control by manipulating dissolved oxygen (So) in order to avoid partial nitrification and thus to reduce N
₂
O peaks. A hierarchical control strategy is proposed, where the higher level is composed of an affine function and the lower level of a Proportional-Integral (PI) controller. A modified version of Benchmark Simulation Model 2 (BSM2G) that includes GHG emissions is used for the evaluation. The simulation results show that the proposed control strategy achieve the reduction of GHG emissions by reducing N
₂
O. However, additional control strategies must also be implemented to take into account the other evaluation criteria of the plant.