Nitrous oxide reduction in wastewater treatment plants by the regulation of the internal recirculation flow rate with a fuzzy controller

The reduction of greenhouse gas emissions due to anthropogenic causes is one of the world's main challenges to face climate change. Wastewater treatment plants are necessary to improve the quality of wastewater before it is discharged into the receiving environment, but they have the disadvantage of generating nitrous oxide emissions during the biological treatment, which is a potent greenhouse gas. Avoiding partial nitrification by increasing dissolved oxygen is one of the ways to reduce these emissions. However, this article proposes to face a reduction of nitrous oxide emissions by reducing oxygen to minimum levels causing heterotrophic microorganisms to reduce nitrous oxide to dinitrogen. To achieve this objective, the present work proposes a regulation of the internal recirculation flow rate of the biological treatment by means of a fuzzy controller. This regulation is added to a usual control strategy in wastewater treatment plants, which achieves satisfactory results with respect to water quality and operational costs but that generates high nitrous oxide emissions. The Benchmark Simulation Model no. 2 Gas is used as working scenario, which includes the two main nitrous oxide emission pathways: heterotrophic denitrification and ammonia oxidizing bacteria denitrification. The proposed internal recirculation manipulation is shown to achieve nitrous oxide reductions of 26.70 and 30.83 % in different time periods with a slight effluent quality improvement and an operational cost reduction.