Strategies for mitigating nitrous oxide production and decreasing the carbon footprint of a full-scale combined nitrogen and phosphorus removal activated sludge system.

Nitrous oxide (NO) emitted from biological nutrient removal activated sludge systems contributes significantly to the total carbon footprint of modern wastewater treatment plants. In the present study, NO production and emissions were experimentally determined in a large-scale plant (220,000 PE) employing combined nitrogen (N) and phosphorus (P) removal. As a modelling tool, the Activated Sludge Model 2d (ASM2d) was extended with modules describing multiple NO production pathways and NO liquid-gas transfers. The new model was calibrated and validated using the results of laboratory experiments and full-scale measurements. Different operational strategies were evaluated following the proposed model-based procedure. Heterotrophic denitrification was found to be the predominant pathway of NO production under both anoxic and aerobic conditions. This behaviour could primarily be attributed to the predominant abundance of heterotrophic denitrifiers over nitrifiers. Simulations revealed that the optimal solution for minimizing liquid NO production is to set the dissolved oxygen concentration in the aerobic zone from 1 to 2 mg O/L and to enhance the mixed liquor recirculation rate (MLR) (>500% of the influent flowrate) while not compromising effluent standards. Regarding the actual conditions, the potential reduction in the carbon footprint was estimated to be 10% by applying the proposed operational strategy. The results suggest that considerable improvements can be achieved without substantial upgrades and increased costs.