Modelling nitrous oxide emissions from nitritation-denitritation IFAS-SBR treating sidestream wastewater

Nitritation-denitritation is an energy and resource-efficient method for nitrogen removal, particularly for ammonia-rich streams such as anaerobically digested wastewater. Integrated Fixed-Film Activated Sludge Sequencing Batch Reactors (IFAS-SBR) can further enhance the efficiency of the process. However, emissions of nitrous oxide (N₂O) - a potent greenhouse gas with global warming potential 300 times stronger than CO₂ – can significantly increase the carbon footprint of the process. Investigating the interactions between operational strategies, biomass types, and N₂O production pathways is thus critical. In this study, a dynamic IFAS-SBR model was used to analyze lab-scale N₂O emission data, and to evaluate the ability of current models to capture N₂O dynamics accurately. The mathematical model includes nitrifier nitrification, nitrifier denitrification, and heterotrophic denitrification pathways applied to both flocculent biomass and one-dimensional biofilm. Sensitivity and identifiability analyses guided a multi-step parameter estimation process to calibrate the model. The calibrated model successfully predicted N₂O dynamics, although it provided only a qualitative description of N₂O emissions when validated against data obtained under a quite different operational scheme. The model was then employed to evaluate how dissolved oxygen (DO) levels, temperature, and influent ammonia concentrations affect N₂O emissions and the efficiency of ammonia removal. This evaluation included scenario analyses where temperatures ranged from 5°C to 25°C, DO levels during aeration varied from 0.3 to 2 mg/L, and influent ammonia concentrations spanned from 700 to 1000 mg/L. Results highlight the critical role of temperature in determining the optimal dissolved oxygen setpoint to minimize N₂O emissions while maximizing ammonia removal efficiency.