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.