Dynamic modelling of N₂ O emissions from a full-scale granular sludge partial nitritation-anammox reactor

Partial nitration-anammox is a resource-efficient pathway for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology may be counter-acted by the emission of N₂ O, a potent greenhouse gas. In this study, mathematical modelling was applied to analyse N₂ O formation and emission dynamics and to develop N₂ O mitigation strategies for a one-stage partial nitritation-anammox granular sludge reactor. Dynamic model calibration for such a full-scale reactor was performed, applying a one-dimensional biofilm model and including several N₂ O formation pathways. Simultaneous calibration of liquid phase concentrations and N₂ O emissions leads to improved model fit compared to their consecutive calibration. The model could quantitatively predict the average N₂ O emissions and qualitatively characterize the N₂ O dynamics, adjusting only seven parameter values. The model was validated with N₂ O data from an independent data set at different aeration conditions. Nitrifier nitrification was identified as the dominating N₂ O formation pathway. Off-gas recirculation as a potential N₂ O emission reduction strategy was tested by simulation and showed indeed some improvement, be it at the cost of higher aeration energy consumption.