Model-Based Evaluation of N₂O Production Pathways in the Anammox-Enriched Granular Sludge Cultivated in a Sequencing Batch Reactor

A mechanistic model was developed as an extension of the Activated Sludge Model No. 1 to describe three nitrous oxide (N₂O) production pathways in a laboratory-scale anammox-enriched granular sequencing batch reactor. Heterotrophic denitrification and two processes mediated by ammonia oxidizing bacteria (AOB), that is, ammonia (NH₄⁺) oxidation via hydroxylamine (NH₂OH) and autotrophic denitrification, were considered. A systematic model calibration and validation protocol was developed to obtain a unique set of kinetic parameters in the extended model. The dynamic nitrate (NO₃^-), nitrite (NO₂^-), NH₄⁺ and N₂O behaviors were accurately predicted (R² ≥ 0.81) under five different nitrogen loading conditions. The predicted N₂O production factor ranged from 1.7 to 2.9%. The model-based analysis also revealed the dominant N₂O production mechanisms in terms of the actual process conditions, that is, NH₄⁺ oxidation via NH₂OH when only NH₄⁺ was supplied, heterotrophic denitrification when only NO₂^- was supplied, and a shift of the dominant mechanism when a mixture of NH₄⁺ and NO₂^- was supplied.