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 (N 2 O) 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 (NH 4 + ) oxidation via hydroxylamine (NH 2 OH) 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 (NO 3 – ), nitrite (NO 2 – ), NH 4 + and N 2 O behaviors were accurately predicted (R 2 ≥ 0.81) under five different nitrogen loading conditions. The predicted N 2 O production factor ranged from 1.7 to 2.9%. The model-based analysis also revealed the dominant N 2 O production mechanisms in terms of the actual process conditions, that is, NH 4 + oxidation via NH 2 OH when only NH 4 + was supplied, heterotrophic denitrification when only NO 2 – was supplied, and a shift of the dominant mechanism when a mixture of NH 4 + and NO 2 – was supplied.