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