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.

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In this study, denitrification of ammonium-reach anaerobic sludge digester liquor was investigated during start-up periods of two laboratory-scale "fill-and-draw" reactors. One reactor was fed with a single carbon source (ethanol), whereas the other reactor was fed with a complex carbon source (fusel oil). During two acclimation experiments, the structure of microbial community involved in denitrification was analyzed using 16S rDNA polymerase chain reaction-denaturing gradient gel electrophoresis fingerprints and fluorescent in situ hybridization. The characteristics of the mixed liquor were additionally supported by regular measurements of nitrate uptake rates. The addition of fusel oil and ethanol resulted in a significant enhancement of the denitrification rate and efficiency combined with the increasing volumetric addition of sludge digester liquor up to 15 % of the reactor volume. The microbiological analyses revealed that the addition of sludge digester liquor as well as both external carbon sources (fusel oil and ethanol) did not affect the structure of microbial communities in a severe way. In both reactors, Curvibacter sp. and Azoarcus sp. were found as the most abundant representatives of denitrifiers.

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