Modeling the Effect of External Carbon Source Addition under Different Electron Acceptor Conditions in Biological Nutrient Removal Activated Sludge Systems

The aim of this study was to expand the International Water Association Activated Sludge Model No. 2d (ASM2d) to predict the aerobic/anoxic behavior of polyphosphate accumulating organisms (PAOs) and “ordinary” heterotrophs in the presence of different external carbon sources and electron acceptors. The following new aspects were considered: (1) a new type of the readily biodegradable substrate, not available for the anaerobic activity of PAOs, (2) nitrite as an electron acceptor, and (3) acclimation of “ordinary” heterotrophs to the new external substrate via enzyme synthesis. The expanded model incorporated 30 new or modified process rate equations. The model was evaluated against data from several, especially designed laboratory experiments which focused on the combined effects of different types of external carbon sources (acetate, ethanol and fusel oil) and electron acceptors (dissolved oxygen, nitrate and nitrite) on the behavior of PAOs and “ordinary” heterotrophs. With the proposed expansions, it was possible to improve some deficiencies of the ASM2d in predicting the behavior of biological nutrient removal (BNR) systems with the addition of external carbon sources, including the effect of acclimation to the new carbon source

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