Modelling N₂O dynamics of activated sludge biomass: Uncertainty analysis and pathway contributions

Nitrous oxide (N2O) is a potent greenhouse gas emitted during biological wastewater treatment. A pseudo-mechanistic model describing three biological pathways for nitric oxide (NO) and N2O production was calibrated for mixed culture biomass from an activated sludge process using laboratory-scale experiments.

The model (NDHA) comprehensively describes N2O producing pathways by both autotrophic ammonium oxidizing bacteria and heterotrophic bacteria. Extant respirometric assays and anaerobic batch experiments were designed to calibrate endogenous and exogenous processes (heterotrophic denitrification and autotrophic ammonium/nitrite oxidation) together with the associated net N2O production.

Ten parameters describing heterotrophic processes and seven for autotrophic processes were accurately estimated (variance/mean <25%). The model predicted accurately NO and N2O dynamics at varying dissolved oxygen, ammonium and nitrite levels, and was validated against an independent set of experiments with the same biomass.

In aerobic ammonium oxidation experiments the nitrifier denitrification and heterotrophic denitrification estimated pathway contributions increased at high nitrite and low oxygen concentrations; while the nitrifier nitrification pathway showed the largest contribution at high dissolved oxygen levels.

The uncertainty of N2O emissions during model calibration is commonly overlooked, which limits the confidence of model-based mitigation strategies. Here we show that the precision of the estimated parameters resulted in a low uncertainty of the N2O emission factors during aerobic ammonium oxidation at DO ≈ 2.0 mg/L (1.2 ± 0.1%) and DO ≈ 0.5 mg/L (4.6 ± 0.6%).