  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 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. Aerobic ammonium oxidation experiments at two oxygen levels
used for model evaluation (2 and 0.5 mg/L) indicated that both the nitrifier
denitrification (42, 64%) and heterotrophic denitrification (7, 17%) pathways
increased and dominated N2O production at high nitrite and low oxygen
concentrations; while the nitrifier nitrification pathway showed the largest
contribution at high dissolved oxygen levels (51, 19%). The uncertainty of the
biological parameter estimates was propagated to N2O model outputs via Monte
Carlo simulations as 95% confidence intervals. The accuracy of the estimated
parameters resulted in a low uncertainty of the N2O emission factors (4.6 +-
0.6% and 1.2 +- 0.1%).
