Systematic calibration of N₂O emissions from a full-scale WWTP including a tracer test and a global sensitivity approach

Nitrous oxide (N₂O) is a greenhouse gas (GHG) emitted during biological nitrogen removal from wastewater treatment plants (WWTPs). Some modelling tools have been proposed to predict N₂O emissions during the design and operation of WWTPs. In this study, the novel ASM2d-N₂O model, which accounts for the production of N₂O in nutrient removal WWTPs, was used to study the associated emissions from a full-scale WWTP with two independent lines. Firstly, the hydraulics of the WWTP was characterized by a residence time distribution test, showing the flow was equally divided into the two treatment lines (49.3 vs. 50.7%), that each reactor worked as an ideal continuous stirred tank reactor and the secondary settler model flux was similar to a plug-flow reactor. The ASM2d-N₂O model was then calibrated using experimental data obtained under dynamic conditions. A global sensitivity analysis was used to select, among 59 model parameters, five candidates that resulted to be related to nitrifying organisms. Different parameter subsets up to four parameters were evaluated, being the subset [µ_NOB, q_{AOB_AMO}, K_{O2_NOB}, K_{NO2_NOB}] the best, achieving 53.3% reduction of the calibration cost function. The model fit obtained provided a reasonably description of nutrients and N₂O emission trends, considering the inherent operational variability suffered in full-scale WWTPs. Finally, a simulation-based study showed that, for the given WWTP and operational conditions, an unbalanced distribution of flow-rate between the two treatment lines did not result in a significant increase on N₂O emissions. The results obtained show that this model can be a suitable tool for predicting N₂O emissions in full-scale WWTPs, and can therefore be used to find operational conditions that help to minimise these emissions.