Evaluation of nitrogen removal and nitrous oxide turnovers in granule-based simultaneous nitrification and denitrification system.

Nitrous oxide (N2O) is an inevitable intermediate generated during the nitrogen removal process of granule-based simultaneous nitrification and denitrification (SND) system. In order to alleviate N2O production while maintaining a desired total nitrogen (TN) removal level in this system, a comprehensive evaluation of the contribution pathways and process parameters affecting N2O turnovers is keenly required. Therefore, mathematical models were applied to evaluate the impact of operating conditions and unravel potential mechanisms on TN removal performance and N2O production. Simulation results show that higher N2O production (11.6 %-14.2 %) occurs at higher dissolved oxygen (DO) concentrations, lower chemical oxygen demand (COD) levels, longer hydraulic retention time (HRT) and larger granule size in the granular SND system. The relative conversion rates of nitrogenous components in different regions within the granule influence N2O turnovers, with the nitrification process occurring only in the region 200 µm inward from the granule surface and denitrification working throughout the entire granule. In the inner region of the granule (0-300 µm), the heterotrophic bacteria (HB) denitrification pathway dominates N2O production as a source of N2O. While in the outer region (300-450 µm), HB denitrification acts as a sink for N2O and regulates N2O turnovers (i.e. production and reduction of N2O) together with the hydroxylamine (NH2OH) pathway that is the main contributor of N2O production. Moreover, simultaneous adjustment of multiple operating parameters within a certain range can lower the N2O production factor (<0.5 %) while achieving the desired TN removal efficiency (>80 %), resulting in a feasible N2O mitigation strategy.