Modeling of N2O Emissions in a Full-Scale Activated Sludge Sequencing Batch Reactor

Nitrous oxide (N₂O) is a greenhouse gas with a significant global warming potential. A dynamic model was developed to estimate the N₂O pro- duction and emission in a full-scale sequencing batch reactor (SBR) municipal wastewater treatment plant (WWTP). Based on the Activated Sludge Model 1 (ASM1), the model considered all known biological and abiotic N₂O production pathways along with the application of a ‘stripping effectivity’ (SE) coefficient for reflecting the non-ideality of the stripping model. N₂O data of two different cycles (types B and C) were used for the model calibration. Cycle B involved the alternation amongst aerated and non-aerated phases, whereas cycle C included a unique long aerobic phase.  Optimizing  the  dissolved  oxygen (DO) and SE parameters for both cycles provided a good fit of the model    (DO = 1.6 mg L⁻¹ and SE = 0.11 for  cycle  B,  and  DO = 1.66 mg  L⁻¹  and SE = 0.11 for cycle C). In both cases, N₂O emission peaks were related to high nitrite concentration in the liquid phase. Nitrifier denitrification was identified as the predominant biological pathway for N₂O generation. Although SBR oper- ation occurred at similar DO and SE values for both cycles, the emission factor was significantly different; 0.8% for cycle B and 1.5% for cycle C, indicating the impact of cycle configuration on the N₂O emission. Thus, optimized SBR operation is essential in order to achieve a low overall carbon footprint through the avoidance of high N₂O emissions and energy requirements.