Impact of organics, aeration and flocs on N₂O emissions during granular-based partial nitritation-anammox

Partial nitration-anammox is a resource-efficient technology for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology are challenged by the emission of N₂O, a potent greenhouse gas. In this study, a granular sludge one-stage partial nitritation-anammox reactor comprising granules and flocs was run for 337 days in the presence of influent organics to investigate its effect on N removal and N₂O emissions. Besides, the effect of aeration control strategies and flocs removal was investigated as well. The interpretation of the experimental results was complemented with modelling and simulation. The presence of influent organics (1 g COD g^-1 N) helped to suppress NOB and significantly reduced the overall N₂O emissions while having no significant effect on anammox activity. Besides, long-term monitoring of the reactor indicated that constant airflow rate control resulted in more stable effluent quality and less N₂O emissions than DO control. Still, floc removal reduced N₂O emissions at DO control but increased N₂O emissions at constant airflow rate. Furthermore, anammox bacteria could significantly reduce N₂O production during heterotrophic denitrification, likely via competition for NO with heterotrophs. Overall, this study demonstrated that the presence of influent organics together with proper aeration control strategies and floc management could significantly reduce the N₂O emissions without compromising nitrogen removal efficiency during one-stage partial nitritation-anammox processes.