Impact of nitrite and oxygen on nitrous oxide emissions from a granular sludge sequencing batch reactor

Maximizing nutrient removal and minimizing greenhouse gas (GHG) emissions is imperative for the future of wastewater treatment. As municipalities focus on minimizing their carbon footprints, future permits could regulate GHG emissions from wastewater treatment plants. This study investigates how nitrous oxide (N₂O) emissions are affected by dissolved oxygen and nitrite concentrations, providing potential strategies to meet possible gaseous emission permits. A lab-scale sequencing batch reactor (SBR) was enriched with aerobic granular sludge (AGS) capable of phosphate removal and simultaneous nitrification-denitrification (SND). N₂O emissions were tracked at varying dissolved oxygen (DO) and nitrite (NO₂^-) concentrations, with >99% SND efficiency and 93%-100% phosphate removal efficiency. Higher DO and NO₂^- concentrations were associated with higher N₂O emissions. Emissions were minimized at a DO concentration of 1 mg L^-1, with an average emission factor of 0.18% of oxidized NH₃-N emitted as N₂O-N, which is lower than factors from many full-scale treatment plants (Vasilaki et al., 2019) and similar to a Nereda® full-scale AGS SBR (van Dijk et al., 2021). This challenges assertions that AGS emits more N₂O than conventional activated sludge, although more research at full-scale with influent quality variations is required to confirm this trend. Molecular analyses revealed that the efficient SND was likely achieved with shortcut nitrogen removal facilitated by a low presence of nitrite oxidizing bacteria and a large population of denitrifying phosphate accumulating organisms, which far outnumbered denitrifying glycogen accumulating organisms.