Nitrite accumulation inside sludge flocs significantly influencing nitrous oxide production by ammonium-oxidizing...

This work aims to clarify the role of potential nitrite (NO₂^-) accumulation inside sludge flocs in N₂O production by ammonium-oxidizing bacteria (AOB) at different dissolved oxygen (DO) levels with focus on the conditions of no significant bulk NO₂^- accumulation (<0.2 mg N/L). To this end, an augmented nitrifying sludge with much higher abundance of nitrite-oxidizing bacteria (NOB) than AOB was enriched and then used for systematically designed batch tests, which targeted a range of DO levels from 0 to 3.0 mg O₂/L at a fixed ammonium concentration of 10 mg N/L. A two-pathway N₂O model was applied to facilitate the interpretation of batch experimental data, thus shedding light on the relationships between N₂O production pathways and key process parameters (i.e., DO and NO₂^- accumulation inside sludge flocs). The results demonstrated (i) the biomass specific N₂O production rate firstly increased and then decreased with DO, with the maximum value of 3.03 ± 0.05 mg N/h/g VSS obtained at DO level of 0.75 mg O₂/L, (ii) the AOB denitrification pathway for N₂O production was dominant (98.0%) at all DO levels tested even without significant bulk NO₂^- accumulation (<0.2 mg N/L) observed in the system, but its contribution decreased with DO, (iii) DO had a positive impact on the hydroxylamine pathway for N₂O production which therefore increased with DO, and (iv) the nitrite accumulation existed inside the sludge flocs and induced significant N₂O production from the AOB denitrification pathway.