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

This work aims to clarify the role of potential nitrite (NO) accumulation inside sludge flocs in NO production by ammonium-oxidizing bacteria (AOB) at different dissolved oxygen (DO) levels with focus on the conditions of no significant bulk NO 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 O/L at a fixed ammonium concentration of 10 mg N/L. A two-pathway NO model was applied to facilitate the interpretation of batch experimental data, thus shedding light on the relationships between NO production pathways and key process parameters (i.e., DO and NO accumulation inside sludge flocs). The results demonstrated (i) the biomass specific NO 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 O/L, (ii) the AOB denitrification pathway for NO production was dominant (98.0%) at all DO levels tested even without significant bulk NO 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 NO production which therefore increased with DO, and (iv) the nitrite accumulation existed inside the sludge flocs and induced significant NO production from the AOB denitrification pathway.