Coarse bubble mixing in anoxic zone greatly stimulates nitrous oxide emissions from biological nitrogen removal process

The biological nitrogen removal process in wastewater treatment inevitably produces nitrous oxide (N₂O), a potent greenhouse gas. Coarse bubble mixing is widely employed in wastewater treatment processes to mix anoxic tanks; however, its impacts on N₂O emissions are rarely reported. This study investigates the effects of coarse bubble mixing on N₂O emissions in a pilot-scale mainstream nitrite shunt reactor over a 50-day steady-state period. Online and offline N₂O monitoring campaigns show that coarse bubble mixing in the anoxic zones significantly elevates N₂O emissions, yielding an extremely high emission factor of 15.5 ± 3.5 %. Intensive sampling and isotopic analyses suggest that the elevated emissions are primarily due to the inhibition of the N₂O denitrification process by oxygen in the anoxic phase introduced by coarse bubbling. Substituting coarse bubble mixing with submersible pump mixing resulted in a substantial reduction of N₂O emissions, decreasing the emission factor by an order of magnitude to 1.2 ± 0.8 %. The findings reveal that a previously overlooked factor, coarse bubble mixing, can significantly stimulate N₂O emissions. The use of coarse bubble mixing in anoxic tanks of biological nitrogen removal warrants caution.