Abstract
The biological nitrogen removal process in wastewater treatment inevitably produces nitrous oxide (N(2)O), a potent greenhouse gas. Coarse bubble mixing is widely employed in wastewater treatment processes to mix anoxic tanks; however, its impacts on N(2)O emissions are rarely reported. This study investigates the effects of coarse bubble mixing on N(2)O emissions in a pilot-scale mainstream nitrite shunt reactor over a 50-day steady-state period. Online and offline N(2)O monitoring campaigns show that coarse bubble mixing in the anoxic zones significantly elevates N(2)O 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 N(2)O 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 N(2)O 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 N(2)O emissions. The use of coarse bubble mixing in anoxic tanks of biological nitrogen removal warrants caution.