Flocs enhance nitrous oxide reduction capacity in a denitrifying biofilm-based system: Mechanism of electron competition

[Display omitted] •Flocs can reduce N2O accumulation by 32 % at carbon loading rate of 30 mg-COD/g VSS/h.•Flocs exhibit higher N2O-reducing capacity due to ignore diffusion restriction.•Flocs promote electrons toward downstream electron pool in biofilm/flocs system.•Fiercer electron competition tends to occur in biofilm systems without flocs. Nitrous oxide (N2O) accumulation induced by insufficient electron donors and substrate diffusion restriction is inevitably occurred in denitrifying biofilm-based systems, yet rare mechanism is intensively explored. This study discovered the mechanism of electron competition affecting N2O reduction capacity in two denitrifying biofilm-based systems, and uncovered the role of flocs for mitigating N2O production with varying exogenous organic carbon levels. The results clearly delineated that substrate diffusion restriction of biofilms deteriorated electron donors availability, leading to fiercer electron competition (ratio of 1.95) in the single denitrifying biofilm system. Such competition also occurred with the decreasing carbon loading rates (from 180 to 30 mg-COD/g VSS/h), further resulting in a higher N2O accumulation of 0.95 mg-N/L. By contrast, flocs from the denitrifying biofilm/flocs system can efficiently reduce overall N2O accumulation by 32 %. Also, flocs promoted the higher proportion of the electrons distributed to nitrous oxide reductase (Nos), indicating a strong N2O-reducing capacity. In addition, electrons from Nos inclinedly distributed to nitrate reductase (Nar) with an increase of 21.4 % under co-occurrence of NO3−, NO2− and N2O in the single denitrifying biofilm system, suggesting the mechanism of electron competition was the competition in upstream electron pool. In contrast, electrons from Nos tended to nitrite reductase (Nir) (5.9 %) instead of Nar (2.1 %) in the denitrifying biofilm/flocs system implied the competition in downstream electron pool. This study unraveled that flocs can be applied in denitrifying biofilm-based systems to alleviate N2O emissions and total carbon footprints at wastewater treatment plants (WWTPs).