Mechanistic insights into nitrogen removal performance and electron competition with mixed electron donor supply in a biofilm electrode reactor

In this study, a unique electrode configuration in the form of an "inverted T" was developed in the biofilm electrode reactor (BER), enabling superior nitrogen removal via the synergistic effect of hydrogen autotrophic denitrification and heterotrophic denitrification. In contrast to the sole heterotrophic denitrification in the biofilm reactor (BR), weak electric stimulation in the BER system promoted <i>in situ</i> hydrogen production as well as electron transport and utilization, resulting in a notable 20% improvement in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mtext>NO</mml:mtext> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> removal efficiency for both influent COD/N ratios. Conversely, notable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mtext>NO</mml:mtext> <mml:mn>2</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> accumulation occurred under both COD/N ratios, with concentrations ranging from 6.0 to 8.0 ​mg/L. The enrichment of non-heterotrophic denitrifiers, such as <i>Thermomonas, Pelomonas,</i> and <i>Hydrogenophaga,</i> was observed in the BER with a relative abundance exceeding 1.0%, contributing to the hydrogen autotrophic denitrification pathway. Based on the outcomes of the multiple electron donor utilization in the coexistence of different electron acceptor combinations, despite H₂ serving as an additional electron donor in the BER, electron competition was still detectable. Notably, nitrite reductase (Nir) emerged as the weakest competitor, resulting in a constrained <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mtext>NO</mml:mtext> <mml:mn>2</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> reduction capacity. Based on the analysis of the electron competition characteristic, the potential <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mtext>NO</mml:mtext> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> metabolic pathway in the BER system was primarily driven by heterotrophic denitrification processes. The introduced electricity in the BER system was favorable for facilitating nitrogen removal through <i>in situ</i> production of hydrogen, direct supply of electrons from the electrode, improvement of functional microbial activity, and enhancement of enzymatic activity.