In situ electron generation through Fe/C supported sludge coupled with a counter-diffusion biofilm for electron-deficient wastewater treatment: Binding properties and catalytic competition mechanism of nitrate reductase

A membrane-aerated biofilm-coupled Fe/C supported sludge system (MABR-Fe/C) was constructed to achieve in situ electron production for NO₃⁻-N reduction enhancement in different Fe/C loadings (10 g and 200 g). The anoxic environment formed in the MABR-Fe/C promoted a continual Fe²⁺release of Fe/C in 120 d operation (average Fe²⁺concentrations is 1.18 and 2.95 mg/L in MABR-Fe/C10 and MABR-Fe/C200, respectively). Metagenomics results suggested that the electrons generated from ongoing Fe²⁺ oxidation were transferred via the Quinone pool to EC 1.7.5.1 rather than EC 1.9.6.1 to complete the process of NO₃⁻-N reduction to NO₂⁻-N in Acidovorax, Ottowia, and Polaromonas. In the absence of organic matter, the NO₃⁻-N removal in MABR-Fe/C10 and MABR-Fe/C200 increased by 11.99 and 12.52 mg/L, respectively, compared to that in MABR. In the further NO₂⁻-N reduction, even if the minimum binding free energy (MBFE) was low, NO₂⁻-N in Acidovorax and Dechloromonas preferentially bind the Gln-residues for dissimilatory nitrate reduction (DNR) in the presence of Fe/C. Increasing Fe/C loading (MABR-Fe/C200) caused the formation of different residue binding sites, further enhancing the already dominant DNR. When DNR in MABR-Fe/C200 intensified, the TN in the effluent increased by 3.75 mg/L although the effluent NO₃⁻-N concentration was lower than that in MABR-Fe/C10. This study demonstrated a new MABR-Fe/C system for in situ electron generation to enhance biological nitrogen removal and analyzed the NO₃⁻-N reduction pathway and metabolic mechanism, thus providing new ideas for nitrogen removal in electron-deficient wastewater.