Precise H₂ supply enables quantitative control of on-demand deep nitrate removal while preserving denitrification...

Precise control of deep nitrate (NO₃^-) removal is increasingly required for industrial water reuse, yet different reuse scenarios demand different target NO₃^- concentrations that cannot be readily achieved by conventional heterotrophic denitrification processes. Here, we demonstrate that membrane-mediated H₂ supply enables on-demand deep hydrogenotrophic denitrification by quantitatively matching H₂ supply with targeted NO₃^- removal. During 180 days of continuous operation in an H₂-based membrane biofilm reactor (H₂-MBfR), effluent NO₃^- concentrations were predictably tuned from 0.1 to 4.5 mg-N/L by progressively reducing H₂ transfer flux, closely matching stoichiometric expectations. Crucially, partial NO₃^- removal under H₂-limited conditions preserved denitrification completeness while avoiding accumulation of NO₂^-, NO, N₂O, or NH₄⁺. Metagenomic analysis further revealed that complete hydrogenotrophic denitrifiers possessing the full enzymatic repertoire for NO₃^- to N₂ reduction dominated the biofilm community (92-97% of MAG abundance). Under H₂ over-supply conditions, excess electrons were channeled into biofilm-derived organic matter production via extracellular protein secretion pathways, consequently elevating effluent COD concentrations-a risk that can be avoided through precise H₂ regulation. These findings establish that membrane-mediated H₂ supply achieved quantitative control of deep denitrification without compromising denitrification completeness, providing a mechanistic basis for balancing desired NO₃^- removal, water quality protection, and operational costs in applications requiring deep yet tailored nitrogen control.