The concept of simultaneous microbial-driven and Pd-catalyzed nitrate (NO3-) reduction was evaluated in terms of NO3--removal efficiency and reduction-product selectivity. Experiments were conducted in three identical H-2-based membrane biofilm reactors (MBfR) operated in parallel: biogenic Pd nanoparticles (PdNPs) associated with biofilm ("Pd-biofilm"), biofilm alone ("Biofilm"), and abiotic PdNPs alone ("Pd-film"). Solid-state characterizations confirmed that the PdNPs in Pd-biofilm were dominated by Pd nanocrystallites similar to those in Pd-film, and molecular microbiological analyses confirm that the microbial community in Pd-biofilm were dominated by S-proteobacteria with denitrifying activity similar to Biofilm. Pd-biofilm accelerated NO3-reduction to NO2-mainly through enzymatic activity and accelerated subsequent NO2-reduction mainly through PdNP catalysis. When H-2 could be delivered at a rate approximately equal to the total demand to reduce NO3-to N-2, active biofilm reduced NO3-/NO(2)exclusively to N-2, and it also attenuated NH4+ formation; as a result, the overall selectivity towards N-2 in Pd-biofilm was nearly 100% and higher than in Pd-film. Thus, coupling PdNP catalysis and microbial denitrification promoted H-2-based NO3-reduction and led to greater selectivity towards N-2 as long as H-2 delivery was controlled. From a practical perspective, delivering H-2 by diffusion through bubbleless membranes enabled accurate control of N selectivity. (C) 2017 Elsevier B.V. All rights reserved.