Mechanistic insights into CO pressure regulating microbial competition in a hydrogen-based membrane biofilm reactor for denitrification.

CO is a proven pH regulator in hydrogen-based membrane biofilm reactor (H-MBfR) but how its pressure regulates microbial competition in this system remains unclear. This work evaluates the CO pressure dependent system performance, CO allocation, microbial structure and activity of CO source H-MBfR. The optimum system performance was reached at the CO pressure of 0.008 MPa, and this pressure enabled 0.18 g C/(m·d) of dissolved inorganic carbon (DIC) allocated to denitrifying bacteria (DNB) for carbon source anabolism and denitrification-related proton compensation, while inducing a bulk liquid pH (pH 7.4) in favor of DNB activity by remaining 0.21 g C/(m·d) of DIC as pH buffer. Increasing CO pressure from 0.008 to 0.016 MPa caused the markedly changed DNB composition, and the diminished DNB population was accompanied by the enrichment of sulfate-reducing bacteria (SRB). A high CO pressure of 0.016 MPa was estimated to induce the enhanced SRB activity and weakened DNB activity.