Unraveling the Influent Surface Loading Effect on Denitrification and Microbial Community Characteristics in a Hydrogen-Based Membrane Biofilm Reactor under Nitrate Stress

Influent surface loading is an important operating parameter of hydrogen-based membrane biofilm reactors (H2-MBfRs) and greatly influences denitrification and microbial community characteristics. However, there are few studies on these effects of influent surface loading. In this study, H2-MBfRs were established to investigate the effect produced by influent loading. When influent loading was 2.16 g m−2 day−1, the maximal NO3−-N removal flux was 1.61 g m−2 day−1 (2.45 times that when the influent loading was 0.72 g m−2 day−1, which was 0.657 g m−2 day−1), and the maximal NO3−-N removal efficiency was 74.36%. High-throughput sequencing indicated that higher influent loading was a powerful driver of changes in biofilm microbial communities. Gammaproteobacteria were the dominant bacteria (52.76%–74.63%), and the H2-MBfRs were enriched with Hydrogenophaga and Rhodobacter (3.95%–17.83% and 1.95%–4.90%, respectively), especially in the outer layer, suggesting that the embodiment of positively regulated denitrification is more favorable at locations with high pollutant loads. Notably, the interactions between Methyloversatilis and other microorganisms, including Rhodobacter, Pajaroellobacter, and Reyranella were more pronounced at 2.16 g m−2 day−1. This study revealed the spatial distributions of dominant microbiota in biofilms in different flow fields, which may provide a theoretical framework for the rational regulation of H2-MBfRs influent loading and improving the stability of denitrification systems.