Biofilm characteristics for providing resilient denitrification in a hydrogen-based membrane biofilm reactor

In a hydrogen-based membrane biofilm reactor (H₂-MBfR), the biofilm thickness is considered to be one of the most important factors for denitrification. Thick biofilms in MBfRs are known for low removal fluxes owing to their resistance to substrate transport. In this study, the H₂-MBfR was operated under various loading rates of oxyanions, such as NO₃-N, SO₄-S, and ClO₄⁻ at an H₂ flux of 1.06 e⁻ eq/m²-d. The experiment was initiated with NO₃-N, SO₄-S, and ClO₄⁻ loadings of 0.464, 0.026, and 0.211 e⁻ eq/m²-d, respectively, at 20 °C. Under the most stressful conditions, the loading rates increased simultaneously to 1.911, 0.869, and 0.108 e⁻ eq/m²-d, respectively, at 10 °C. We observed improved performance in significantly thicker biofilms (approximately 2.7 cm) compared to previous studies using a denitrifying H₂-MBfR for 120 days. Shock oxyanion loadings led to a decrease in total nitrogen (TN) removal by 20 to 30%, but TN removal returned to 100% within a few days. Similarly, complete denitrification was observed, even at 10 °C. The protective function and microbial diversity of the thick biofilm may allow stable denitrification despite stress-imposing conditions. In the microbial community analysis, heterotrophs were dominant and acetogens accounted for 11% of the biofilm. Metagenomic results showed a high abundance of functional genes involved in organic carbon metabolism and homoacetogenesis. Owing to the presence of organic compounds produced by acetogens and autotrophs, heterotrophic denitrification may occur simultaneously with autotrophic denitrification. As a result, the total removal flux of oxyanions (1.84 e⁻ eq/m²-d) far exceeded the H₂ flux (1.06 e⁻ eq/m²-d). Thus, the large accumulation of biofilms could contribute to good resilience and enhanced removal fluxes.