Innovative nitrogen transformation: Coexistence of DNRA and denitrification under high alkalinity in a hydrogen-based membrane biofilm reactor

Nitrate (NO₃⁻) contamination has become a significant global environmental issue. Traditional nitrate reduction processes typically require external pH control to maintain neutral conditions and prevent nitrite accumulation. In this study, a hydrogen-based membrane biofilm reactor (H₂-MBfR) was constructed without external pH regulation. The reactor relied on the alkalinity generated by the nitrate reduction process itself, maintaining a highly alkaline environment with stable denitrification and up to 60% ammonium conversion at pH levels reaching 11.70. The DNRA process was found to be independent of substrate type, inversely proportional to electron supply, and exhibited the highest reaction rate at pH 11, as confirmed by both ex-situ and in-situ batch experiments. Microbial community analysis indicated that Meiothermus was the predominant genus within the biofilm. This research reveals a novel nitrogen transformation phenomenon, demonstrating the coexistence of DNRA and denitrification processes under high alkalinity conditions in the H₂-MBfR system. These findings offer new insights into nitrate reduction processes and suggest potential advancements in wastewater treatment and resource recovery.