Microbial Community and Nitrate Removal Trends in Corncob and Woodchip Denitrifying Bioreactors under Sequential Hydraulic Operating Phases

Abstract

Denitrifying bioreactors rely on carbon media to support microbial communities that remove nitrate from the subsurface drainage water. Woodchips (WCs) and corncobs (CCs) are commonly used media, but their associated microbial communities remain poorly compared. We evaluated the nitrate removal efficiency (NRE) and 16S rRNA gene-based microbial community composition in six laboratory-scale upflow bioreactors containing WC or CC media. Bioreactors were operated sequentially at hydraulic retention times of 8, 16, and 4 h to assess the performance and community responses across operating conditions. CC bioreactors achieved higher NRE than WC bioreactors during the 8- and 16 h phases, while no significant differences were observed during the 4 h phase. Microbial communities differed between media types and shifted across operational phases, reflecting the combined influence of the substrate properties, media replacement, hydraulic conditions, and reactor history. Although WC systems supported greater taxonomic richness, this diversity was not associated with a higher NRE. In contrast, CC systems showed stronger relationships between the media-enriched taxa and NRE. These results demonstrate that substrate-associated microbial community composition is linked to nitrate removal performance and should be considered alongside the hydraulic operation and reactor history when evaluating denitrifying bioreactors.