Microbial Mechanisms of Biofilm Reactor Degradation of Refractory Phenolic Compounds Driven by Dissolved Oxygen Gradient

Abstract This study investigated how progressively enhanced oxygen supply affected mixed-phenolic-compound removal and microbial niche differentiation in biofilm reactors. Four 250 mL reactors containing suspended sludge and polyurethane (PU) carriers treated synthetic wastewater with six phenolic compounds at total phenol (TPh) concentrations of 10–300 mg/L. Oxygen availability was varied through sealed, limited-gas exchange, partial-gas exchange, and fully open configurations rather than fixed dissolved oxygen set points. At 300 mg/L TPh, R3 and R4 achieved final removal efficiencies of 99.99% and 99.62%, respectively, whereas R1 and R2 deteriorated under high loading. In a representative 48 h batch test, R4 achieved 79.92% TPh removal at 16 h compared with 66.45% for R3, indicating faster early stage removal under the strongest oxygen-supply condition. 16S rDNA sequencing showed distinct suspended-sludge and PU-biofilm communities, with Rhodanobacter predominating in the R4 PU biofilm. PICRUSt2 and FAPROTAX predicted shifts toward carbohydrate and amino acid metabolism, electron-transfer-related functions, and oxidative aromatic-compound transformation under enhanced oxygen availability. These findings support oxygen-supply management as a useful strategy for high-strength phenolic wastewater treatment while identifying the need for validation with real wastewater and direct transformation-product analysis.