Dissolved oxygen regulation enhances organic micropollutant removal in wastewater treatment bioreactors

Organic micropollutants (OMPs) are ubiquitously detected in wastewater and pose potential risks to aquatic ecosystems and human health, making their effective removal a critical objective of wastewater treatment processes. Dissolved oxygen (DO) is a central operational parameter that governs microbial metabolism in biological wastewater treatment processes; however, its long-term role in controlling OMP removal remains insufficiently understood. Here, three bioreactors were operated for 166 days under staged DO conditions ranging from 0.8 to 4.5 mg/L to systematically evaluate the effects of DO on the removal of eight representative OMPs and associated microbial responses. Operating at a low DO level maintained stable removal of conventional pollutants while significantly enhancing the biodegradation of several OMPs, including dimetridazole, ofloxacin, trimethoprim, and sulfamethazine. Despite only minor changes in overall community composition, intermediate and rare taxa exhibited pronounced sensitivity to DO variation, suggesting their potential involvement in OMP biodegradation under low-oxygen conditions. Enzyme activity measurements combined with metagenomic and transcriptomic analyses further revealed that low DO promoted higher activity, abundance, and expression of redox-related co-metabolic enzymes, particularly peroxidases and cytochrome P450 enzymes. These results demonstrate that DO regulates OMP removal primarily by reshaping microbial functional potential and redox metabolism. Overall, this study provides both mechanistic understanding and practical guidance for applying DO regulation to achieve enhanced micropollutant removal in wastewater treatment systems.