Reflux ratio modulates algal and bacterial metabolic balance to enhance refractory pollutant removal

Algal-bacterial symbiotic systems can achieve carbon emission reduction and energy conservation while degrading pollutants, demonstrating significant potential in treating refractory wastewater. However, catabolite repression triggered by excessive readily degradable substrates may suppress microbial degradation of target pollutants, limiting system efficiency. To investigate the role of metabolic balance and develop regulation strategies, this study employed Disperse Red 3B as the target pollutant in a constructed algal-bacterial cascade reactor, where metabolic intermediates were recirculated at different ratios (10%, 13%, 15%) to actively manipulate internal metabolic flow. Results demonstrate that metabolic balance is the key determinant of system performance: at the optimal 13% reflux ratio, the system achieved an efficient synergistic pathway of algal-initiated partial transformation (ring-opening and chromophore cleavage) and bacterial-driven mineralization, with decolorization and COD removal reaching 92% and 96%, respectively. Deviation from this balance significantly reduced performance. This study confirms metabolic balance as decisive for algal-bacterial synergy and validates a method to actively guide it via reflux regulation, offering a new paradigm for optimizing wastewater treatment from a metabolic perspective.