Effect of surfactant on bioflocculation and organic redirection in high-rate A-stage processes

Bioflocculation is a critical process that promotes carbon redirection by capturing influent organics within sludge flocs. However, this process may be compromised by surfactants that are typically detected in municipal sewage. We evaluated the interplay between surfactant concentration and carbon redirection in high-rate activated sludge (HR-AS) and membrane bioreactors (HR-MBR). Our findings reveal that surfactant-induced inhibition not only suppresses bioflocculation but also increases membrane fouling by altering the production of extracellular polymeric substances (EPSs). Specifically, a 50 mg L^-1 nonionic surfactant concentration suppressed bioflocculation, significantly reducing both organic recovery (3.5% for HR-AS and 4.2% for HR-MBR) and oxidation (4.8% for HR-AS and 3.3% for HR-MBR) (p < 0.05). Notably, the increases hydrodynamic shear in the HR-MBR promoted the formation of denser sludge flocs. These structures limited surfactant diffusion and mitigated inhibitory effects more effectively than in the HR-AS. Microbial community analysis indicated that EPS producers, such as Clostridium sensu stricto 1 and Nakamurella, became dominant under surfactant-induced stress. Furthermore, surfactants increase cake fouling, because increased EPS content strengthened interactions between biofoulants and the membrane surface, thereby increasing total cake resistance. These findings provide the first evidence of surfactant-mediated effects on carbon redirection, providing a new approach for the operational control of A-stage processes for municipal wastewater treatment.