Simultaneously improve autotrophic denitrification and membrane fouling mitigation under low sludge concentration in sulfur-fed membrane bioreactor

Sulfur-driven autotrophic denitrification (SAD) based membrane bioreactor (SMBR) provides a more integrated approach and cost-effective operation for deep and efficient nitrogen removal from municipal secondary effluent. However, SMBR faces the same challenge as conventional membrane reactors do that membrane fouling usually occurs, which restricts SMBR extensive development and applications. In this study, we found that low sludge concentration simultaneously enhanced autotrophic denitrification and alleviated membrane fouling in SMBR. Further analysis showed that low sludge concentration induced the development of loose-structure and denitrifier-rich bio-cakes, while a gel-layer with high extracellular polymeric substances (EPS) content was formed under high sludge concentration. Meanwhile, high autotrophic denitrification rate was obtained under low sludge concentration likely due to the less flocs attachment onto sulfur powder surface. Furthermore, we found low sludge concentration enhanced the microbial diversity within bio-cakes, and weakened the microbial discrepancy between bio-cakes and suspended sludge. Microbial composition results confirmed that the bio-cakes formed in SMBR were mainly composed of autotrophic denitrifiers and EPS producers. Compared with high sludge concentration, low sludge concentration promoted the accumulation of autotrophic denitrifiers on the membrane surface. Microbial assembly mechanism analysis suggested that the stochastic pattern shaped the microbial composition of the bio-cake under low sludge concentration. Net-work results demonstrated low sludge concentration intensified the microbial interactions within bio-cakes. The findings provide mechanistic insights into the membrane fouling and autotrophic denitrification in SMBR.