A Microalgal-bacterial Consortium Reshapes Biofilm Architecture to Enhance Stable Flux in a Gravity-Driven Membrane Bioreactor.

Global expansion of mariculture generates large volumes of saline wastewater that require sustainable and decentralized treatment to protect coastal ecosystems. Gravity-driven membrane bioreactors (GMBRs) are an attractive low-energy option for this purpose; however, severe membrane fouling in saline environments usually limits water flux. In this study, a microalgal-bacterial consortium (MBC) was introduced to reshape biofilm architecture and improve the long-term performance of GMBRs. Four ceramic membrane-integrated GMBRs were operated for 160 days to treat synthetic mariculture wastewater, including two MBC systems with bacteria-to-microalgae inoculation ratios of 3:2 and 5:2 and two bacteria-only controls. The optimized MBC system (5:2) achieved a stable flux of 14.9 LMH, which was 2.4 times that of the conventional GMBRs (∼6 LMH), while also showing superior nutrient removal (TN removal: 67.9-73.3%, effluent TN: 4.65-5.73 mg/L; TP removal: 92.0-96.3%, effluent TP: 0.1-0.3 mg/L). The improved performance was associated with the formation of larger, more porous aggregates (>20 μm), the development of a synergistic MBC architecture, and enhanced degradation of extracellular polymeric substances. Confocal laser scanning microscopy and scanning electron microscopy revealed a dynamic "loose-dense-loose" structural evolution of the biofilm in the MBC systems, which prevented the dense and irreversible layering observed in the conventional systems. Microbial community analysis further showed that the optimized system enriched functional bacteria (e.g., Nitrosomonas and Nitrospira) while maintaining high microalgal viability. These findings demonstrate that MBC is an effective strategy for improving GMBR performance and offers a sustainable approach for mariculture wastewater treatment.