Enhancing separation performance of gravity-driven membrane bioreactor via membrane packing density optimization: Comprehensive insights from biological responses and hydrodynamic insights

The gravity-driven membrane bioreactor (GDMBR) demonstrates substantial potential in addressing the growing need for energy-efficient decentralized wastewater treatment and water reuse solutions. Currently, the underlying mechanism of how membrane packing density regulates the hydrodynamic environment and consequently shapes the biofilm structure remains poorly understood. Herein, this study systematically investigated the effects of different membrane packing densities on flux stability, pollutant removal, and biofilm characteristics during a 70-day experiment treating domestic wastewater. The results indicated that lower packing densities contributed to superior hydraulic performance, with the stable flux increasing from 1.37 LMH at the highest packing density to 2.19 LMH at the lowest density. Additionally, the optimized membrane packing density maintained high-quality effluent with effective nutrient retention in different hydrodynamic conditions, exhibiting the potential for irrigation reuse. Particularly, when the fiber spacing (S) was below twice the characteristic bubble diameter (D_char), it induced sludge bridging and bubble shielding, creating hydrodynamic dead zones that triggered bacterial self-protection. Conversely, crossing this boundary (S > 2D_char) eliminated channel clogging, facilitating effective shear scouring and eukaryotic predation (e.g., Rhogostoma), which increased biofilm porosity and significantly reduced hydraulic resistance. This work provides a critical foundation for optimizing the long-term hydraulic performance of GDMBR systems in practical engineering applications.