Quorum sensing on the activated performances of gravity-driven membrane (GDM) system at low temperatures: Ammonia removal and flux stabilization

Gravity-driven membrane (GDM) filtration technology offers a low-energy, low-maintenance solution for water purification due to its powerful bio-cake, but faces predicament in contaminant removal under low temperatures. Quorum sensing (QS), a microbial communication process mediated by signal molecules such as C6-HSL (NHexanoyl-L-homoserine lactone), has been shown to influence microbial behavior and enhance biofilm formation, with promising potential for consolidation of GDM operation at low temperature. This study investigates the application of C6-HSL to enhance ammonia nitrogen and organics removal in low-temperature GDM systems. C6HSL was administered at varying dosages (0/50/250/500 nM/d) at 5 C-degrees, with further experiments conducted at 25 C-degrees to assess the impact of QS regulation on GDM performance under room temperature. The results demonstrated that the optimized presence of C6-HSL (500 nM/d) improved the removal efficiency of dissolved organic carbon, ammonia nitrogen, and raised the stable flux (increase ratios at low vs. room temperature: similar to 19 % vs. similar to 7 %, similar to 38 % vs. similar to 9 %, and similar to 7.9 % vs. 0.4 %, respectively). Analysis of the biofouling layer's structural characteristics, composition, and biological activity indicated that the increased C6-HSL effectively promoted microbial growth and migration/movement, increased the production of extracellular polymeric substances (EPS) and soluble microbial products (SMP), and accelerated biofilm formation. The resulting thicker (similar to 172.98 vs. similar to 119.21 mu m), rougher (similar to 62.1 vs. similar to 55.0 nm) cake layer, equipped with abundant channels for water penetration, contributed to increased contaminant degradation, but also stabilized the flux with slight growth (similar to 3.26 vs. similar to 3.02 LMH) under low temperature. Notably, the enhancement effect of QS regulation was more pronounced at low temperature than at room temperature, attributing to the optimized release and diffusion of signal molecules and the more substantial stimulation of metabolic and enzymatic activity in nitrifying bacteria. This study provides valuable insights and technical support for decentralized water treatment in winter via reliable GDM process operation in challenging temperature conditions.