Microbial behavior drives structure and composition changes during bio-cake evolution and implications for membrane fouling mitigation

Biofouling significantly affects membrane system performance, however, microbial ecological processes and behaviors within the bio-cake layer during operation remain poorly understood. In this study, four lab-scale gravity-driven membrane (GDM) ultrafiltration systems were operated in parallel to monitor flux variations and membrane fouling characteristics throughout GDM operation. Results showed that a gradual decrease and stabilization in flux (44.69, 7.19, 3.78, 3.1 L/(m² h), respectively), accompanied by bio-cake layer homogenization, compaction, and an increase in extracellular polymeric substances (EPS) content over time (8.7, 46.99, 79.93, 69.64 mg/m², respectively). Microbial community succession was primarily driven by ecological drift and other stochastic processes. Distinct bacterial community structures emerged at different stages, with interspecies relationships tending toward competition. Predation shifted from nematodes and flagellates in early stages to flagellates dominating in later stages. N-doctanoyl-L-homoserine lactone (C8-HSL) and N-dodecanoyl-L-Homoserine lactone (C12-HSL) were identified as the dominant quorum sensing signal molecules, with alanine, aspartate, and glutamate metabolism identified as key metabolic pathways. EPS production evolved from microbial metabolites in early stages to quorum sensing-induced in later stages. Aestuariicella hydrocarbonica and Methylotenera sp. were identified as principal quorum sensing bacteria. C8-HSL significantly determined the flux of GDM. Regulating microbial community behavior was more effective in mitigating fouling than shaping community structure. This study is the first to investigate microbial ecological processes and behavior changes within the bio-cake layer throughout GDM operation, providing valuable insights into balancing biofouling mitigation and performance enhancement.