Salinity-Driven Microbial Community Engineering for Safer Ultrafiltration Water Reuse

Ultrafiltration is central to water reclamation but faces two critical challenges: microbial regrowth that threatens biostability and pathogen invasion that undermines biosafety. Here, we proposed an ecological strategy that transformed backwash from a cleaning procedure into a microbiome engineering tool, thereby simultaneously addressing both challenges. Our findings provided evidence for the major microbial sources in permeate, including membrane breakthrough, detachment from the membrane permeate side, and from downstream pipeline surfaces. High-salinity backwash (100 mM NaCl) suppressed the latter two dominant sources, reducing permeate total cell counts (TCC) by more than 50%. It also enhanced the removal of assimilable organic carbon (AOC), thereby limiting microbial regrowth in the permeate by 32% during 40 day storage. Under pathogen shock loading, the salinity-driven biocake layer accelerated pathogen inactivation, reduced pathogen accumulation by 86.9%, and thereby prevented pathogen leakage into the permeate. The mechanism analysis revealed that NaCl reshaped the biocake microbiome, enhancing deterministic assembly. This functionally specialized consortium showed strengthened cooperation and upregulated key metabolic pathways, enabling synergistic AOC degradation. In addition, it suppressed pathogen invasion through superior carbon competitiveness and secretion of antimicrobial metabolites. This work provided an ecological engineering approach to enhance both biostability and biosafety in ultrafiltration-based water reuse systems.