Indicator Compliance in Bulk Water Does Not Reflect Biofilm-Associated Microbial Community Composition or Potential Risk in UV-C LED-Treated Rainwater Harvesting Systems

Abstract Rainwater harvesting (RWH) systems are increasingly used to supplement potable and nonpotable water supplies but remain understudied with respect to biofilm-associated microbial risk. This study used a pilot-scale, temperature-controlled pipe loop to investigate how pipe material (copper and cross-linked polyethylene, PEX) and point-of-use ultraviolet-C light-emitting diode (UV-C LED) disinfection influence bulk water quality and biofilm microbial communities. UV-C LED treatment reduced Escherichia coli and total coliforms to below detection limits (<0.5 most probable number/100 mL, MPN/100 mL) across all pipe materials and temperatures, matching conventional mercury-vapor ultraviolet performance in the single warm-copper configuration where both technologies were directly compared. Despite compliant bulk water quality, putative opportunistic pathogen-associated genera, including Legionella, Pseudomonas, and Mycobacterium, persisted within pipe-surface biofilms. Pipe material strongly influenced biofilm ecology independent of UV treatment or temperature. PEX supported greater microbial diversity and greater absolute Legionella abundance, whereas copper supported lower overall diversity but a higher relative abundance of pathogen-associated genera. These findings demonstrate that compliance with traditional fecal indicator organisms does not necessarily reflect biofilm-associated microbial risk in RWH systems and highlight the importance of combining point-of-use disinfection with material-aware biofilm management.