Proliferation of opportunistic premise plumbing pathogens in water treatment plants: Occurrence patterns and influencing factors.

Opportunistic premise plumbing pathogens (OPPPs), including Mycobacterium spp., Legionella spp., and Pseudomonas aeruginosa, pose emerging biosafety concerns in urban water supply systems, particularly for immunocompromised populations. While comprehensive quantitative microbial risk assessment (QMRA) requires dose-response data, this study systematically tracks the proliferation dynamics of these pathogens across full-scale treatment trains in four drinking water treatment plants (DWTPs) over four seasons. Water samples were collected from the intake, coagulation-sedimentation, sand filtration, activated carbon filtration, and finished water stages and analyzed via quantitative PCR (qPCR) with propidium monoazide (PMA) pretreatment to target membrane-intact cells. The results demonstrated that conventional treatment processes inadequately control OPPPs, with the ozone-biological activated carbon (O3-BAC) stage identified as a critical hotspot for pathogen regrowth, which is likely driven by increased microbial activity and organic carbon utilization. Disinfection via chlorination exhibited variable efficacy as measured by PMA-qPCR: while Legionella spp. were relatively susceptible, P. aeruginosa demonstrated higher apparent persistence under chlorination, remaining detectable in finished water. Correlation analyses further linked the occurrence of OPPP to specific water quality parameters and shifts in the microbial community structure. These findings underscore the need for optimized treatment strategies that account for seasonal variations and target OPPP regrowth niches, thereby advancing biosafety control in the drinking water supply.