Comprehensive effects of pretreatments and pipe materials on water quality stability and potential microbial risks in...

Maintaining chemical and biological water stability throughout reclaimed water distribution systems (RWDSs) is critical for safe water reuse. This study conducted a 140-d continuous investigation of simulated RWDSs equipped with ductile iron and polyethylene pipes under influents pretreated by biological activated carbon (BAC) filtration and chlorine disinfection. Overall, microbial communities exhibited remarkable differences at sampling niches (influents, effluents, and pipe wall biofilms) at the significance of p < 0.01, irrespective of pretreatment strategies. NST and βNTI analysis further revealed that stochastic processes governed biofilm and effluent community assembly, diminishing pretreatment-derived deterministic selection pressures in influents. As for water quality, chlorine disinfection effectively suppressed HPC in influents (>90% removal) and significantly reduced pathogen loads, but led to severe corrosion accompanied with pronounced iron and manganese release (rising 40∼78-fold) in ductile iron pipe. The application of polyethylene pipe eliminated the corrosion drawback while simultaneously achieved chlorine's disinfection strengths, keeping a low HPC level of 7 × 10⁴ CFU/mL after 140-d operation. Conversely, BAC filtration exhibited consistent performances in both ductile iron and polyethylene pipes, maintaining HPC at 3∼5 × 10⁵ CFU/mL by achieving over 67% AOC removal and showing advantages in corrosion resistance. Moreover, a normalized performance scoring method was used to quantitatively assess the combined effects of pretreatments and pipe materials on chemical and biological stability of RWDSs. BAC pretreatment was preferred in existing ductile iron and other metallic distribution networks, while both BAC or chlorine pretreatments combined with plastic pipes, according to different requirements and scenarios, are promising alternatives for new installed or scheduled RWDSs. This study moves beyond a single focus on water quality towards a more integrated assessment of the whole system, and provides suitable application scenarios for better RWDSs management.