To manage potential microbial risks and meet increasingly strict drinking water health standards,UV treatment has attracted increasing attention for use in drinling water systems in China.However,the effects of UV treatment on microbial control and disinfection byproducts (DBPs) formation in real municipal drinking water systems are poorly understood.Here,we collected water samples from three real drinking water systems in Beijing and Tianjin to investigate the impacts of UV treatment on microbial control and DBP formation.We employed heterotrophic plate count (HPC),flow cytometry (FCM),quantitative PCR analysis,and high-throughput sequencing to measure microorganisms in the samples.Different trends were observed between HPC and total cell count (measured by FCM),indicating that a single indicator could not reflect the real degree of biological re-growth in drinking water distribution systems (DWDSs).A significant increase in the 16S rRNA gene concentration was observed when the UV system was stopped.Besides,the bacterial community composition was similar at the phylum level but differed markedly at the genera level among the three DWDSs.Some chlorine-resistant bacteria,including potential pathogens (e.g.,Adnetobacter) showed a high relative abundance when the UV system was ted off.It can be concluded that UV treatment can mitigate microbial re-growth to some extent.Finally,UV treatment had a limited influence on the formation of DBPs,including trihalomethanes,haloacetic acids,and nitrogenated DBPs.The findings of this study may help to understand the performance of UV treatment in real drinking water systems.