Flow configuration shapes microbiome assembly and function in full-scale drinking water BAC filters

To address the challenges of traditional down-flow biological activated carbon (BAC) filters, up-flow filters have been increasingly applied in drinking water treatment plants (DWTPs), yet their microbial characteristics and underlying assembly mechanisms are not fully explored. This study presents the first comprehensive comparison of bacterial and eukaryotic communities, functional traits, ecological interactions and assembly mechanisms in up-flow and down-flow BAC filters across 18 full-scale DWTPs spanning diverse geographic and operational contexts in China. Despite site-specific variability, distinct bacterial and eukaryotic community structures were observed between the two configurations (ANOSIM, R=0.345-0.353, P < 0.05), highlighting the strong influence of filter design on microbiomes. Functional gene profiling revealed significant enrichment of high-abundance pathways related to carbon, sulfur, and nitrogen metabolism in up-flow filters (P<0.05), indicating elevated biogeochemical activity. HAllA and network analyses revealed the pivotal role of eukaryotes in structuring microbial interactions and uncovered distinct cross-domain interaction patterns between filter types. Community assembly analysis showed deterministic processes dominated BAC microbiomes, with significantly stronger homogeneous selection in up-flow systems (P < 0.05). Together, these findings provide new ecological insights into BAC filter microbiomes and support the broader adoption of up-flow designs to enhance treatment performance and microbial stability in full-scale DWTPs.