Impact of flow configuration on microbial ecological processes and health risks in biological activated carbon filters

Microorganisms are crucial for stable operation of biological activated carbon (BAC), however, understanding microbial community assembly process and microbiome functional traits remains limited, especially in up-flow BAC. This study employed 16S rRNA sequencing and genome-based metagenomics to analyze ecological processes and genomic characteristics of microbial communities in bench-scale BAC filters over 310 days. Flow configuration was a critical factor influencing BAC performance under steady-state, and the organic matter removal efficiency of the up-flow BAC (42.74 ± 3.00 %) was superior to that of down-flow BAC (33.64 ± 3.18 %). Although up-flow increased microbial diversity, no significant differences were found between up-flow (4.67 ± 0.22) and down-flow BAC (4.62 ± 0.12) during the stable phase. Flow configuration influenced microbial community assembly during the stable phase of BAC, with up-flow driven by stochastic processes (with proportion of 88.75 %) and down-flow by deterministic processes (74.32 %). Microbial network and interactions within BAC were more influenced by operational time than by flow configuration. This study identified 436 high-quality metagenome-assembled genomes (MAGs), with 7 associated with nitrification, particularly 3 from comammox Nitrospira clade A. Antibiotic resistance gene (ARG) hosts in down-flow BAC were primarily Proteobacteria, while in up-flow BAC, hosts shifted from Proteobacteria to Actinobacteria in the stable phase. Pathogenic bacteria Acinetobacter junii and Enterobacter cloacae genomes contained ARGs and mobile genetic elements. Nitrifying and pathogenic bacteria MAGs were more abundant in up-flow BAC. This study highlights the microbial ecological processes and genomic characteristics in BAC, offering insights for ensuring drinking water quality.