Flow velocity-dependent methane emissions in sewage pipelines: Mechanistic insights from substrate metabolism to microbial trait dynamics

Abstract

Urban sewer networks contribute significantly to greenhouse gas emissions through methane release, but the mechanisms controlling these emissions, particularly how flow velocity affects them, remain poorly understood. This study systematically investigated the impacts of flow velocity on methane emissions in sewage pipelines and revealed the underlying mechanisms through multiple analytical approaches. Results demonstrated that increasing flow velocity from 0.4 to 0.8 m/s significantly reduced cumulative methane production by 72.9 % within 72 h, with methane emission flux decreasing from 0.292 to 0.127 mg/(m²·h). Three-dimensional fluorescence spectroscopy analysis revealed that higher flow velocity limited substrate consumption, as evidenced by Region-IV fluorescence intensity being 48.1 % higher at 0.8 m/s than at 0.4 m/s. Microbial analysis showed that higher flow velocity significantly altered community structures, reducing key functional bacteria (e.g., Tolumonas decreased from 2.85 to 2.14 %) and methanogens (e.g., Methanobacterium decreased from 24.45 to 22.03 %), while diminishing community resilience and functional complementarity. Moreover, functional gene analysis further demonstrated that higher flow velocity downregulated genes involved in substrate hydrolysis (e.g., pgm), volatile fatty acids synthesis (e.g., porA), and methane production (e.g., mcrA). Correlation analysis identified flow-induced changes in microbial community composition as the primary driver of methane production variability. These findings provide novel insights into flow velocity-dependent methane emission mechanisms in sewer systems.