Sewer biofilms as drivers of greenhouse gas emissions in urban sewer systems: mechanisms and control strategies.

Urban sewer systems function as active subsurface bioreactors, where dynamic biofilms drive significant methane (CH4) and nitrous oxide (N2O) emissions. Redox-stratified microenvironments within biofilms enable concurrent fermentation, methanogenesis, methane oxidation, and nitrogen transformations, linking hydraulic conditions to greenhouse gas generation. CH4 derives mainly from anaerobic micro-zones regulated by substrates and retention time, whereas N2O forms under micro-aerobic conditions via nitrification and incomplete denitrification. Current mitigation strategies, including physical cleaning, chemical dosing, and electron acceptor adjustments, often target isolated pollutants, neglecting inter-gas trade-offs. Methanotroph-mediated methane oxidation, including aerobic and nitrite-dependent anaerobic pathways, provides an additional biological route for mitigating sewer CH4 emissions. This review synthesizes biofilm-mediated emission mechanisms and control measures, advocating a shift from single-pollutant interventions toward precision, system-wide strategies. Integrating high-frequency monitoring, multi-omics, and dynamic modeling is essential to reconcile drainage functionality with climate goals and advance low-carbon, resilient urban wastewater infrastructure.