Urban Methane from Sewers: Reactor Engineering, Biological Sinks, and Carbon Markets

Sewer networks are largely absent from greenhouse gas inventories, yet they behave as anaerobic bioreactors and emit an estimated 1.18–1.95 Tg CH 4 yr −1; worldwide. This Viewpoint examines sewer methane through complementary lenses of microbial ecology and process engineering. A reactor-engineering perspective links sediment- and biofilm-derived methanogenesis through measurable kinetic parameters and treats emissions as the net result of methanogenesis and the often-ignored activity of methanotrophs. A three-tiered monitoring framework that integrates proxy sensing, mobile surveys, and hotspot measurements is proposed to address current data gaps. Beyond monitoring, embedding sewer methane mitigation within MRV-compliant carbon credit schemes could transform emission control from an operational cost into a revenue stream. Four categories of control measures are evaluated, spanning hydraulic management, chemical dosing, physicochemical treatment, and biological intervention. Integrating microbial ecology, process engineering, monitoring technology, and environmental economics offers a practical route to managing sewer emissions within the wastewater sector's net-zero efforts.