Techno-economic viability of a pilot-scale ex-situ biomethanation process integrated into a WWTP for urban bus fuelling

This study presents the operation and comprehensive techno-economic assessment (TEA) of a pilot-scale ex-situ biomethanation system integrated within an urban wastewater treatment plant (WWTP) in Barcelona. A 4.7 m 3 anaerobic trickling bed reactor (TBR) was operated for over 650 days, utilizing local digester biogas and green hydrogen to produce high-purity biomethane (>95% CH 4 ) for urban bus fuelling. During stable continuous operation, the reactor achieved a methane production rate (MPR) of 3.4 ± 0.1 Nm 3 CH4 /m 3 bed /day, and an energy efficiency of 69.4 ± 0.7%, with temperature identified as the primary limiting factor. The TEA integrated life cycle costing (LCC) across three production scenarios: fossil natural gas baseline, ex-situ biomethanation (S1), and membrane separation upgrading (S2); and two distribution models: direct bus fuelling (M1) and grid injection (M2). At a baseline electricity price of 0.12 €/kWh and 30% annual load factor (LF), the levelised cost of biomethane (LCOB) was 126.0 €/MWh HHV for S1 and 47.8 €/MWh HHV for S2, with electricity representing approximately 79% of S1 operational expenditure. Sensitivity analysis identified five convergence conditions for economic competitiveness: electrolyser CAPEX below 500 €/kWe, electricity prices below 0.05 €/kWh, LF above 50%, by-product valorization, and access to biomethane selling premiums. While membrane separation offers superior cost-effectiveness under current conditions, biomethanation provides 55% higher biomethane yield through CO 2 (g) conversion and stronger decarbonization potential. Overall, enhanced governmental support mechanisms are essential to improve profitability and enable the deployment and demonstration of biomethanation technologies at a semi-industrial scale.