Circularity assessment of wastewater plant using sludge for biomethane production for transportation

A membrane bioreactor wastewater treatment plant (WWTP) in Mikkeli, Finland, was assessed for environmental performance and circularity by combining life cycle assessment (LCA) with eco-efficiency metrics. The WWTP treated 3.16 × 10 6 m 3 of influent in 2023, producing sludge that generated 4.60 × 10 5 m 3 of biomethane via anaerobic digestion. LCA (ISO 14040/44) quantified global warming potentials (GWP100) of 0.862 kg CO 2 e/m 3 for treatment and 0.050 kg CO 2 e/m 3 for biomethane after accounting for a 0.290 kg CO 2 e/m 3 natural gas offset. Environmental impacts were monetized at €52/t CO 2 e to calculate the Weighted Environmental Impact (WEI), and combined with local value-added prices to derive the General Eco-Efficiency Index (GEI) per ISO14045. Annually, the system achieved a WEI of − €142 511 and a GEI of €7.80 × 10 6 , with biomethane production boosting circularity by 10.4%, (up to 56% maximum theoretical increase). This WEI & GEI framework offers a transparent, scalable tool for benchmarking environmental and economic performance across diverse circular economy platforms. • Case study of Circular Economy Platform in Mikkeli, Finland. • Piloting MBR WWTP converting sludge to biomethane for transport. • Assessed MBR wastewater treatment via global warming potential (GWP). • Calculated weighted environmental impacts (WEI) and eco-efficiency index (GEI). • WEI & GEI derived per ISO 14040/44 life cycle and ISO 14045 eco-efficiency standards. • Biomethane raises circularity by 10.4%. • Potential circularity increase up to 56%.