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Biomethane produced from waste-derived biomass (biowaste) is a clean and renewable fuel, which offers substantial reductions of greenhouse gas emissions and resource consumption. Biomethane is currently produced via the "biogas road", which includes the anaerobic digestion of wet biowaste and a successive upgrading of obtained biogas, with good environmental performance. An alternative production strategy is the "syngas road", which includes the gasification of dry or semi-dry biowaste followed by cleaning, conditioning, methanation, and final upgrading of obtained syngas. It is still at a demonstration level but appears of great interest for the highest values of energy efficiency and carbon utilisation. The study acquired technical data from existing plants of both these strategies, and developed a quantitative environmental assessment by means of holistic tools of sustainable engineering: material and substance flow analyses and an attributional life cycle assessment. The technical and environmental performance of the two biomethane roads have been then compared, in terms of energy and process efficiency as well as potential impacts in the main midpoint categories (global warming, respiratory inorganics, and non-renewable energy). The syngas road appears to have higher levels of carbon utilisation and better environmental performance, even though an extended sensitivity analysis shows different results if alternative plant configurations and energy mix are considered. This suggests that R&D studies and policies of economic incentives have to be further implemented for both the analysed strategies, even because they deal with different kind of biowaste, having different availability in different countries. (C) 2019 Elsevier Ltd. All rights reserved.