Environmental performance of manure co-digestion with natural and cultivated grass - A consequential life cycle assessment

The aim of this study was to assess the environmental consequences of increased manure-based biogas production relying on grass as a co-substrate (from both unused and cultivated boreal grasslands). Through consequential life cycle assessment, three biogas scenarios were investigated: i) mono-digestion of dairy cow manure, ii) manure co-digestion with reed canary grass cultivated specifically for bioenergy production and iii) manure co-digestion with unused grass from semi-natural grasslands. A full balance of biogenic carbon was considered including soil carbon changes and indirect land use changes. Mono digestion of manure showed a potential for an improved environmental performance for global warming and phosphorus-eutrophication, in comparison to conventional manure management, but yielded more than 2 times lower energy production compared to co-digestion. Co-digestion with grass from semi natural grasslands showed an even 41% better potential to reduce global warming and resulted 2 times lower phosphorus-eutrophication compared to mono-digestion, provided that the grass would have otherwise been left un-harvested on land. Because of the indirect land use change associated with an additional demand for land, and the need for additional fertilizers, co-digestion with cultivated grass showed a 26% worse global warming, 2 times higher acidification, 4 times higher nitrogen-eutrophication and 36% worse phosphorus-eutrophication performance compared to natural grass. Results highlighted that grass co-digestion with manure does lead to an enhanced performance of the global warming and phosphorus-induced eutrophication impacts. This conclusion, however, did not apply for nitrogen-related impacts categories (acidification and nitrogen-induced eutrophication). Results were strongly affected by the choice of the indirect land use change factor for modelling and the energy source displaced. In a nutshell, this study highlighted the environmental relevance of considering energy grass and in particular semi-natural grasslands for the production of manure-based biogas, though it showed the necessity to improve the nitrogen balance of the supply chain of these scenarios, and to carefully consider the counterfactual use of the grass stream. (C) 2017 Elsevier Ltd. All rights reserved.