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.