Grass as a C booster for manure-biogas in Estonia: a consequential LCA Sirli Pehme 1,* , Lorie Hamelin 2 , Eve Veromann 1 1 Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences 2 Department of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark  Corresponding author. E-mail: sirli.pehme@emu.ee ABSTRACT The aim of this study was to assess the environmental consequences of using grass (from both unused and cultivated boreal grasslands) as a co-substrate to dairy cow manure for biogas production. Environmental impact categories assessed were global warming, acidification and nutrient enrichment (distinguishing between N and P). Scenarios studied were: traditional management of dairy cow manure, mono- digestion of manure, manure co-digestion with reed canary grass and manure co-digestion with residual grass from semi-natural grass- lands. The latter scenario showed the best environmental performance for the global warming category, for other categories it did not show clear benefits. Using reed canary grass specially produced for biogas purpose resulted in a climate change impact just as big as the reference manure management, mainly as a result of indirect land use changes. Increased impacts also occurred in the acidification and eutrophication (N) categories for the reed canary grass scenario, reflecting the impacts of the cultivation process. The main conclusion was that future strategies for manure-biogas production in Estonia should not rely upon land-dependent biomass, even if the availability of arable land in Estonia is, under current conditions, not considered to be an issue. Keywords: anaerobic digestion, land use changes, dairy manure, reed canary grass, natural grass 1. Introduction Biogas production from manure has a good potential to simultaneously produce a renewable and flexible en- ergy carrier, while reducing the environmental impacts of manure management (mainly due to the reduced emis- sions from raw manure storage) and recycling biomass macronutrients (as well as the slowly degradable carbon) (Hamelin 2013). Although the energy produced from manure-biogas in the European Union (EU) is currently far below its full potential (Hamelin et al. 2014), a drastic increase of biogas production is nevertheless planned in the EU (Beurskens and Hekkenberg 2011), as well as in Estonia (Melts et al. 2013). However, due to the too low carbon (C) and carbon-to-nitrogen (C/N) content of animal manure, it is usual practice to supplement manure with C-rich co-substrates for anaerobic digestion. Grass, especially reed canary grass, has been considered to have great potential for biogas production mainly due to its relatively high yield and the fact that arable land resource is available in Estonia (Ministry of Economic Affairs and Communications 2010; Värnik et al. 2011). Two grass options were considered in this study: i) reed canary grass (this being one of the dedicated energy crops suggested to grow in Nordic countries) and ii) the residual grass from semi-natural grasslands, which is clearly underused currently but has a considerable biogas potential (Melts et al. 2013). The goal of this consequential life cycle assessment (LCA) study was to quantify the environmental consequences of implementing, in Estonia, a manure-biogas strategy relying on grass as a co-substrate (options i) and ii), as opposed to managing manure conventionally and not harvesting the grass from semi-natural areas, nor producing energy grass. The focus is on dairy cow manure, this being presenting the highest share from all manure types in Estonia (Luostarinen 2013). 2. Methods 2.1. LCA approach The life cycle impact assessment methodology used for this study was the EDIP2003 method described in Hauschild and Potting (2005) and the functional unit upon which all input and output flows were expressed was ”the management of 1 tonne of dairy cow manure ex-animal (i.e. the manure as freshly excreted by the ani- mals)”. Four impact categories were considered: global warming, acidification and nutrient enrichment (distin- guishing between N and P). Background data were based on Ecoinvent v.2.2 database (Frischknecht and Rebitzer 2005). Foreground data were mainly based on the Estonian situation, partly combined with Danish data. Proceedings of the 9th International Conference on Life Cycle Assessment in the Agri-Food Sector 970