Environmental consequences of different carbon alternatives for increased manure-based biogas

Manure-biogas is a renewable energy resource rather untapped in Europe in comparison to its full potential. Given the current and emerging renewable energy targets, considerable increases in its production can be expected. This consequential life cycle assessment (LCA) study investigated the environmental consequences of different co-substrate strategies for reaching drastic increases in manure-biogas production in Denmark. Six co-substrates not already fully used for biogas were considered: energy crops, straw, household food waste, commercial food waste, garden waste and the solid fraction deriving from source-segregation of animal urine and feces. Soil carbon changes as well as direct and indirect land use changes were included in the LCA. Source-segregated manure stood out as the environmentally best co-substrate, followed by garden waste. Co-substrates already in use for energy recovery (straw, household and commercial food wastes) displayed a more modest environmental performance while energy crops, here represented by maize silage, was the only option giving rise to net greenhouse gas emissions. This was essentially due to the indirect land use change emissions related to this scenario, which were quantified to 357 t CO2 eq. ha−1 displaced.