Environmental impact assessment of biochar in anaerobic digestate

Abstract

Climate change and unsustainable crop production are among the most prominent environmental issues and viable solutions must be looked for. A circular economy approach by recycling biochar-digestate back to agriculture as a fertilizer can be one of the means for tackling these problems. This thesis presents a Gate-to-Cradle life cycle assessment (LCA) for the global warming potential (GWP) of biochar-amended digestate in agriculture compared to mineral fertilizer.

The LCA was performed as case a study for Lindum where digestate was obtained from The Magic Factory (TMF) and biochar from Lindum’s facility. The biochar-to-digestate ratios of 0.07% w/w, 6.25% w/w, and 12.5% w/w were able to mitigate greenhouse gases (GHGs) by 88 kg CO2-eq./daa (80%), 323 kg CO2-eq./daa (296%), and 592 kg CO2-eq./daa (542%), respectively. The two emission-mitigating keys were the reduced N2O fluxes and the C storage from biochar and digestate. C storage counterbalanced other emissions such as spreading, transport, agricultural emissions, and pyrolysis resulting in total environmental benefits. One of the LCA hotspots is the amount of biochar applied, because of its significant contribution to C abatement and reduced N2O fluxes. Another important thing emphasized in this thesis is the agricultural benefits of applying biochar to digestate such as nutrient retention and increased soil C.

Furthermore, this thesis exhibits clear results that biochar-amended digestate can serve benefits for the climate. In addition, biochar in digestate can achieve positive agricultural effects which are important for sustainable food production.