Environmental impacts of alternative co-substrates for biogas production : a comparative life cycle assessment

Abstract: In recent years, the production of biogas from animal manure has gained increased attention in Denmark, as it has been identified as an important resource to reach the goal of a fossil free society by 2050. In addition, manure management with biogas production has been recognized as a viable way to reduce environmental impacts from animal production systems. Yet, because the methane production potential of animal manure is low, biogas plants depend on the addition of high energy organic wastes as co-substrates to manure, to make their operations profitable. The latter are in short supply and are already being imported in countries like Denmark. The use of different co-substrates and their biogas potential has been investigated, but there is presently a lack of knowledge about the environmental impacts of using one cosubstrate versus another. Therefore, this study assessed the environmental impacts of three co-substrates to pig slurry, which are currently underexploited; namely extruded wheat straw, the organic fraction of municipal solid waste, and the solid fraction of liquid-solid separated slurry. A comparative LCA was carried out, where the conventional manure management scheme of slurry storage and subsequent application to arable fields was compared to this three different ways of biogas production. Upon the analysis, extruded wheat straw was identified as a superior co-substrate. This is due to its low nutrient content, high methane yield potential, and low water content, which resulted in the lowest environmental impacts for eutrophication and the most savings for climate change potential. The second best co-substrate was identified to be the solid fraction of separated slurry and lastly the OFMSW had the most environmental impacts out of all scenarios, due to its relationship to energy production from incineration. A sensitivity analysis, where different methane yield potentials were tested for each co-substrate, was performed and the results proved to be robust. However, increase detail to the model is necessary to provide more confidence to the results, since system expansion activities proved to be crucial for the performance of each scenario.