The Ecological Potential of Manure Utilisation in Small-Scale Biogas Plants

The main sources of greenhouse gas emissions, accelerating global climate change, are heat and electricity generation. To lower these emissions, an expansion of renewable energy usage is required. Biogas plants, a flexible renewable power source, are one possibility, and are already widely established in the European energy system. This study focuses on the utilisation of raw manure in closed systems to reduce direct CO2 eq. emissions. It is the first to compare manure treatment in different types of small-scale biogas applications and under the impact of increasing temperatures resulting from climate change. The environmental impact in terms of four impact categories is evaluated by means of a life cycle assessment. Two cases are investigated: a biogas plant with either subsequent combustion in a combined heat and power plant or the direct usage of biogas as a simplified and less expensive application. The analysis shows that the first case yields ‑173 kg CO2 eq. per m³ of manure, whereas the simplified one causes 20.9 kg CO2 eq. per m³ of manure. If the first case is scaled with the currently existing number of small-manure plants in Germany, emissions of 464 mil. t CO2 eq. are mitigated per year. With increasing average annual temperatures, higher manure credits are generated and so the emissions of both plant options are reduced to ‑264 and ‑69.5 kg CO2 eq. per m³ of manure, respectively, ascribing the direct biogas usage reductions of GHG emissions. Consequently, both systems have the potential for reducing emissions due to improved manure management and can contribute to mitigating climate change.