Environmental impact of manure treatment and utilization system with biogas plant by life cycle assessment method: Comparison with other systems

The aim of this study were to evaluate the environmental impacts of manure utilization system at Biogas plant compared to Composting system (composting depot) and Separating system (solid composting at depot and liquid composting at slurry store), using the life cycle assessment method. The functional unit was defined as annual manure utilization from 100 dairy cows. The processes of each system associated with manure treatment equipments construction, manure treatment and manure application, and these processes were included in system boundary. Especially, manure application processes were set manure spreader for compost, slurry spreader with splash plate (SP) for liquid compost, slurry tanker with splash plate, band spread (BS) and slurry injection (SI) for digested slurry. The environmental impact categories to evaluate were defined as global warming potential (GWP) and acidification potential (AP). As the results, the GWP were estimated 346t-CO2eq at Composting system, 625t-CO2eq at Separating system and 86-90t-CO2eq at Biogas plant. It showed that the Biogas plant was relatively low GWP for manure utilization system regardless of types of application. The AP were estimated 10t-SO2eq at Composting system, 18t-SO2eq at the Separating system, 13-24t-SO2eq at Biogas plant, and the item which gave the largest impact was NH3(98-99%) at all systems. Especially, the large part of NH3 emission from the Biogas plant was caused at manure (digested slurry) utilization process. However, it was showed that the AP impact was decreasing at the cases of digested slurry application with BS or SI compared with the case of using SP. These results led to the conclusion that the Biogas plant prevents global warming gas emissions, and the digested slurry application with BS or SI was a necessary condition for decreasing AP impact (NH3 emission) at the Biogas plant utilization.