Using a life cycle assessment method to determine the environmental impacts of manure utilisation: Biogas plant and composting systems

The aim of this study was to use life cycle assessment methods to determine the environmental impacts of manure utilisation by a biogas plant and by a typical manure composting system. The functional unit was defined as the average annual manure utilisation on a dairy farm with 100 cows. The environmental impact categories chosen were emissions of greenhouse gases (GHG) and acidification gases (AG). The GHG emissions were estimated as: 345.9 t CO ₂-equivalents (e) for solid composting (case 1), 625.4 t CO ₂-e for solid and liquid composting (case 2), and 86.3-90.1 t CO ₂-e for the biogas plant system. The AG emissions were estimated as: 10.1 t SO₂-e for case 1, 18.4 t SO₂-e for case 2, and 13.1-24.2 t SO₂-e for the biogas plant system. These results show that a biogas plant system produces low GHG emissions, but comparatively high AG emissions with land application. It is suggested that land application using band spread or shallow injection attachments will decrease AG emissions (NH ₃) from biogas plant systems.