Life cycle environmental impacts of compressed biogas production through anaerobic digestion of manure and municipal organic waste

Abstract

Compressed biogas (CBG) has proven to reduce greenhouse gas (GHG) emissions compared to fossil diesel, but due to diverging methodologies, results of different studies lie far apart. The usage of the co-produced digestate can lead to high acidification and eutrophication impacts, but this aspect is often not considered, neither is the way different substitution methods for the digestate affect the results. In this work we address these issues performing a Life Cycle Assessment (LCA) of CBG production through anaerobic digestion of animal manure and municipal organic waste (MOW). GHG savings of 93%–131% compared to the fossil diesel benchmark in the Renewable Energy Directive were found, thus complying with the 60% reduction target. The ammonia emissions during the digestate application make acidification and terrestrial eutrophication the main environmental burdens of CBG from MOW, leading to a 142% and a 715% increase for acidification and terrestrial eutrophication respectively, compared to fossil diesel. When CBG is produced from manure and the avoidance of manure storage emission is considered, a 257% and a 700% lower acidification and terrestrial eutrophication impact was found.Our results suggest that better manure-storage techniques are required and that field emissions due to digestate application, which is sensitive to pH and nitrogen content, should be reduced. It was further demonstrated that including the digestate fertilizing emissions in the system boundaries of the LCA of CBG production and substitution of both mineral fertilizer production and usage have a great impact on the outcome, but also introduce uncertainty of the results. The outcome of this work was used to suggest recommendations on the methodology for future LCA studies on CBG production and EU biofuel regulations.