Environmental Sustainability Assessment of Pig Manure Mono- and Co-Digestion and Dynamic Land Application of the Digestate

In this study, the environmental impacts of pig manure direct land application and anaerobic digestion (mono- and co-digestion) for biomethane production followed by digestate land application were evaluated by life cycle assessment, with a focus on the dynamic land application of digestate in terms of the nutrient profiles, soil nutrient status and environmental regulations. Ireland was used as a case study where co-digestion of manure with grass silage is viable, and an Irish emission inventory was established. The results indicate that pig manure and grass silage co-digestion performs best in 9 of the 11 impact categories assessed. Compared with direct land application, mono-digestion of pig manure decreases direct greenhouse gas emissions by 48% (190 tonne CO2e). Co-digestion with grass silage increases the total energy recovery by 226% (1592 MWh) than mono-digestion. The nitrogen available for plants in digestate (41.8%) is a little lower than that in raw pig manure (43.2%) due to the higher ammonia emissions after anaerobic degradation. The environmental impacts of the three pig manure management methods are greatly affected by soil nutrient conditions and regional environmental regulations, which determine the organic fertilizer application rates and consequently the required land areas and transport distances. In this case study, the land application area for digestate is determined by phosphorus acceptance of soils rather than nitrogen or potassium. This study provides practical insights to farmers, the gas industry, and policy makers to conduct effective organic fertilizer application, select optimal AD plant location, and maximize environmental benefits.