Improved environmental sustainability and bioenergy recovery through pig manure and food waste on-farm co-digestion in Ireland

Ireland is under a great pressure to fulfill its commitment to renewable energy generation and greenhouse gas (GHG) emissions mitigation. Food waste (FW) and pig manure (PM) co-digestion may provide a promising solution. A life cycle assessment was performed in this study to compare the environmental impacts of the FW and PM co-digestion scenario with existing PM and FW management practices in Ireland (Baseline), which are direct land application of PM, and composting and mono-digestion of FW. The nutrient availability, hotspots, and the effect of FW proportion in co-digestion substrates were analyzed. The results indicate that compared with the Baseline, the co-digestion scenario performs better in 9 of the 11 environmental categories assessed. The nitrogen availability of the digestate in the co-digestion scenario (43%) is higher than that of the organic fertilizers in the Baseline (34%). The methane emission during digestate storage, ammonia emission during digestate land application, and energy consumption during anaerobic digestion are hotspots with significant effects on environmental impacts, and mitigation methods are proposed. Based on an average size pig farm (762 sows) producing 16,000 t/yr PM, the global warming potential (GWP) becomes negative only when the FW proportion is over 2000 t/yr. This research can provide references to farmers, the gas industry and policy makers for effective fertilization, optimized on-farm AD plant operation and environmentally sustainable management of biowastes.