Novel farm-scale bioenergy and waste management: A life cycle assessment of anaerobic co-digestion and manure composting

This 13-month study assessed energy production from on-farm anaerobic co-digestion of liquid dairy manure (DM) and food waste (FW) with a life cycle assessment (LCA) conducted to determine the environmental impact of the on-farm system. The co-digestion substrates generated 1.83 million m3 biogas/yr, with 56.2 % methane (CH4) in the biogas. Only 72.2 % of the biogas was used in the 240-kW combined heat and power generator, producing 224 kW/h (1.98 million kWh/yr), which is enough energy to electrify 190 US households. Excess biogas was flared. Lab-scale tests showed that co-digestion was 77 % higher (987 mL CH4/g VS) than DM mono-digestion (224 mL CH4/g VS), with the field conditions (36.3 °C) having 44 % less energy production than lab conditions (35 °C), possibly due to variability in food waste processing substrates additions over time. The DM solids (4703 m3/yr) were separated prior to digestion and composted, with lab scale studies showing only 20 % more energy (280 mL CH4/g VS), if the more recalcitrant manure solids were included. Together, the co-digestion and composting systems achieved 81 % reduction in greenhouse gas emissions, offsetting 19,256 tons CO2/yr. The LCA showed reductions in acidification (320 %), eutrophication (447 %), carcinogens (42 %), and ecotoxicity (347 %), but also highlighted the importance of FW transportation distance. The poultry and cranberry food processing wastes were 50 and 167 km from the farm, with 15 % additional emission reductions possible with use of local FW sources. The findings highlight the effectiveness of co-digesting manure with food waste to produce renewable energy and reduce emissions at the farm-scale.