Environmental impact of milk and electricity production from dairy farms with biogas plants of different size and feeding system

Anaerobic digestion is an effective method to decrease the environmental impact of milk production and to generate renewable energy. A dairy farm with an annexed biogas plant produces three outputs, milk, meat and electricity, and should be considered as a single system for the evaluation of the environmental impact. The aim of this study was to investigate to what extent the size and feeding methods of the biogas plants in three dairy farms with different features affect the environmental impact related to the milk and electricity produced. The first farm was characterized by a biogas plant of 300 kW fed only with cattle manure, the second farm had a plant of 526 kW fed with cattle manure, maize silage and waste of maize grain production, while the third farm had a biogas plant of 999 kW fed with cattle and poultry manure, maize and triticale silage, and fats from slaugh-terhouse waste. The Life Cycle Assessment involved the environmental impact of Fat Protein Corrected Milk (FPCM), in the first instance with meat and electricity from biogas considered with an economic allocation, and then considered a credit. Successively, the environmental consequences of electricity were evaluated considering milk and meat as credits. With the economic allocation, Carbon Footprint (CFP) of one kg of FPCM was 0.82, 0.85 and 0.72 kg CO2 eq in the first, second and third farm, respectively. In all the three farms, electricity generation from biogas determined a reduction in the environmental impact of milk production thanks to a credit of emissions provided by the production of renewable energy. Milk CFP in the three farms was 0.82, 0.91 and 0.13 kg CO2 eq respectively, with mitigation from biogas of 11%, 11% and 85%. The best performances were obtained in the third farm thanks to higher levels of electricity generation, covered digestate storage and the large use of agri-industrial waste. Electricity CFP in the farms was-0.74,-0.68 and-0.06 kg CO2 eq/kWh respectively. Environmental consequences of electricity production from biogas plants were compensated in all the farms thanks to the credits provided by milk; this compensation was as greater as the ratio between milk and electricity production and the first farm reached better outcomes. In the context of reducing the environmental impact of animal foods and implementing an ecological transition to reach national and European goals for renewable energy production, the work could be beneficial to assess which combination between dairy and energy products should be promoted.