Closing nutrient loops through decentralized anaerobic digestion of organic residues in agricultural regions: A multi-dimensional sustainability assessment

Decentralized anaerobic digestion (AD) of manure and organic residues is a possible strategy to improve carbon and nutrient cycling within agricultural regions, meanwhile generating renewable energy. To date, there has been limited adoption of decentralized AD technology in industrialized countries owing to low profitability for plant operators. There remains a need to demonstrate the wider sustainability of small-scale, decentralized AD in order to justify policy support for such a strategy. This study applies a multi-dimensional assessment of the environmental, economic and social sustainability of two scenarios of decentralized, farm-scale AD of pig slurry and organic residues in Southern Sweden. The environmental dimension was assessed by means of an expanded boundary life cycle assessment, in which trade-offs between fertilizer replacement, soil organic carbon accumulation, digestate/manure storage and application, transport and soil emissions were evaluated. The economic dimension was assessed through modelling of the net present value and internal rate of return. Finally, the social dimension was assessed by means of a stakeholder perception inquiry among key stakeholders in the field. It was concluded that the overall environmental balance of decentralized AD was favorable, while also the net present value could be positive. Fertilizer replacement, soil organic carbon and digestate storage effects were identified as important factors that should be accounted for in future life cycle assessments. A key issue for interviewed stakeholders was product quality assurance. Wider application of multi-dimensional sustainability assessment, capturing important nutrient cycling effects, could provide an evidence base for policy to support sustainable deployment of decentralized AD.