Optimizing transport to maximize nutrient recycling and green energy recovery

Abstract

A circular biobased economy must be able to sustainably manage multiple resources simultaneously. Nutrient (nitrogen, phosphorus, and potassium) recycling and renewable energy production (biogas) can be compatible practices but require substantial transport of heavy organic waste. We combine a spatial optimization model and Life Cycle Assessment (LCA) to explore how Sweden could maximize its use of excreta resources. We use $10\times 10$ km${}^2$ resolution data on the location of animal and human excreta and crop demand and model both optimal biogas plant locations and transport of nutrients to and from these plants. Each type of biogas plant (given 4 realistic mixes of excreta) is then evaluated for global warming potential, primary energy use and financial resource costs. Moving excreta through biogas plants, as opposed to simply reapplying on fields, to meet crop nutrient demands comes at a similar cost but the climate and primary energy savings are substantial. As much as 91% of phosphorus and 44% of nitrogen crop demand could be met via optimally transported excreta and the country would avoid about 1 450 kt of CO₂-eq, save 3.6 TWh (13 000 tera-joules) of primary energy, and save 90 million euros per year. Substituting mineral fertilizers with recycled nutrients results in savings across all indicators, but the added energy and avoided greenhouse gas emissions associated with biogas production make a large difference in the attractiveness of nutrient recycling. Although the numeric values are theoretical, our results indicate that carefully coordinated and supported biogas production could help maximize multi-resource benefits.