Trade-offs between nutrient circularity and environmental impacts in the management of organic waste
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Measuring the circularity of resources is essential to assessing the performance of a circular economy. This work aims at proposing an indicator that quantifies how effective a system is at extending the lifetime of its waste components after they have been discarded. The developed indicator was applied to study the circularity of nutrients within a system that handles the organic waste (OW) generated in the Spanish region of Cantabria. A superstructure was developed to determine the optimal configuration of the system. It is composed of alternative unit processes for (1) the management of OW and (2) the application of the recovered products as soil amendment to grow corn. A multiobjective mixed integer linear programming problem was formulated under two policy scenarios with different source separation rates. The problem was optimized according to six objective functions: the circularity indicators of carbon, nitrogen, and phosphorus, which are maximized, and their associated environmental impacts to be minimized (global warming, marine eutrophication, and freshwater eutrophication). The model was fed with the life cycle assessment results obtained with the Environmental Assessment System for Environmental TECHnologies (EASETECH) version 2.3.6 and the nutrient flows in the agriculture subsystem, which were calculated with Denitrification–Decomposition (DNDC) version 9.5. It was concluded that improving nutrient circularity paradoxically leads to eutrophication impacts and that increasing the SSR of OW has a positive effect on the carbon footprint of the system.