Nutrient circularity from waste to fertilizer: A perspective from LCA studies

Nutrient circularity, an exemplification of circular economy (CE), is situated in the waste/wastewater-agriculture nexus. Recycling nutrient elements from waste streams to fertilizer products amplify the sustainable management of resources and intersect technical and biological loops, a concept developed for CE. Such a complex system needs to be directed by robust assessment methods such as life cycle assessment (LCA) to identify trade-offs and potentials. This review aims to provide a comprehensive outlook of the current state of nutrient circularity and a critical analysis on the applicability of LCA to nutrient CE pathways. Our worked has summarized CE pathways including direct land application, traditionally integrated processes in wastewater treatment plants, and targeted nutrient recycling technologies. Despite the restrictions on inputs streams, recycling technologies demonstrated a relative low selectivity. LCA is a powerful instrument to guide nutrient circularity; however, system modeling settings can confine the applicability of LCA for CE pathways. Given that LCA studies can only partially capture the CE characteristics, a deliberate methodological selection of functional unit, allocation method and impact indicators is required for the specific CE aspect under investigation. Lower data scale limits the LCA ability to assess CE practices that requires systemic analyses. Hence, full scale assessment is of necessity since it incorporates potential gains and drawbacks from the material upscaling, process efficiency changes and possible industrial symbiosis. The findings of this review lay a robust groundwork for future research, pinpointing areas of focus in LCA modeling within nutrient circularity. This is particularly vital for the Global South to ensure knowledge transfer and prompt action.