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.

Purpose: As LCA is widely applied for benchmarking and decision-making, the need to clarify the ambiguity within fundamental methodological issues is imperative. Nutrient substitution, a subcategory of substitution, where credits are given for secondary fertilizer, is one of the common means to solve multi-functionality in LCA studies. This review aims to unravel the unique challenges associated with nutrient substitution, given the increasing relevance attributed to this topic. Methods: A systematic review of LCA studies available in Scopus and Web of Science (WoS) has been conducted. Studies about the recovery of nutrients from waste streams to produce fertilizer were scrutinized. As this review focuses on nutrient substitution methodology, only studies applying substitution for secondary fertilizer were included. PRISMA checklist has been used for reporting and completeness check of the review. Results are demonstrated from system modeling and explicit substitution procedure perspectives, supplemented by an investigation on sensitivity analysis. Results and discussion: As a general caveat, poor documentation and low transparency have been observed. Substitution has been used to model attributional (ALCA) and consequential LCA (CLCA) systems. The choice of functional unit combined with nutrient substitution in ALCA could attribute impacts to other functions than those studied. The determination of system boundary, especially the incorporation of the Use on Land (UoL) stage and avoided UoL emissions, is not always in accordance with the selected system modeling. Furthermore, there is no consensus on calculating the nutrient substitution rate. Single and aggregated factors comprising internal product quality, external-environmental, and external-societal variables have been identified. A prevalent observation among most studies is the absence of a sensitivity analysis pertaining to the nutrient substitution rate. Conclusion: The consistency of nutrient substitution cannot be achieved without an unambiguous definition and connotation of substitution and system modeling. The exclusion of the UoL phase not only limits the scope of a study but also fails to reflect quality differences between primary and secondary products. The key lies in elevating awareness regarding the intricacies of nutrient substitution, which consequently necessitates a rigorous definition and integration of influential factors when calculating substitutability.

Wind power forms the major contributor to Germany’s goal of transforming the energy sector and becoming climate-neutral until 2045. The increasing installation of wind turbines comes with an increasing demand for rare-earth elements, especially neodymium, praseodymium and dysprosium, to produce high-performing magnets. However, these elements are considered to be critical raw materials because of their supply risk and economic importance. The European Commission aims to ensure supply chain resilience by improving the circularity of these critical raw materials. After an average of 20 years, wind turbines transition into their End-of-Life phase. This work aims to map the present and future potential of NdFeB magnets used in wind turbines in Germany to be introduced into a circular economy resulting in material amounts of potentially recycled magnets and secondary rare-earth elements considering different potential End-of-Life pathways.