Toward Consistent Life Cycle Assessment for Nutrient Circularity: A Framework Based on Avoided Burden and Substitution for Decision-Making

As essential inputs for sustaining soil fertility and food production, fertilizers are increasingly applied to meet the rising demand for agricultural outputs. Environmental concerns over energy use and resource depletion have driven efforts in the fertilizer industry toward sustainable production. Recent geopolitical tensions have also revealed supply chain vulnerabilities, making the development of alternative feedstocks for fertilizer production an urgent priority. Waste and wastewater streams represents nutrient-rich resources suitable for producing secondary fertilizers, helping reduce discharge burdens and support resource recovery in line with circular economy (CE) principles. However, environmental benefits from a circular system are not automatically guaranteed; sustainable nutrient circularity depends on a deep understanding of the CE system dynamics and methodology in environmental assessment. Life cycle assessment (LCA) is a standardized method for evaluating the potential environmental impacts of a product relative to a functional unit (FU). In applying LCA to circular end-of-life systems such as nutrient circularity, a significant challenge arises from multifunctionality. Substitution, commonly employed to address this issue, assigns avoided burdens to non-FU function. However, it is not the sole method to assign credits. In consequential LCA, avoided burden captures changes in environmental impacts driven by the FU. Inconsistencies in modeling avoided burden are widespread, risking greenwashing and undermining the reliability of decision-making. This dissertation explores nutrient circularity, defined as the recycling of nutrients from waste and wastewater streams into fertilizers, and presents a consistent framework for modeling avoided burden to support robust decision-making. It consists of three sequential publications, each addressing distinct yet interconnected dimensions of the topic. The first publication (literature review) offers a comprehensive assessment of the current state of nutrient circularity and critically examines the role of LCA in evaluating nutrient CE pathways. The literature review synthesizes existing CE pathways, encompassing direct land application, integrated processes that are commonly employed in wastewater treatment plants, and targeted nutrient recycling technologies. These CE pathways establish a basis for systematically categorizing their characteristics to support their accurate representation in LCA studies. The second publication (literature review) deepens the methodological discussion by analyzing substitution and avoided burden, highlighting both theoretical foundations and the practical challenges involved in quantifying the substitution potential of secondary fertilizers. It identifies that although substitution is frequently used in LCA, its application often lacks coherence with other methodological choices, such as FU and system boundary, resulting in an arbitrary assignment of avoided burdens. Additionally, consensus is lacking on how to calculate the fertilizer technical substitution rate. A range of parameters, from internal product quality to external environmental and societal factors, is identified as key determinants in shaping the comparison between virgin and secondary fertilizers. Building on identified gaps in the literature, the third paper presents a unified framework to clarify the distinction and calculation of avoided burden and substitution in LCA. It pinpoints that avoided burden reflects broader system-level effects than substitution. The stepwise framework supports scientifically rigorous LCA design in nutrient circularity by aligning functional units, system boundaries, research objectives, and modeling choices, improving consistency and reliability for decision-making. This framework also contributes a method based on plant nutrient uptake for calculating avoided burden and provides 65 technical substitution potentials across nine secondary fertilizer classes. By distinguishing product quality from environmental influences and reducing sampling bias through statistical analysis, it improves the robustness of life cycle inventory data. A case study on phosphorus recycling in Germany illustrates the framework’s application through 14 scenarios combining three functional units, two LCA modeling approaches, and three levels of system boundaries. The study highlights the importance of product-specific LCA aligned with the proposed framework. Results show that different system boundaries yield different avoided burdens, even for the same functional unit, and that system modeling choices primarily shape how these burdens appear. Substitution potential influences results only certain scenarios, while inclusion of the use-on-land (UoL) phase notably influences toxicity indicators, offering a basis for comparing secondary and commercial fertilizer quality. This dissertation clarifies the distinct role of avoided burden and substitution in nutrient circularity, addressing methodological ambiguities and promoting consistent application in LCA. By including various forms of avoided burden and their environmental co-benefits and co-costs, the framework helps prevent greenwashing and supports methodological rigor. Given the role of avoided burden in crediting systems, quality criteria are essential to avoid non-additional or short-term claims. The framework supports the further development and integration in harmonized LCA (e.g., Environmental Product Declaration), carbon credits, and system redesign under Safe and Sustainable by Design (SSbD). It also enhances phosphorus accounting, encouraging future research on improving the phosphorus cycle through circular economy strategies. In addition, it strengthens phosphorus flow representation in LCA, opening new avenues for further research focused on improving the phosphorus cycle through nutrient circularity.