The principles of the circular economy are transforming how we view waste materials. The use of life cycle assessment (LCA) methodology to quantify the environmental impacts of waste management has been subject to assumptions and approaches that influence the interpretation of results. This study analyzed the implications of the transition from a linear economy where waste is valueless and managed for disposal to a circular 'cradle-to-cradle' economy where waste is valorized as a resource. A LCA study was carried out viewed from two stakeholder perspectives (i) waste disposal; and (ii) waste valorization by nutrient recovery. The composting of two domestic wastes was considered: wasted food and green (garden) waste. For the waste disposal perspective the zero-burden-assumption was adopted and a functional unit of kg waste was used. For the waste valorization perspective the upstream impact was included and a downstream functional unit of kg available nitrogen (N) was used. From the waste disposal perspective using a functional unit of kg waste, composting caused less impact than landfill. However, from the nutrient recovery perspective using a functional unit of kg available N, the business as-usual synthetic fertilizer system had the lowest impact. This was because of the upstream impact of producing and processing food. How waste is perceived, and the subsequent assumptions and methodological choices made at the goal and scope stage of an LCA have a significant impact on the results and interpretations of an LCA study. When the focus of a system is waste valorization in a circular economy, a functional unit of per kg waste is not appropriate. It was concluded that to properly determine the impacts of valorizing waste materials in the circular economy, the upstream impacts must be considered and a functional unit that reflects the function of the downstream, secondary processing is required. (C) 2017 Elsevier Ltd. All rights reserved.
