RESEARCH AND ANALYSIS
global paper life cycle from virgin inputs to end-of-life waste treatment. The discussion of environmental performance metrics led to three distinct conclusions.
efficiency metrics would be more useful if they counted in all these types of waste reuse. The term reuse is defined here as any further use of wastes including recycling, energy recovery, and non-energy recovery—all the uses that potentially substitute virgin inputs and contribute to the overall resource efficiency of production and consumption. The reuse of wastes can be included in efficiency metrics by considering the reuse potential of waste flows. The concept of a reuse potential has been suggested and explored by Park and Chertow (2014) for coal-combustion by-products. The waste reuse potential depends on the material properties and contex- tual factors and may change over time. A major challenge is data availability, especially since materials can have multiple uses. The reuse potential is operationalized as a value between 0 (no reuse possible) and 1 (full reuse possible) (Park and Chertow 2014). In part, standards for waste utilization are already found in documentation on best available techniques (Suhr et al. 2015). The reuse potential could be included numerically in material efficiency calculations. In addition, for complex material systems, such as analyzed in this article, the ideal total flow pattern could be assessed by assuming the full exploitation of the reuse potential of each flow. A Sankey dia- gram could serve to display both actual flows (as in this article) and ideal flows based on maximum waste reuse. The latter idea coincides with one of the first uses of the Sankey diagram by its namesake. In 1898, Sankey used two diagrams to compare ac- tual and ideal flows of energy flows in a steam engine (Schmidt 2008; Sankey 1898)—the same could be done for material flows. The assessments of recycling rates and material efficiency are closely related since both rely on the identification of a potential. For postconsumer waste, the reuse potential can be more precisely specified as a technical recycling potential andcon- cerns the amount of wastes actually available to be collected as resources for reprocessing. For material efficiency, the reuse potential of a number of process wastes should be considered. In other words: The performance of a system of production and consumption ought to be judged by the extent to which wastes that can be used as resources are actually used as resources. Im- portantly, a waste qualifies as a resource if it can (beneficially) substitute virgin inputs. This rule holds both for postconsumer waste and for industrial waste. The technical recycling poten- tial of waste paper was already calculated in this article. The calculation of the reuse potential of industrial wastes requires detailed knowledge of the relevant flows and the establishment of standards for waste reuse and is left for further study, as is the challenge of prioritizing between different types of waste reuse. Conclusions This study calculated detailed global paper flows and critically discussed recycling and material efficiency metrics. The material balance was presented as a Sankey diagram and displays, for the first time, material flows in all stages of the
1. The currently common recycling metric divides paper for recycling by total paper production. This metric does not directly stimulate avoidance of virgin inputs and associ- ated impacts. A better indicator is the RIR, which divides paper for recycling by total fibrous inputs. 2. Recycling metrics are more meaningful if the achievable potential is known. The technical recycling potential is con- strained by additions to stock and losses to sewage. As- suming effective control of contamination, the fraction of paper for recycling in total fibrous inputs can still be almost doubled. 3. Material efficiency should consider both final products and reused wastes as outputs of a process. The reuse of wastes can be contrasted with the reuse potential , which depends on material properties and contextual factors. The fulfillment of the reuse potential may be included in material efficiency metrics. Further research should build on the above three conclu- sions. This study provided a start by mapping the global flows of paper. Future work could assess the reuse potential of the dif- ferent waste flows and their fulfillment in a circular economy. In addition, the material balance can be used as the basis for a variety of environmental assessments.
Acknowledgments The authors express their gratitude to Simon Weston and two anonymous reviewers for their comments.
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