Designing Value Chains of Plastic and Paper Carrier Bags for a Sustainable and Circular Economy

Abstract

Progress toward a sustainable and circular economy (SCE) is crucial to mitigate the large-scale exploitation of natural resources and the pile-up of man-made materials in the environment. Carrier bags form a large component of discarded and mismanaged plastic waste whose value can be retained through a SCE. A novel design framework developed in our previous work, has been applied to design optimal value chains based on a superstructure network containing cradle-to-cradle life cycles of some currently available carrier bag alternatives. Employing the design framework allows systematic exploration of a large number of scenarios with combinations of numerous alternatives, by virtue of using global optimization methods. Life-cycle metrics such as global warming potential and life-cycle cost have been supplemented with a novel general circularity metric that is formulated to measure (a) economic returns and (b) ecological regeneration. These metrics constitute the objectives of the framework, and pareto optimal solutions are found to quantify trade-offs present while designing for these SCE objectives, without assigning arbitrary weights. In doing so, the long-standing paper versus plastic dilemma is quantified using systematic cradle-to-cradle design approaches, and optimal value-chain reforms for SCE are found. The prospects of biobased polylactic acid bags are also studied. We are also able to affirm that advancing technologies for mechanical recycling and policy action for societal-change and value-chain reforms can lead to win–win solutions in the SCE objective domains. Some general conclusions derived from the study are as follows: low-density polyethylene bags (with 10 reuses) perform best in minimizing global warming potential and life-cycle cost, single-use high-density polyethene bags with recycling give the highest economic circularity, whereas paper bags being landfilled lead to the most ecological circularity as they readily compost to return nutrients to the environment. Trade-off solutions are found from pareto surfaces in order to achieve the best possible compromise, and it contains combination of paper and reusable plastic bags. It is observed that increasing substitutability of virgin material with recycled material favors use of biobased polylactic acid bags. Hot spot sectors for atmospheric emissions and circularity are also found for optimal value chains, which can help direct research toward activities of importance.