Life cycle assessment of household plastic waste recycling in urban areas of Japan: evaluating municipal and supermarket recycling systems for sustainable urban transitions

Household plastic waste (HPW) management is important for urban sustainability transitions. However, the environmental performance of HPW recycling depends on the combined effects of different recycling systems and technologies. This study assesses the environmental impacts of HPW recycling in urban areas of Japan using a life cycle assessment framework that integrates municipal and supermarket recycling systems. Five scenarios were developed, complemented by a sensitivity analysis of alternative recycling structures. Scenario analysis shows that expanding the supermarket recycling system from 5% to 20% results in only marginal reductions in recycling-chain environmental impacts, with global warming potential (GWP) decreasing by 1.2%. Although supermarket recycling improves source separation and material quality, its contribution to overall environmental performance remains limited when downstream recycling structures are unchanged. Sensitivity analysis of the municipal recycling system reveals substantial environmental benefits when thermal recycling (TR) is progressively reduced. When the share of TR decreases from 66% to zero, GWP decreases by approximately 15%, and eutrophication potential (EPMC) decreases by up to 38%. In the supermarket recycling system, increasing material recycling (MR) to approximately 86% decreases all indicators, whereas further increases in MR diminish or reverse the benefits across several indicators. Overall, the results demonstrate that sustainable urban transitions in HPW recycling cannot be achieved solely by expanding the supermarket system. Instead, coordinated adjustments to both recycling systems and the allocation of recycling technology are required to achieve meaningful environmental improvements. This study provides recycling-chain-level evidence to support urban policymakers in designing more sustainable recycling systems.