*1.1. Importance and Motivation*

With rapid urbanization and industrialization, humans consume increasing goods and services which cause the growth of direct and indirect urban solid waste generation [1]. Urban solid waste often has harmful impacts on human health and the ecological environment. Urban solid waste management, regarding the treatment of solid, liquid and/or atmospheric wastes before they are released into the environment is an issue of growing global concern [2]. In China, solid waste generation shows a trend of growth, and the corresponding utilization-disposal rate is trending downward. In 2011, the amounts of industrial solid waste and household garbage reached 3.62 billion and 0.16 billion Mg, respectively. The disposal rate and utilization rate were about 25.88% and 54.24%, respectively. In 2019, the amounts of industrial solid waste and household garbage increased to 3.86 billion and 0.24 billion Mg, respectively, whereas the disposal rate and utilization rate were about 24.31% and 53.33%, respectively. Investment in environmental protection occupied about 1.21% of GDP, while investment in solid waste production was much less [3]. Strategies that can help reduce the negative impacts of large amount of urban solid waste are desired [4].

Yang, Y.; Wu, X. A Factorial Ecological-Extended Physical Input-Output Model for Identifying Optimal Urban Solid Waste Path in Fujian Province, China. *Sustainability* **2021**, *13*, 8341. https://doi.org/ 10.3390/su13158341

**Citation:** Liu, J.; Li, Y.; Huang, G.;

Academic Editors: Julian Scott Yeomans and Mariia Kozlova

Received: 4 July 2021 Accepted: 23 July 2021 Published: 26 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The formulation of sound strategies requires the cooperation of numerous economic sectors [5]. When considering the city as a complex system, various economic sectors have direct or indirect relationships. The urban system can be treated as a network in which sectors are comparable to nodes and intersectoral transactions correspond to edges [6]. There are often sectors that are essential to reduce solid waste production as transfer centers [7]. These sectors are located in the middle of the supply chain path and simultaneously multiple supply chains of different path lengths [8]. The intermediate inputs of these sectors indirectly promote upstream production, while the intermediate outputs of these sectors are broadly used by downstream sectors resulting in more generation of direct solid waste production. Therefore, it is crucial to analyze urban solid waste generation from a systematic perspective aimed at recognizing the direct/indirect relationships among economic sectors, as well as assessing the direct solid waste (i.e., waste generated in the process of production), and indirect solid waste (i.e., when sector *i* receives products from other sectors, direct solid waste in the process of products production is the indirect solid waste of sector *i*) embodied in goods flowing within the regional and national scale economic system. This is helpful for global cities to achieve sustainable development target.
