## 3 Literature Overview and Indicator Selection  
### Literature Overview  
Various indicators have been proposed in the literature for measuring circular economies at three levels: macro (global, national, regional, city), meso (industrial symbiosis, eco-industrial parks), and micro (single firm, product) [19; 20; 21]. This study focuses on meso-level indicators that \"focus on the industry, consumption activity or particular material level helping to detect waste of materials, pollution sources and opportunities for efficiency gains in specific sectors or consumption domains\" [22].  
A review by [PERSON] et al. [23] retrieved a comprehensive set of 61 circularity indicators at the three levels. Several meso-level indicators have been developed to assess economic and environmental performance connected to existing industrial parks, such as the one in Kalundborg (Denmark) [24], or the five located in the Jiangsu province (China) [25]. On the other hand, qualitative and quantitative methods are used by scholars to evaluate IS impact on the companies involved and surrounding communities in terms of enhanced sustainability [26]. Many authors have suggested IS performance indicators with different properties and relations to business and sustainability goals; [PERSON] and [PERSON] [27] identified eight studies presenting indicators or sets of indicators. The main purposes of the indicators are to evaluate the level, or density, of symbiotic relationships within industrial areas, the degree of waste recycling or reuse as raw material (including hazardous materials), and the reduced environmental impact. A wider IS indicator taxonomy [28] shows that indicators investigating environmental and economic benefits are the most used, while social issues are still poorly considered; other indicators consider the structural characteristics of IS, such as network properties or the spatial scale of IS relationships.  
Some hybrid indicators integrating more than one sustainability and/or structural dimension have also been developed. [PERSON] and [PERSON] (2014) [29] use a hybrid indicator, composed of one economic and three environmental indicators, based on the eco-efficiency concept introduced by the World Business Council for Sustainable Development; eco-efficiency considers the relationship between the value of the output obtained (product or service) and the environmental impact generated. An ecosystem approach is adopted by [PERSON] et al. [19], who designed five classes of indicators to assess the effectiveness of waste exchanges and quantify the specific contributions of individual firms/waste exchanges to network operations. [PERSON] et al. [18] developed an Industrial Symbiosis Indicator (ISI) for monitoring the evolution of IS networking within an eco-industrial park, intending to provide a decision-making tool for IS stakeholders. The combination of qualitative and quantitative indicators to evaluate the technical feasibility, economic viability, and environmental benefits of potential resource exchanges are proposed by [PERSON] et al. [20]. The key performance quantitative indicator system set up by [PERSON] and [PERSON] [30] consists of four subsystems including economic, environmental, and social indicators, and a general indicator system considering IS structure, activity, knowledge transfer, resilience, and adaptability issues.