*2.1. Circular Economy and Multitude of Related Concepts*

Circular economy is a concept that has not been clearly defined in the literature so far. However, different propositions share much in common and converge towards the same paradigm [28]. Kirchherr et al. (2017) [29] view the CE as a market-based economic system that supports business models implementing the ideas of reducing, alternatively reusing, recycling, and recovering materials in the production, distribution, and consumption processes. Such reorientation of the economic system at all levels (products, companies, consumers, cities, regions, countries) shall lead to the environmental viability, welfare, and social equity for the current and future generations. The circular economy is defined in opposition to the linear 'make-take-waste' model and is understood as an extension of the concept of green economy or bioeconomy [30–33] and linked to a cleaner economy, a low emission economy, industrial symbiosis [34], industrial ecology, eco-industry [35,36], cradleto-cradle economy [37], Tech-Ökonomie [38], zero-waste economy, 'regenerative by design' economy [39], natural capitalism [40], green engineering, ecological modernisation [41], or sustainable development in general [42–46].

The bio-based CE is an economy where materials and energy are produced and derived from renewable biological sources [47,48]. Moreover, biological resources are managed in a way that their value is maintained at the highest level as long as possible [49]. Bioeconomic orientation of the CE is particularly suitable in sectors such agriculture [50], fertilizers [51], forestry [52], marine economy, pulp and paper, food production and retail [53], feedstock [54], cosmetics, biofuels, bioplastics [55], construction, furniture as well as bio-waste management [56,57], and wastewater treatment [58]. Metic et al. propose a concept of dual circularity, noting the existence of distinct, yet overlapping, thematic areas of a technology-focused CE and bio-based CE [59]. The area where 'bio' fuses with 'tech' includes, among others, such topics as microbial production, enzyme technology, and Green Chemistry [60].

Regardless of the definition, the implementation of the principles of a circular economy and the transformation towards less wasteful systems, a more effective and sustainable use of natural resources, and the reduction of pollutant emissions, including greenhouse gases, is becoming one of the key challenges worldwide [61]. Institutional, economic, environmental, organisational, social, technological, supply chain related drivers, barriers, and critical success factors determining the transition to a CE are discussed from different perspectives and at different levels of analysis [62]. Changing the economic systems is not possible in the short term horizon, and the practices that lead to the implementation of the circular economy postulates are introduced gradually [63]. Monitoring the progress of the performance at micro, meso, and macro levels [64] towards the circular economy is a complex and demanding task, mainly because of the multidimensionality and vagueness of the concept [65,66].
