**2. The circular economy**

The concept of CE originates in different schools of thought, including industrial ecology, general systems theory, and ecological and environmental economics [14]. Its conceptual roots can be traced back to notions put forth decades ago, such as the "Spaceship economy," [15] the irreversible degradation of natural resources when used by economic activities [16], the economy of loops [17], and the analogy between ecosystems and industrial systems [18]. The contemporary conceptualizations of CE include features from relevant concepts, including, but not limited to, the regenerative design [19], industrial symbiosis [20], "cradle to cradle" design [21], and performance economy [22].

Over the past 10–15 years, the CE has been attracting increasing attention from academia, companies, citizens, and policymakers [23]. It is regarded as a potential solution to the challenges of resource depletion and environmental degradation caused by the unsustainable linear "take–make–waste" paradigm that has dominated the contemporary economic systems [1, 2]. To address these challenges, the CE promotes system innovations that aim to maximize resource value, promote the cascading use of renewable resources and minimize waste generation to reduce negative environmental impacts and build natural, social, and economic capital [1, 24].

Overall, there is a general understanding that the CE is connected to sustainability and sustainable development. Geissdoerfer et al. [23] identified three different general

### *The Role of Biochar Systems in the Circular Economy: Biomass Waste Valorization and Soil… DOI: http://dx.doi.org/10.5772/intechopen.104389*

types of relationships between the CE and sustainability; 1) conditional, where the CE is seen as one of the main conditions to attain sustainability, 2) beneficial, where the CE is regarded as beneficial in regard to sustainability, and 3) trade–off, where the CE is seen as a concept that can generate both benefits and costs in terms of sustainability. Having this study as a point of departure, Suárez-Eiroa et al. [25] suggested that there is a close relationship between the CE and sustainability and that the CE is at least beneficial for achieving sustainable development, as it can address some of the causes of current sustainability-related problems. The relevance of CE for achieving sustainable development was also confirmed by Schroeder et al. [26], who demonstrated that CE practices can contribute to achieve a significant number of Sustainable Development Goal targets. Despite these perspectives, the exact relationship between the CE and sustainability and sustainable development remains still unclear and debatable [27, 28].

Moreover, there is a lack of consensus in defining the CE. Kirchherr et al. [5] provided evidence of the heterogeneity in the definitions of the CE, by identifying 114 different definitions within academic articles, policy documents, and reports. The scholars also found that only a few of the identified definitions show explicit linkages between the CE concept and sustainable development. They also highlighted that the social dimension of sustainable development is highly overlooked, compared to the environmental and economic dimensions.

There is also a lack of consensus in conceptualizing the CE principles. A principle is defined "as a basic idea or rule that explains or controls how something happens or works" [29]. Reike et al. [30] analyzed 69 academic articles and identified that divergent approaches in conceptualizing the CE principles dominate the literature. More specifically, the scholars focused on the R-principles of the CE and found varying numbers of these R-imperatives, ranging from 3Rs (Reduce–Reuse–Recycle) through 5Rs (Reduce–Reuse–Remanufacture–Recycle–Recover) to the more nuanced 10Rs (Refuse, Rethink, Reduce, Reuse, Repair, Refurbish, Remanufacture, Repurpose, Recycle, Recover). In addition, they revealed that different authors ascribe different meanings in their conceptualizations of the R-principles and that some authors apply a clear hierarchy when defining them, while others are more vague and suggestive.

Apart from the R-principles, alternative CE principles have also been proposed in the literature. Suárez-Eiroa et al. [25] used the term operational principles to define theoretical strategies that explain how CE operates. They proposed seven operational principles: (1) Adjusting inputs to the system to regeneration rates, (2) Adjusting outputs from the system to absorption rates, (3) Closing the system, 4) Maintaining the value of resources within the system, (5) Reducing the system's size, (6) Designing for CE, and (7) Educating for CE. Moreover, Bocken et al. [31] introduced the three principles: (1) Narrowing loops, (2) Slowing loops, and (3) Closing loops, to guide business strategists and designers in the transition from a linear to a CE. In a recent study, Velenturf and Purnell [28] proposed 10 principles for the design, implementation, and evaluation of sustainable CE. These are: 1) Beneficial reciprocal flows of resources between nature and society, 2) Reduce and decouple resource use, 3) Design for circularity, 4) Circular business models to integrate multi-dimensional value, 5) Transform consumption, 6) Citizen participation in sustainable transitions, 7) Coordinated participatory and multi-level change, 8) Mobilize diversity to develop a plurality of circular economy solutions, 9) Political economy for multi-dimensional prosperity, and 10) Whole system assessment.

According to Kalmykova et al. [32], the divergent approaches in defining and conceptualizing the CE can hamper the advancement of the CE. However, the CE is an evolving and dynamic field that involves different stakeholders with different interests and priorities and thus the adoption of a single unifying definition is perhaps impossible and undesirable, as it would disregard some interests and fail to capture recent developments [33]. This, of course, is not a reason to stop striving for greater conceptual clarity on the CE. In this context, it is important to define explicitly the concept and its principles early in a study.

In this chapter, we embrace the definition proposed by Kirchherr et al.6(p229):

*"A circular economy describes an economic system that replaces the 'end-of-life' concept with reducing, alternatively reusing, recycling and recovering materials in production/distribution and consumption processes. It operates at the micro level (products, companies, consumers), meso level (eco-industrial parks) and macro level (city, region, nation and beyond), with the aim to accomplish sustainable development, thus simultaneously creating environmental quality, economic prosperity and social equity, to the benefit of current and future generations. It is enabled by novel business models and responsible consumers."*

We adopt this definition as a basis for exploring the role of biochar systems in the CE, as we consider it as one of the most comprehensive and insightful definitions of the CE in the literature. It highlights that the transition toward the CE requires the implementation of the model at three system levels (micro, meso and macro level). Moreover, it clearly relates the CE with the three dimensions of sustainable development (social, economic, environmental) and indicates that the CE has a key role as a means to achieve sustainable development. It is also important that it has an explicit reference to the 4Rs (Reduce–Reuse–Recycle–Recover) principle of the CE.
