**5. The role of biochar systems in the circular economy**

To explore the role of biochar systems in the CE, the definition of the CE by Kirchherr et al. [5] (see Section 2) is used as a conceptual basis. More specifically, it is examined how the studied biochar systems can satisfy key elements of the definition.

The definition has an explicit reference to the 4Rs (Reduce–Reuse–Recycle– Recover) principle highlighting that, in the CE, a top priority is given on reducing the use of materials, and then on reuse, recycling, and recovery. On-site remediation with biochar (S3) can contribute to both reduction and reuse of materials, as the remediated soil can be reused on-site preventing the use of virgin soil for backfilling. Moreover, on-site remediation can generate significant fuel savings, as it involves less transportation compared to off-site remediation (S2) and landfilling of contaminated soil with the incineration of wood waste (S1). Off-site remediation cannot offer the same benefits as on-site remediation, as the remediated soil is not

used on-site for backfilling. Nonetheless, the remediated soil can be reused in other applications (e.g., construction of noise barriers), preventing the use of virgin soil for these applications. In addition, both off-site and on-site remediation recover energy from the sorted wood waste and at the same time prevent the landfilling of the contaminated soil. Hence, it is evident that both biochar systems, especially S3, contribute to fulfilling the 4Rs principle of the CE.

The definition also indicates that a multi-level implementation of the CE model at the micro, meso and macro level is required for the transition to the CE. The versatility of biochar systems offers opportunities for the operationalization of the CE model at different system levels. The studied systems in this chapter demonstrate how biochar systems could form the basis of circular models for valorizing different waste streams in urban areas (macro level). Nonetheless, similar systems based on pyrolysis of biomass waste or other biomass feedstocks could also be developed in symbiosis with other industrial facilities in eco-industrial parks (meso level). For example, biomass waste (e.g., from a paper or pulp mill) could be pyrolyzed to supply heat and/ or electricity for industrial processes within the eco-industrial park, while the produced biochar could be used as a resource for the manufacture of other materials, such as concrete, steel or activated carbon (see Section 3). In addition, biochar systems offer circular economy pathways at the micro level. It has been reported that decentralized biochar systems using agro-industrial wastes could be deployed in farms and small and medium enterprise (SME) activities to generate bioenergy and produce biochar that can be used as amendment of agricultural soils [12] or feed supplements for poultry or ruminants [45]. For example, a pyrolysis-biochar system could be integrated into an olive–grove farm in symbiosis with an olive mill, where residues from the olive grove and oil extraction are used as feedstock for the pyrolysis to produce heat and power for olive milling operations and biochar for amending the soil in the olive grove [37].

Moreover, according to the definition, the ultimate goal of operationalizing the CE model at different levels is to achieve sustainable development. One aspect of this goal is the creation of environmental quality. The assessment of the environmental performance of the biochar systems described in this chapter highlighted that these systems have great potential to improve environmental quality. First, they can contribute substantially to climate change mitigation through carbon sequestration in the biochar. Moreover, when compared to the conventional "dig and dump" system, the biochar system for on-site remediation can provide additional greenhouse gas emission savings, as it delivers fuel and virgin material savings. Apart from contributing to climate change mitigation, the assessed biochar systems can also provide additional environmental benefits, as they perform better than the "dig and dump" system in 10 out of 12 analyzed impact categories (see Section 4.2.2). However, there are also trade–offs associated with these systems, as they cause more impacts in the impact categories of ionizing radiation and fossils. The reason is that the technology for pyrolysis of wood waste used in this specific case requires considerable amounts of auxiliary electricity, which in Sweden is derived to a large extent from nuclear power, which is associated with these environmental impacts. Furthermore, the efficacy of biochar to stabilize certain metal(loid)s was not as high as for PAHs, and, in general, the extent of potential ecological and human health risks from the reuse of the remediated soil is still unknown.

To understand the role of biochar systems in CE it should be noted that CE, as defined here, does not imply "re–circulation of everything." One of the key benefits of biochar is to remove carbon from the atmosphere, thus contributing to climate change mitigation, by turning biomass into a stable material with a long lifetime in

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

soils. Thus, the carbon cycle from atmospheric carbon dioxide to organic matter and back to the atmosphere is not closed, but slowed down, fitting into the CE concept of "slowing loops" [31].

Apart from environmental quality, other aspects of the desired goal to achieve sustainable development are the creation of economic prosperity and social equity. These aspects were not included in the scope of the above–described assessment, as it was focused only on the environmental sustainability of the studied systems. Nevertheless, it has been reported in the literature that biochar systems can generally have positive economic effects, as they can create new revenue opportunities, cut costs by reducing resource use and improving logistics, and create new business opportunities [34, 37]. Moreover, they can deliver social benefits, as they can create employment, promote food security through improved crop production from enhanced soil productivity, and offer energy diversification and security of supply [34, 37]. Moreover, the creation of new job opportunities and the associated increase in income are important factors for poverty reduction, which can help in reducing inequalities in society [59].

The above–mentioned environmental, social, and economic benefits of biochar systems are good indications that these systems have the potential to contribute to achieving sustainable development, which is the ultimate goal of the CE. Nevertheless, further research is required to identify and assess potential risks and drawbacks with these systems. From an environmental perspective, it is essential to investigate further various types of biochar systems to ascertain whether they could create risks to environmental quality. For example, in the case of the studied biochar systems, in this chapter, further research could be directed toward identifying and assessing the magnitude of potential risks associated with the reuse of the remediate soils within urban environments. From a social and economic perspective, further research is needed to identify and assess potential socio-economic implications of biochar systems, including those described in this chapter.
