**6. Conclusion**

The CE has emerged as an alternative development model to the unsustainable "take–make–waste" approach that characterizes the contemporary economic systems. The transition toward the CE requires the implementation of new innovative technological solutions that can foster CE principles and help operationalize the CE model at different system levels. One emerging technology that can have a role in the transition toward the CE is biochar systems. These are multifunctional systems that can be deployed for biowaste treatment, and bioenergy and biochar production. As the produced biochar has versatile physicochemical properties, it can be used in various applications. Perhaps, the most prominent application of biochar, is its incorporation into soils, as it can contribute to climate change mitigation through carbon sequestration and at the same time amend the properties of soils. Overall, the multifunctionality of biochar systems, in combination with the versatility of the produced biochar, makes them suitable to function as a basis for developing circular models of waste management.

This chapter describes two biochar systems that could be developed for valorizing wood waste and contaminated soil in an urban area in Sweden. In the studied systems, wood waste is converted via pyrolysis into syngas and biochar. The syngas is used as the energy source for district heating supply. The produced biochar is applied to

contaminated soil, either on-site or off-site, to sequester carbon and at the same time to remediate the soil to enable its reuse and prevent its landfilling.

The environmental performance of the two biochar systems was assessed and compared to the conventional "dig and dump" system, where the wood waste is incinerated for energy recovery and the contaminated soil is disposed of in a landfill. The assessment was carried out by combing LCA with MEFA and SFA. The MEFA showed that the biochar system for on-site remediation could provide large fuel and virgin soil savings, compared to the biochar system for off-site remediation and the "dig and dump" system. The LCA revealed that the two biochar systems performed better than the "dig and dump" system in 10 out of 12 analyzed impact categories. The two biochar systems performed remarkably well in the climate change category, as they can achieve net negative GHG emissions, because of carbon sequestration in the biochar. Between the two biochar systems, on-site remediation with biochar performs better than off-site in all impact categories, as the former provides fuel and virgin soil savings. However, there are also trade–offs with the biochar systems, as the pyrolysis of wood waste contributes to ionizing radiation and fossils depletion due to increased consumption of auxiliary electricity. Moreover, the SFA showed that the efficacy of biochar to stabilize certain metal(loid)s is not as good as for PAHs. Hence, the extent of potential risks (e.g., ecological and human health) associated with the reuse of biochar-remediated soils is still unknown.

Based on the findings from the assessment of the studied biochar systems and using the definition of the CE by Kirchherr et al. [5] as a conceptual basis, it was highlighted that these systems can have an important role in the transition toward the CE. It was established that these systems, especially the one for on-site remediation, fulfill the 4Rs principle of the CE. It was also suggested that the versatility of biochar systems creates opportunities for operationalizing the CE model at different system levels. Furthermore, based on the findings of the environmental assessment and findings from the literature, it was inferred that the biochar systems have the potential to provide environmental, social, and economic benefits and thus to contribute to achieving sustainable development, the ultimate goal of the CE. Nevertheless, further research is required to assess whether the reuse of the biochar-remediated soil creates potential risks to ecosystem quality and human health. Moreover, further research could assess potential social and economic implications from the development of these systems.
