**4.1 Systems description**

**Figure 1** depicts three different systems for the management of urban wood waste and contaminated soil. System 1 (S1) depicts how these two waste streams are currently managed in the urban area of Helsingborg in southern Sweden, which was the case study area for the research project. Systems 2 and 3 (S2 and S3) depict two alternative options for managing wood waste and contaminated soil based on biochar systems. More details for each system are provided below.


### **Figure 1.**

*The three studied systems for the management of wood waste and contaminated soil.*

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

The biochar is mixed with contaminated soil (6% biochar, 94% soil, weightto-weight), which is transported to the WM facility from excavation sites in Helsingborg. It is assumed that the excavated soil is transported for treatment to the facility due to technical or/and legislative restrictions that do not allow its mixing with biochar on-site and its direct reuse for backfilling. Instead, virgin soil (gravel) is used to backfill the excavated sites and the biochar-soil mix is reused in other applications (e.g., for noise barrier construction).

• *S3: On-site remediation with biochar.* The main difference between S3 and S2 is that the produced biochar is transported to the excavation sites and there it is mixed with the contaminated soil (6% biochar, 94% soil). The biochar-soil mix is then reused on-site for backfilling.

### **4.2 Environmental performance**

### *4.2.1 Methods*

The environmental performance of the three above–described systems was assessed by combining three Industrial Ecology tools, that is, Material and Energy Flow Analysis (MEFA), Substance Flow Analysis (SFA), and Life Cycle Assessment (LCA).

The goal of the MEFA was to map and quantify material and energy flows in the three systems in order to provide an understanding of the functioning of the systems and create the quantitative basis for the application of the LCA. The system boundaries of the MEFA included all processes for managing the contaminated soil (e.g., excavation and mixing) and wood waste (e.g., incineration and pyrolysis) and transportation between processes. However, they did not include the composting of the green waste, as the focus of the assessment was on the sorted wood waste, and the leaching of PAHs and metal(loid)s from the landfilled contaminated soil (S1) or the reused soil (S2 & S3), as it was studied through an SFA. The time boundary of the assessment was annual. The estimation of the material and energy flows was done by combining primary data and data from the literature.

The LCA was a comparative process-based LCA and its goal was to assess the life cycle environmental impacts of the studied systems. The system boundaries of the LCA were the same as those of the MEFA. They also included upstream impacts from the supply of backfill material, downstream impacts from the disposal of wood waste incineration ash, and impacts from capital goods (e.g., machinery). The functional unit was set as "1 year of operation of the pyrolysis plant (0.8 t/h dry wood, 1250 t/year biochar)." This functional unit is equivalent to the treatment of 5,650 t wood waste for district heating and remediation of 12,240 m3 contaminated soil with biochar. To handle allocation issues and keep the functional unit constant the system expansion approach was followed. The modeling of the Life Cycle Inventory (LCI) was carried out using the LCA software Brightway2 [56] based on the Ecoinvent database (version 3.6 – cut-off) [57]. For the Life Cycle Impact Assessment (LCIA), the ILCD 2.0 impact assessment method [58] was used. From the 15 impact categories, the toxicity-related impact categories carcinogenic effects, non-carcinogenic effects, and freshwater ecotoxicity were not included, as the fate of the contaminants in the soil was investigated separately through an SFA.

The SFA was conducted to map and quantify the flows of the contaminants (PAHs and metal(loid)s) in the landfilled contaminated soil and the remediated soil. The analysis was carried out taking a life cycle perspective, as the system boundaries included flows from all the processes included in the LCA. In addition, they included leaching of the contaminants from the soils, which was excluded from the MEFA and LCA. The amounts of contaminants leaching from the soils were calculated within a 100-year timeframe, using data from leaching experiments that were performed in the context of the "Biochar-RE: Source" research project and assuming a certain degree of water infiltration in the soils.

## *4.2.2 Results*

The main results from the application of the MEFA are summarized in **Table 1**. The analysis revealed that on-site remediation with biochar (S3) can deliver significant fuel (diesel and biodiesel) savings, as it involves less transportation of materials than the "dig and dump" system (S1) and off-site remediation (S2). Moreover, on-site remediation minimizes the use of virgin material (gravel) for backfilling, as the remediated soil is directly reused on-site. By contrast, in S1 and S2, virgin material is required for backfilling. In addition, the analysis indicated that the pyrolysis of wood waste can supply less heat to the district heating network than incineration and that a considerable amount of auxiliary electricity is needed for the operation of the pyrolysis plant.

**Table 2** presents the results of the LCA for the three systems and **Figure 2** shows the environmental impacts of S2 and S3, normalized to S1 (S1 = 100%), as well as the contribution of each process. Overall, biochar systems (S2 & S3) perform better than the "dig and dump" system (S1) in 10 out of 12 environmental impact categories. When comparing off-site (S2) and on-site remediation (S3), the former has lower environmental impacts in all impact categories. The main reason is that S3 entails less transportation of materials and saves virgin soil. Notably, both biochar systems have negative scores for climate change, as carbon sequestration in the biochar is 2.3 and 4.5 times higher than direct greenhouse gas emissions in S2 and S3, respectively. The biochar systems S2 and S3 had more impacts than S1 only in the impact categories Ionizing radiation and Fossils. The principal cause is the increased consumption of electricity for the operation of the pyrolysis plant, as a significant share of electricity in Sweden is from nuclear power, which is associated with these two impacts.


### **Table 1.**

*Main material and energy flows of the three systems.*

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


*The negative scores for Climate change mean that the uptake of greenhouse gases is larger than direct emissions to the atmosphere (Data source: Papageorgiou et al. [55]).*

### **Table 2.**

*Life cycle environmental impacts of the three systems.*

Moreover, transportation and the incineration of wood waste are the most significant contributors in almost all impact categories for S1 (c.f., **Figure 2**). For the biochar systems S2 and S3, pyrolysis of wood waste and heat substitution are significant contributors. Heat substitution represents the additional heat that needs to be generated to compensate for the reduced heat production in S2 and S3, as pyrolysis produces less energy than incineration because a large share of the initial energy content in the biomass remains in the biochar. For S2, transportation is another significant contributor, as off-site remediation requires transportation of large quantities of materials, for example, virgin soil for backfilling.

**Figure 2.**

*Life cycle environmental impacts of the biochar systems (S2 and S3), normalized to the "dig and dump" system (S1) (S1 = 100%) with process contributions (Data source: Papageorgiou et al. [55]).*

The results of the SFA for PAHs are summarized in **Table 3**. The analysis showed that for all PAHs, except benzo(a)pyrene, the leached amounts from the contaminated soil and the biochar-remediated soil are significantly higher than their life cycle emissions from the other processes of the systems. However, the leached amounts of PAHs constitute only a small part of their initial content in the soils. The analysis showed that remediation with biochar can stabilize PAHs in the soil, as less than 0.1% of the initial content of these contaminants in the soil will leach out within a 100-year period.

For the metal(loid)s the results of the SFA are presented in **Table 4**. Contrary to PAHs, the leached amounts of most metal(loid)s from the landfilled or remediated soil are lower than their life cycle emissions. The only exceptions are Mo and Ba. Moreover, the analysis showed that less than 0.8% of the initial content of metal(loid) s in the contaminated soil leaches out, except for Ba where 1.1% leaches out in S2 and S3, and Mo where 4.7% and 25% of the initial content leaches out in S1, S2, and S3, respectively. Furthermore, the SFA indicated that the application of biochar can reduce the leaching of Cu, Zn, Ni, and Hg, while it does not have the same positive


### **Table 3.**

*Results of the SFA for PAHs.*


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

### **Table 4.**

*Results of the SFA for metal(loid)s.*

effects for the other metal(loid)s. A sensitivity analysis showed that the results for metal(loid)s were sensitive to the assumed degree of water infiltration in the soils, contrary to the results for PAHs, which showed low sensitivity.

Overall, the SFA showed that the treatment of contaminated soils with biochar is effective for stabilizing PAHs. For metal(loid)s, however, the results of the SFA were more varied and sensitive to modeling assumptions. Therefore, further investigation is required to evaluate the effectiveness of this technique for remediating contaminated soils with metal(loid)s and identify and assess potential ecological and human health risks associated with it.
