## 3 Results and Discussion
### Vaporization Options for Sorghum  
#### 3.4.1 Biogas and Energy Production  
The biogas tests (n = 2) presented in Table 4 for both cultivars were performed on biomass collected from area 2, since, in this area, growth and yield were similar to those expected on uncontaminated agricultural soil (Table 3). The biogas productivity and yield obtained with each of the two cultivars of sorghum were different. Concerning Trudan HL, the time to obtain 91% of the total biogas production was between 9 and 16 days. For Biomass 133, this time was between 21 and 25 days. Additionally, the maximum biogas and, more specifically, the maximum biomethane production (Nm\({}^{3}\) T\({}^{-1}\) FM) obtained with Trudan HL was 1.2 * higher than with Biomass 133. Interestingly, [PERSON] et al. [57] found the opposite, with Biomass 133 resulting in a slightly higher biomethane production than Trudan HL, when cultivated in uncontaminated soil, indicating that this is not inherent to the cultivars and is probably more related to the cultivation conditions. In that study, biomethane yields of 268 and 251 Nm\({}^{3}\) T\({}^{-1}\) ODM were found for Biomass 133 and Trudan HL, respectively, which are higher than the values found in the present study of 174 and 223 Nm\({}^{3}\) T\({}^{-1}\) ODM, respectively. These lower biomethane yields may be an effect of the presence of TE in the biomass used in the present study or simply the result of the variation in biomass composition, due to different cultivation conditions between the two studies. Since Trudan HL resulted in the highest biogas yield while also having the higher TE content in its biomass, it seems the latter explanation is more adequate, indicating that TE might not have had a detrimental effect on biogas production. Nevertheless, more investigations are needed for comparing sorghum grown on contaminated and uncontaminated locations with similar soil texture, nutrient availability, and climatic conditions, to allow for a proper assessment of the effect of TE presence in the biogas yield of sorghum biomass issued from phytoattenuation. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Sorghum Biomass 133** & **Sorghum Trudan HL** \\ \hline Biogas production (Nm\({}^{3}\) t\({}^{-1}\) FM) & 205 \(\pm\) 2 & 325 \(\pm\) 11 \\ Biogas production (Nm\({}^{3}\) t\({}^{-1}\) ODM) & 329 \(\pm\) 20 & 398 \(\pm\) 7 \\ Average CH\({}_{4}\) level \% & 53 & 56 \\ Average H\({}_{2}\)S level ppm & 22 & 18 \\ Electricity production (kWhe t\({}^{-1}\) FM, 41\% efficiency) & 439 \(\pm\) 2 & 737 \(\pm\) 16 \\ Heat production (kWht t\({}^{-1}\) FM, 44\% efficiency) & 469 \(\pm\) 2 & 787 \(\pm\) 18 \\ Potential CH\({}_{4}\) production (Nm\({}^{3}\) CH\({}_{4}\) ha\({}^{-1}\)) & 5924 & 5862 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average biogas production (n = 2, \(\pm\)SD), biogas quality, and energy production for the two cultivars of sorghum cultivated on the area 2. Based on the obtained yields, Trudan HL seemed more suitable for biogas production than the other cultivar. Nevertheless, the total biogas production per hectare for each cultivar was also used for comparison, as this can be a more important parameter for choosing the best cultivar from an agricultural point of view. The biogas production was 11,241 and 10,524 Nm\({}^{3}\) ha\({}^{-1}\), which led to a methane production of 5924 and 5862 Nm\({}^{3}\) CH\({}_{4}\) per hectare (Table 4) for the cultivars Biomass 133 and Trudan HL, respectively. These values are similar to the ones reported for other crops, such as sunflower, sugarbeet, and corn [58; 59], and result in Biomass 133 being the most suited crop for biogas production, due to its higher biomass yield, which compensates its slightly lower biogas production potential. On an energy basis, the total production of biogas from Biomass 133 could cogenerate each year around 24,000 kWhe ha\({}^{-1}\) of electricity and 25,000 kWht ha\({}^{-1}\) of heat (Table 4).