## 2 Materials and Methods
### Biogas Production and Energy Value  
The analysis was subcontracted to the laboratory Innolab (Oostkamp, Belgium). For both sorghum cultivars, samples from the biomass harvested on area 2 were sent to  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & **Area 1** & **Area 2** & **Area 3** & **S1** \\ \hline Clay (\%) & 7.7 & 10.7 & 11.7 & - \\ \hline Silt (\%) & 69 & 70.5 & 78.6 & - \\ \hline Sand (\%) & 23.3 & 18.8 & 9.8 & - \\ \hline Soil texture (USDA texture triangle) & Fine silt & Fine silt & Fine silt & - \\ \hline Carbon (g kg\({}^{-1}\)) & 76.3 & 82.4 & 96.8 & - \\ \hline Organic matter (g kg\({}^{-1}\)) & 131.3 & 141.7 & 166.5 & - \\ \hline C/N\({}^{*}\) & 28.4 & 28 & 31.5 & - \\ \hline CEC\({}^{+}\) (meq 100 g\({}^{-1}\)) & 17.6 & 17.9 & 21.2 & - \\ \hline CaO\({}^{++}\) g kg\({}^{-1}\) & 11.7 & 11.5 & 12.5 & - \\ \hline N kjeldahl TNK\({}^{++}\) (g kg\({}^{-1}\)) & 2.69 & 2.94 & 3.07 & - \\ \hline Phosphorus Olsen (g kg\({}^{-1}\)) P\({}_{2}\)O\({}_{5}\) & 0.216 & 0.259 & 0.258 & - \\ \hline pH-H\({}_{2}\)O & 8.08 & 8.06 & 8.12 & - \\ \hline \multirow{6}{*}{Pb} & \multirow{2}{*}{Zn} & 6685 \(\pm\) 509 & 6084 \(\pm\) 132 & 8980 \(\pm\) 340 \\  & (0.060\(\%\)\(\pm\) 0.004) & (0.090\(\%\)\(\pm\) 0.008) & (0.070\(\%\)\(\pm\) 0.012) & 300 \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Pb} & 774 \(\pm\) 18 & 592 \(\pm\) 12 & 1043 \(\pm\) 12 \\  & (0.007\(\%\)\(\pm\) 0.003) & (0.009\(\%\)\(\pm\) 0.003) & (0.007\(\%\)\(\pm\) 0.006) & 100 \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Cd} & 6.3 \(\pm\) 0.1 & 5.0 \(\pm\) 0.1 & 9.0 \(\pm\) 0.1 \\  & (0.47\(\%\)\(\pm\) 0.07) & (0.62\(\%\)\(\pm\) 0.08) & (0.48\(\%\)\(\pm\) 0.12) & 2 \\ \hline Total TE (mg kg\({}^{-1}\)) & \multirow{2}{*}{Cu} & 87.0 \(\pm\) 2.3 & 76.0 \(\pm\) 1.5 & 101.0 \(\pm\) 1.3 \\  & (0.53\(\%\)\(\pm\) 0.07) & (0.59\(\%\)\(\pm\) 0.09) & (0.52\(\%\)\(\pm\) 0.09) & 100 \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{As} & 33.0 \(\pm\) 1.0 & 29.0 \(\pm\) 0.1 & 43.0 \(\pm\) 1.2 \\  & (0.043\(\%\)\(\pm\) 0.004) & (0.050\(\%\)\(\pm\) 0.008) & (0.030\(\%\)\(\pm\) 0.007) & 30 \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Cr} & 116 \(\pm\) 12 & 114 \(\pm\) 3 & 100 \(\pm\) 4 \\  & (0.020\(\%\)\(\pm\) 0.002) & (0.040\(\%\)\(\pm\) 0.007) & (0.030\(\%\)\(\pm\) 0.004) & 150 \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Ni} & 52 \(\pm\) 1 & 51 \(\pm\) 1 & 53 \(\pm\) 1 \\  & (0.19\(\%\)\(\pm\) 0.05) & (0.21\(\%\)\(\pm\) 0.03) & (0.20\(\%\)\(\pm\) 0.04) & 50 \\ \hline \hline \end{tabular}  
* C/N\({}^{*}\): carbon/nitrogen, CEC\({}^{*+}\): cationic exchange capacity, CaO\({}^{++}\): calcium oxide, TNK\({}^{++}\): total nitrogen kjeldahl.  
\end{table}
Table 1: Main physico-chemical properties, total, and NH\({}_{4}\)NO\({}_{3}\) extractable (% of total, expressed between parenthesis) concentrations of trace elements (TE) in the sediment of the field site for each area and TE threshold values (S1) for sediment management. TE concentrations are expressed in mg kg\({}^{-1}\) dry sediment (DS). Values are means \(\pm\) SD.  
the laboratory for analysis. Biogas production tests were performed, according to the standard n\({}^{\circ}\)4630 \"Vergarung organischer Stoffe, Substratcharakterisering, Probennahme, Stoffdatenerbebung, Garvesuche,\" during 50 days at 38 \({}^{\circ}\)C with 3 L of inoculum; the biogas production and the biogas composition were recorded daily. The results were expressed in both normo (N) m\({}^{3}\) t\({}^{-1}\) of fresh matter (FM) and organic dry matter (ODM) for the biogas production. Concerning the biogas composition, they were expressed in percentage (%) and part per million (ppm) for methane (CH\({}_{4}\)) and hydrogen sulfur (H\({}_{2}\)S), respectively. In addition, taking into account the biogas quality and methane energy value given by the laboratory, calculations of potential co-energy production of electricity and heat for the cultivars were also delivered.