## 2 Materials and Methods
### Preparation of Substrates
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Unit** & **Orange Peels (OP)** & **Orange Pulp (PL)** &
\begin{tabular}{c} **Brewery Spent** \\ **Grain (BSG)** \\ \end{tabular} \\ \hline Chemical oxygen demand (COD) & g/L & 10.2 \(\pm\) 0.3 & 11.8 \(\pm\) 0.23 & 33.6\(\pm\) 0.68 \\ Volatile fatty acids (VFA) & mg/L & 611 \(\pm\) 20.2 & 432.5 \(\pm\) 15.2 & 608 \(\pm\) 10.9 \\ pH & - & 4.35 \(\pm\) 0.2 & 4.33 \(\pm\) 0.1 & 5.01 \(\pm\) 0.3 \\ Alkalinity (ALK) & mg/L & 27.1 \(\pm\) 7.28 & 35.3 \(\pm\) 6.08 & 242.0 \(\pm\) 2.83 \\ Total solids (TS) & g/kg & 235.09 \(\pm\) 0.50 & 155.18 \(\pm\) 0.23 & 937.78 \(\pm\) 0.32 \\ Volatile solids (VS) & g/kg & 224.41 \(\pm\) 0.87 & 149.28 \(\pm\) 0.47 & 878.21 \(\pm\) 0.77 \\  & & Macro-elements & & \\ Total nitrogen (TN) & mg/L & 92.1 \(\pm\) 4.2 & 111.0 \(\pm\) 6.1 & 739.4 \(\pm\) 51.4 \\  & g/kg & 0.39 \(\pm\) 0.05 & 0.72 \(\pm\) 0.04 & 0.79 \(\pm\) 0.01 \\ Sodium (Na) & mg/L & 57.0 \(\pm\) 3.2 & 45.7 \(\pm\) 2.3 & 82.9 \(\pm\) 5.3 \\  & g/kg & 0.24 \(\pm\) 0.05 & 0.29 \(\pm\) 0.01 & 0.88 \(\pm\) 0.03 \\ Magnesium (Mg) & mg/L & 171.7 \(\pm\) 7.7 & 115.8 \(\pm\) 8.7 & 259.8 \(\pm\) 7.9 \\  & g/kg & 0.73 \(\pm\) 0.02 & 0.75 \(\pm\) 0.01 & 0.70 \(\pm\) 0.035 \\ Phosphorus (P) & mg/L & 199.7 \(\pm\) 4.9 & 369.8 \(\pm\) 4.7 & 419.1 \(\pm\) 49.7 \\  & g/kg & 0.85 \(\pm\) 0.05 & 2.74 \(\pm\) 0.02 & 4.47 \(\pm\) 0.61 \\ Potassium (K) & mg/L & 2349 \(\pm\) 36.1 & 1687 \(\pm\) 10.1 & 1131 \(\pm\) 8.7 \\  & g/kg & 9.99 \(\pm\) 0.6 & 10.87 \(\pm\) 1.3 & 102.1 \(\pm\) 0.37 \\ Calcium (Ca) & mg/L & 1688 \(\pm\) 13.1 & 887.8 \(\pm\) 13.7 & 1248 \(\pm\) 21.7 \\  & g/kg & 7.18 \(\pm\) 1.3 & 5.72 \(\pm\) 0.05 & 1.33 \(\pm\) 0.09 \\ Sulfur (S) & mg/L & 203.3 \(\pm\) 11.7 & 156.4 \(\pm\) 9.8 & 207.8 \(\pm\) 12.7 \\  & g/kg & 0.86 \(\pm\) 0.05 & 1.01 \(\pm\) 0.02 & 2.21 \(\pm\) 0.03 \\  & & Micro-elements & & \\ Chromium (Cr) & mg/L & 0.108 \(\pm\) 0.05 & 0.114 \(\pm\) 0.02 & 2.96 \(\pm\) 0.05 \\  & mg/kg & 0.46 \(\pm\) 0.01 & 0.73 \(\pm\) 0.01 & 3.16 \(\pm\) 0.01 \\ Manganese (Mn) & mg/L & 1.23 \(\pm\) 0.06 & 0.525 \(\pm\) 0.01 & 34.4 \(\pm\) 0.3 \\  & mg/kg & 5.21 \(\pm\) 0.01 & 3.39 \(\pm\) 0.1 & 3.73 \(\pm\) 0.3 \\ Iron (Fe) & mg/L & 39.75 \(\pm\) 3.3 & 17.37 \(\pm\) 1.2 & 144.5 \(\pm\) 7.8 \\  & mg/kg & 169.10 \(\pm\) 10.17 & 111.90 \(\pm\) 10.7 & 154.1 \(\pm\) 6.9 \\ Nickel (Ni) & mg/L & 0.21 \(\pm\) 0.01 & 0.19 \(\pm\) 0.05 & 0.57 \(\pm\) 0.04 \\  & mg/kg & 0.91 \(\pm\) 0.20 & 1.24 \(\pm\) 0.07 & 0.61 \(\pm\) 0.03 \\ Copper (Cu) & mg/L & 2.71 \(\pm\) 0.01 & 1.43 \(\pm\) 0.03 & 3.69 \(\pm\) 0.03 \\  & mg/kg & 11.51 \(\pm\) 0.05 & 9.18 \(\pm\) 0.7 & 3.94 \(\pm\) 0.03 \\ Zinc (Zn) & mg/L & 6.35 \(\pm\) 0.04 & 6.17 \(\pm\) 0.5 & 4.62 \(\pm\) 5.1 \\  & mg/kg & 27.01 \(\pm\) 1.2 & 39.77 \(\pm\) 4.2 & 5.85 \(\pm\) 4.9 \\  & mg/L & 0.003 \(\pm\) 0.001 & 0.027 \(\pm\) 0.001 & 0.050 \(\pm\) 0.001 \\ Arsenic (As) & mg/kg & 0.013 \(\pm\) 0.001 & 0.175 \(\pm\) 0.002 & 0.053 \(\pm\) 0.001 \\  & mg/L & 0.002 \(\pm\) 0.001 & 0.002 \(\pm\) 0.001 & 0.015 \(\pm\) 0.001 \\ Cadmium (Cd) & mg/kg & 0.008 \(\pm\) 0.003 & 0.011 \(\pm\) 0.003 & 0.016 \(\pm\) 0.001 \\  & mg/L & 0.150 \(\pm\) 0.004 & 0.16 \(\pm\) 0.02 & 0.37 \(\pm\) 0.02 \\ Mercury (Hg) & mg/kg & 0.640 \(\pm\) 0.04 & 1.02 \(\pm\) 0.001 & 0.80 \(\pm\) 0.03 \\  & mg/L & 0.049 \(\pm\) 0.001 & 0.029 \(\pm\) 0.001 & 0.026 \(\pm\) 0.01 \\ Lead (Pb) & mg/kg & 0.21 \(\pm\) 0.001 & 0.19 \(\pm\) 0.01 & 0.28 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the co-substrates used in the experiment (average values and standard deviations are given).