## 3 Results and Discussion
### Uses of Produced VFAs
#### 3.2.3 Anaerobic Digestion Process  
A two-stage anaerobic digestion process is a process that consists of two reactors. The first acidogenic reactor is identical to the system described in this work, where hydrogen and VFAs are produced, followed by a methanogenic reactor, where the produced VFAs  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**pH** & **NaOH (kg/kg\%s)** & **H2 (MJ/kg\%s)** & **VFAs (kg/kg\%s)** & **Eq. Electric Energy (MJ/kg\%s)** & **Eq. Biodiesel (kg/kg\%s)** \\ \hline
4.5 & 0.022 & 1.41 & 0.41 & 2.23 & 0.124 & 7.26 \\
5 & 0.085 & 1.30 & 0.34 & 2.41 & 0.102 & 6.16 \\
5.5 & 0.169 & 1.83 & 0.41 & 1.82 & 0.123 & 7.27 \\
6 & 0.129 & 2.15 & 0.55 & 2.86 & 0.165 & 9.32 \\
6.5 & 0.175 & 0.95 & 0.54 & 3.02 & 0.162 & 8.86 \\
7 & 0.192 & 0.66 & 0.56 & 3.62 & 0.169 & 9.34 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Consumed NaOH, produced H2 and VFAs followed by the equivalents of VFAs conversion through MFC, biodiesel production and anaerobic digestion, for the pH values examined in dark fermentation, per kg of VS of substrate used in the dark fermentation step.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**pH** & **CE (\%)** & **Chemical Energy from VFAs Oxidation (MJ/kg VS)** \\ \hline
4.5 & 26.20 & 8.51 \\
5.0 & 33.44 & 7.22 \\
5.5 & 22.64 & 8.04 \\
6.0 & 26.84 & 10.65 \\
6.5 & 30.12 & 10.01 \\
7.0 & 34.31 & 10.55 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Coulombic efficiency and chemical energy from the oxidation of different mixtures of VFAs produced under different pH values, per kg of VS of substrate used in the dark fermentation process.  
are converted to methane and carbon dioxide. In the second stage, apart from the VFAs, most of the substrate not converted in the acidogenic reactor is typically also consumed. For comparison of the different VFAs utilization processes, only the methane that can be produced from the VFAs produced in the first stage will be calculated, but it should be mentioned that the methane that would be produced from the rest of the substrate is expected to be much higher because of the lipid and protein content that remains mostly unaffected at the first stage of the process. The maximum methane that can be produced can be calculated through a COD balance, where the COD present in the form of VFAs will be equal to the COD of the methane produced, with YCH4/VFAs = 0.35 NL/g COD. 55.5 KJ/gCH4 heat of combustion was used for the calculations. Through the ideal gas assumption, the conversion can be calculated as YCH4/VFAs = 13.9 KJ/g CODVFAs. The COD of each VFA mixture per pH examined can be calculated through the stoichiometry of its oxidation. For a compound with chemical formula: C\({}_{\text{n}}\)H\({}_{\text{a}}\)O\({}_{\text{b}}\), COD can be calculated from Equation (3).  
\[\text{COD}\Bigg{(}\frac{\text{gO}_{2}}{\text{g}_{\text{compound}}}\Bigg{)}= \frac{\left(\text{n}+\frac{\text{a}}{4}-\frac{\text{b}}{2}\right)32}{12\text{n }+\text{a}+16\text{b}} \tag{3}\]  
The biomethanization efficiency of VFAs, especially acetic acid, is almost 100%, because of the low growth yield of acetoclastic methanogens that does not allow significant conversion of chemical energy to biomass [39]. The expected energy content of the produced methane per kg of VS of substrate used in the dark fermentation process is presented in Table 4.