## 3 Results and Discussion
### Effect of Pretreatment and Initial pH on VFAs Production in Batch Fermentation
In the anaerobic digestion with non-pretreated food waste slurry, the highest total VFAs concentration was 16.40 g/L and obtained at pH 12, day 16. The rate of VFAs accumulation during the initial days was higher at higher alkalinity; for instance, it was highest at pH 12 and the highest yield of VFAs at the end of fermentation (0.558 g VFA/g VS) was obtained at this pH. [PERSON] et al. (2018) similarly reported higher VFAs yield at higher pH. The accumulated amount of produced methane in each pH was less than 10 mL (approximately 8-20% of total biogas). The final pH of reactors was less than 5 in all reactors (Table 2) which can explain the low methane production due to inhibition of methanogens at low pH. The VFAs accumulation trend for both thermally and thermochemically pretreated food waste slurry was similar (Figure 8d-f)). Total VFAs concentration ramped up during the initial days and after that increased gradually until the last day of fermentation. The final total VFAs concentration for the thermochemically pretreated food waste slurry at each pH level was higher than the final values for thermally pretreated food waste slurry at respective pH levels. However, except for pH 12, the final total VFAs concentration at all other pH levels was lower than the values for the not pretreated food waste slurry. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Pretreatment Mode** & **Substrate** & **Initial pH** & **Final pH** & **Total Gas Produced (mL)** & **Methane (mL)** \\ \hline \multirow{8}{*}{No pretreatment} & \multirow{4}{*}{excess sewage sludge} & 5 & 5.09 \(\pm\) 0.03 & 13.0 \(\pm\) 0.5 & 8.9 \(\pm\) 0.6 \\  & & 8 & 7.30 \(\pm\) 0.02 & 177.9 \(\pm\) 4.2 & 103.5 \(\pm\) 4.1 \\  & & 10 & 7.37 \(\pm\) 0.03 & 88.6 \(\pm\) 9.7 & 60.0 \(\pm\) 9.9 \\  & & 12 & 8.30 \(\pm\) 0.60 & 11.5 \(\pm\) 4.3 & 0.7 \(\pm\) 0.3 \\ \cline{2-6}  & \multirow{4}{*}{food waste slurry} & 5 & 4.38 \(\pm\) 0.02 & 38.0 \(\pm\) 2.3 & 8.2 \(\pm\) 1.1 \\  & & 8 & 4.50 \(\pm\) 0.00 & 89.7 \(\pm\) 14.2 & 9.2 \(\pm\) 0.5 \\  & & 10 & 4.66 \(\pm\) 0.02 & 111.3 \(\pm\) 1.4 & 10.5 \(\pm\) 0.5 \\  & & 12 & 4.97 \(\pm\) 0.03 & 34.4 \(\pm\) 5.3 & 2.8 \(\pm\) 0.9 \\ \hline \multirow{8}{*}{Thermal pretreatment} & \multirow{4}{*}{excess sewage sludge} & 5 & 6.72 \(\pm\) 0.06 & 69.0 \(\pm\) 3.6 & 8.4 \(\pm\) 1.8 \\  & & 8 & 7.37 \(\pm\) 0.01 & 78.7 \(\pm\) 22.6 & 48.6 \(\pm\) 3.6 \\ \cline{1-1}  & & 10 & 7.51 \(\pm\) 0.04 & 100.2 \(\pm\) 3.9 & 16.9 \(\pm\) 1.9 \\ \cline{1-1}  & & 12 & 9.38 \(\pm\) 0.06 & 19.0 \(\pm\) 6.4 & 0.9 \(\pm\) 0.3 \\ \cline{1-1}  & \multirow{4}{*}{food waste slurry} & 5 & 4.87 \(\pm\) 0.13 & 94.2 \(\pm\) 16.6 & 8.4 \(\pm\) 0.2 \\ \cline{1-1}  & & 8 & 5.09 \(\pm\) 0.11 & 108.1 \(\pm\) 11.2 & 12.5 \(\pm\) 0.8 \\ \cline{1-1}  & & 10 & 5.18 \(\pm\) 0.08 & 88.2 \(\pm\) 39.2 & 2.6 \(\pm\) 1.0 \\ \cline{1-1}  & & 12 & 5.19 \(\pm\) 0.14 & 62.3 \(\pm\) 8.7 & 2.7 \(\pm\) 2.3 \\ \hline \multirow{8}{*}{Pretreatment with NaOH at 90 \({}^{\circ}\)C for 30 min} & 5 & 6.65 \(\pm\) 0.03 & 85.6 \(\pm\) 12.1 & 5.7 \(\pm\) 0.6 \\ \cline{1-1}  & & 8 & 7.21 \(\pm\) 0.09 & 81.4 \(\pm\) 6.1 & 18.9 \(\pm\) 1.2 \\ \cline{1-1}  & & 10 & 7.48 \(\pm\) 0.05 & 35.7 \(\pm\) 7.4 & 9.6 \(\pm\) 1.4 \\ \cline{1-1}  & & 12 & 9.29 \(\pm\) 0.03 & 7.3 \(\pm\) 2.8 & 1.6 \(\pm\) 0.5 \\ \cline{1-1} \cline{2-6}  & \multirow{4}{*}{food waste slurry} & 5 & 4.87 \(\pm\) 0.08 & 68.4 \(\pm\) 12.9 & 6.4 \(\pm\) 1.3 \\ \cline{1-1}  & & 8 & 5.09 \(\pm\) 0.02 & 98.0 \(\pm\) 11.8 & 12.0 \(\pm\) 1.8 \\ \cline{1-1}  & & 10 & 5.18 \(\pm\) 0.02 & 87.6 \(\pm\) 3.7 & 3.8 \(\pm\) 0.8 \\ \cline{1-1}  & & 12 & 5.19 \(\pm\) 010 & 87.0 \(\pm\) 4.0 & 6.8 \(\pm\) 1.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Batch reactors’ initial pH, final pH, and gas products volume. The next phase of the experiment was designed based on the aforementioned results, however, the suggestion of an industrial partner was another factor for designing the process.