## 3 Results and Discussion
### Effect of Pretreatment and Initial pH on VFAs Production in Batch Fermentation  
Effects of the chosen thermal (90 \({}^{\circ}\)C for 30 min), thermochemical pretreatments (90 \({}^{\circ}\)C and NaOH for 30 min), and initial pH (5, 8, 10, and 12) on VFAs production from excess sewage sludge and food waste slurry were evaluated in batch reactors and the total VFAs concentrations. The averages of three replicates are reported in Figure 8.  
For the excess sewage sludge without pretreatment as the substrate, the highest total VFAs concentration at the end of the fermentation (day 25) occurred at pH 12 with the value 14.73 g/L (VFAs yield of 0.46 g VFA/g VS by day 25) followed by 9.73 g/L, at pH 5. The total VFAs concentration was almost zero for pH 8 and 10 on day 25. The final pH levels of the reactors are reported in Table 2. The pH in reactors with initial pH levels of 8 and 10 dropped to 7.3, which made the system more suitable for biogas production [32]. Thus, the reason for the low level of VFAs in the system could be due to their conversion to biogas. These results seem reasonable given the gas production data (Figure 9). The accumulated biogas volume at pH 8 was 177.9 mL (58.17% methane) which is almost 14 times the biogas volume at either pH 5 or 12. The accumulated biogas volume at pH 10 was 88.59 mL with 67.77% methane content. Figure 7: The changes in the sCOD of the non-pretreated, chemically, and thermochemically pretreated excess sewage sludge and food waste slurry. (Chemicals: NaOH and Ca(OH)\({}_{2}\), temperatures: 70 and 90 \({}^{\circ}\)C). Similar to the case of excess sewage sludge without pretreatment, the highest final total VFA concentration for the thermally pretreated excess sewage sludge was achieved at pH 12 with value 11.83 g/L, although the maximum VFAs concentration during the fermentation, 13.94 g/L, belonged to pH 10 at day 6 (Figure 8b). The VFAs accumulation trend at pH 8, 10, and 12 were quite similar to the trend of not pretreated excess sewage sludge but the trend at pH 5 was totally different. The total VFA concentration at pH 5 increased till day 6 and reached its maximum, 7.68 g/L, and thereafter decreased gradually until the end of fermentation (day 25). The final total concentration was 2.38 g/L, roughly 20% amount of the VFAs from not pretreated excess sewage sludge at pH 5. The accumulated biogas volume at pH 5 was 69 mL compared to 13 mL from not pretreated excess sewage sludge. This huge increase in biogas production indicated that the lower amount of VFAs was due to their conversion to biogas. The pH at the end of fermentation was 6.72, which is suitable for biogas production. Comparing the amount of NH\({}_{4}\)\({}^{+}\)-N in the thermally pretreated excess sewage sludge system with the not pretreated excess sewage sludge system suggests that a higher NH\({}_{4}\)\({}^{+}\)-N amount provides higher buffering capacity that maintains the pH for better biogas production. For instance, the NH\({}_{4}\)\({}^{+}\)-N amounts for the system with thermally pretreated excess sewage sludge and the system with non-pretreated excess sewage sludge at day 9 were 813.33 mg/L and 206.66 mg/L, respectively. [PERSON], et al. [33] reported that in anaerobic digestion VFAs in the system could be neutralized by proper NH\({}_{4}\)\({}^{+}\)-N concentration. Figure 8: Effect of pretreatment on VFAs production from excess sewage sludge and food waste slurry. Total VFAs concentration trends for fermentation with thermochemically pretreated excess sewage sludge were quite similar to trends of VFAs from thermally pretreated excess sewage sludge, however, the final total VFAs concentration was slightly higher. The final total VFAs concentration was 13.99 g/L at pH 12 (Figure 8c). Comparing the total VFAs concentrations (Figure 8a-c) indicates that pretreatment did not improve the accumulation of VFAs in the system in most pH levels; even in the case of pH 5, it reduced total VFAs significantly.