## 3 Results and Discussion
### Acidogenic Fermentation  
#### 3.2.1 pH and Organic Load Effects on BSG Acidogenic Batch Fermentation  
Several batch assays were performed to study the effect of pH (5.0, 5.5 and 6.0) and organic load (OL) (10, 20, 30 and 40 gCOD L\({}^{-1}\)) on the acidogenic fermentation of BSG hydrolysate (after neutralization). In order to evaluate the overall batch assays performance, stoichiometric and kinetic parameters were calculated and are presented in Table 4. The maximum concentration of fermentation products (FP) obtained for each assay is shown in Table 4. The results show that the concentration of FP increases with OL and pH. The highest FP concentration (23.1 \(\pm\) 2.2 gCOD L\({}^{-1}\)) was reached at OL of 40 gCOD L\({}^{-1}\) and pH 6.0. According to Table 4, there is no clear trend of the impact of pH and OL on the yield of conversion of sugars into FP (Y\({}_{\text{FP/S}}\)). Except for pH 5.5 and OL of 40 gCOD L\({}^{-1}\), the Y\({}_{\text{FP/S}}\) values are between 0.52 and 0.69 gCOD-FP gCOD\({}^{-1}\). These values are in the same range of those reported for cheese whey by [PERSON] et al. [19] (0.55-0.68 gCOD gCOD\({}^{-1}\) at pH 4.5-6.0) and by [PERSON] et al. [34] for glucose (0.66-0.59 gCOD gCOD\({}^{-1}\), at pH 4.5-5.5, respectively). In all assays, all the sugars (glucose, xylose and arabin) were totally consumed. However, when the DF was calculated based on the influent total sugars' concentration (gCOD L\({}^{-1}\) basis), most of the values obtained were close or higher than 1, suggesting that other compounds present in the BSG hydrolysate are contributing to acids production. Thus, the DF was calculated using influent soluble COD values. These values ranged from 0.41 to 0.58 gCOD-FP gCOD\({}^{-1}\) for all the conditions tested and were not significantly affected either by the OL or by the pH (Table 4). This suggests that efficiency of fermentation is not dependent on the OL and pH, in the range of values used. Regardless of the pH range tested, generally, both maximum volumetric and specific FP production and sugar consumption rates (r\({}_{\text{FPmax}}\), q\({}_{\text{FPmax}}\), \(-\)r\({}_{\text{smax}}\), \(-\)q\({}_{\text{smax}}\), respectively) increased with the OL and pH. The highest maximum volumetric and specific rates (r\({}_{\text{FPmax}}\), q\({}_{\text{FPmax}}\), \(-\)r\({}_{\text{smax}}\), \(-\)q\({}_{\text{smax}}\), respectively) were reached at pH 6.0 under an OL of 40 gCOD L\({}^{-1}\) (Table 4). The increase of specific rates (q\({}_{\text{FPmax}}\), \(-\)q\({}_{\text{smax}}\)) with OL means that the system follows a substrate limiting kinetics in the range of organic concentration used in the feed. Thus, further increase on OL values could be supported by the culture. By increasing the OL, the concentration of furfural and 5-HMF also increased (since they are proportionally present in the feed). This suggests that these compounds were not inhibitory for the culture and thus the acid hydrolysis did not negatively impact the process efficiency. In terms of global volumetric productivity (r\({}_{\text{FP}}\)), the highest value was reached at pH 5.0 and OL of 40 gCOD L\({}^{-1}\). The profile of the FP varies with the pH and OL. The highest FP diversity was achieved at pH 5.0 and OL of 40 gCOD L\({}^{-1}\), where all acids were produced except HVal (Figure 3a,b). Nevertheless, regardless of pH or OL, HBut and HAc were the dominant acids produced in all experiments followed by HProp. As the main sugar present in the BSG hydrolysate is xylose, it was already expected to obtain butyrate and acetate as the main acids produced as previously reported by [PERSON] et al. [35].