## 3 Results and Discussion
### Acidogenic Fermentation
#### 3.2.2 Continuous Acidogenic Fermentation of BSG Hydrolysate  
In a previous study [8], raw BSG was used to produce VFA in a semicontinuous process operation mode. However, reactor operating constraints resulting from the use of solid waste (namely solid accumulation inside the reactor) limits the process operationality and thus its application at large scale. To overcome these operation limitations and to evaluate the feasibility of operating a continuous process with hydrolyzed BSG, an EGSB was set up. This reactor was continuously fed with hydrolyzed BSG for 30 days at an OLR of 8.11 \(\pm\) 0.87 gCOD L\({}^{-1}\) d\({}^{-1}\) (OL of 20.1 \(\pm\) 2.6 gCOD L\({}^{-1}\)) and HRT of 2.5 \(\pm\) 0.2 days. This HRT value was chosen since it allows the biomass to have time to react with the BSG hydrolysate, promoting HOrgs production [4; 44], while the OLR was imposed to maintain the same ratio of biomass concentration to reactor volume applied in the batch tests (20-30% \(v/v\)). Since as previously observed in the batch assays, the pH did not significantly affect the acidogenic fermentation of BSG hydrolysate, the EGSB in our study was operated without pH control, which will contribute to lower the operating costs when scaling up the process. When comparing the results obtained in the present study with those obtained in a previous study [8] using raw BSG and a fed-batch reactor operated at an OLR of 5.9 gCOD L\({}^{-1}\) d\({}^{-1}\) and HRT of 19 d, the FP concentration obtained in the present study was lower (9.00 \(\pm\) 1.59 vs. 15.8 \(\pm\) 2.0 gCOD-FP L\({}^{-1}\)). This difference may be explained by the significantly lower HRT used in this study compared with the previous one. This difference has impact in the FP volumetric productivity of both processes. Indeed, the FP volumetric productivity (r\({}_{\text{FP}}\)) reached in this study, in the last 10 days of operation, was 147 \(\pm\) 24 mgCOD-FP L\({}^{-1}\) h\({}^{-1}\) (Table 5), which was much higher than that reported by [PERSON] et al. [8] with raw BSG (34.9 \(\pm\) 4.4 and 37.0 \(\pm\) 5.9 at OLR of 5.9 gCOD L\({}^{-1}\) d\({}^{-1}\) and HRT of 19 and 41 d, respectively, and 91.3 \(\pm\) 9.1 mgCOD-FP L\({}^{-1}\) h\({}^{-1}\) at OLR of 22.1 gCOD L\({}^{-1}\) d\({}^{-1}\) and HRT of 16 d). Thus, though a lower FP concentration was obtained, the lower HRT used in this study has a beneficial effect on the process productivity. On the other hand, BSG hydrolysis promotes sugar availability, which enables easier conversion into organic acids, which also contributes to the increase in the volumetric productivity. Furthermore, the volumetric and specific FP productivities (r\({}_{\text{FP}}\) and q\({}_{\text{FP}}\), respectively) obtained for the EGSB were higher (147 \(\pm\) 24 mgCOD-FP L\({}^{-1}\) h\({}^{-1}\) and 13.8 \(\pm\) 1.7 mgCOD-FP gVSS\({}^{-1}\) h\({}^{-1}\), respectively) than those obtained for the batch assays operated at the closest conditions, 42.2 \(\pm\) 5.9, 79.3 \(\pm\) 14.9 mgCOD-FP L\({}^{-1}\) h\({}^{-1}\) and 2.72 \(\pm\) 0.44, 4.45 \(\pm\) 0.98 mgCOD-FP gVSS\({}^{-1}\) h\({}^{-1}\), respectively, at pH 5 and OL of 10 and 20 gCOD L\({}^{-1}\). These results demonstrate that FP production from hydrolyzed BSG in continuous mode is a better option than the production in batch. The FP yield (Y\({}_{\text{FP/S}}\)) obtained (0.58 \(\pm\) 0.1 gCOD-FP gCOD\({}^{-1}\)) in the EGSB (Table 5) is in the range of the values obtained in the batch tests at pH 5 and of those reported by [PERSON] et al. [34] when using glucose as substrate (0.58 versus 0.66 gCOD-FP gCOD\({}^{-1}\) at pH 5.5 and 4.5, respectively). [PERSON] et al. [19], who studied the pH effect on acidogenic fermentation of cheese whey using a CSTR, also reported an FP yield of 0.59 \(\pm\) 0.11 gCOD-FP gCOD\({}^{-1}\) at pH 4.5, despite the differences in HRT. Furthermore, the DF achieved in