## 3 Results and Discussion
### Acidogenic Fermentation
#### 3.2.2 Continuous Acidogenic Fermentation of BSG Hydrolysate
\begin{table}
\begin{tabular}{l c} \hline \hline
**OLR** & 8.64 \(\pm\) 0.60 \\ (gCOD L\({}^{-1}\) d\({}^{-1}\)) & \\ \hline
**FP produced (FP)** & 9.00 \(\pm\) 1.59 \\ (gCOD L\({}^{-1}\)) & 147 \(\pm\) 24 \\ \hline
**r\({}_{\text{FS}}\)** & 360 \(\pm\) 22 \\ (mgCOD L\({}^{-1}\) h\({}^{-1}\)) & 13.8 \(\pm\) 1.7 \\ (mgCOD-FP gVSS\({}^{-1}\) h\({}^{-1}\)) & 34.2 \(\pm\) 5.5 \\ \hline
**FP profile [HAc/HProp/ETOH /HBut/HIsoov/HVal]** & 22/1.2/4/3/69/2 \\ (\%COD basis) & 49:51 \\ \hline
**Y\({}_{\text{FP/S}}\)** & 0.58 \(\pm\) 0.1 \\ (gCOD-FP gCOD\({}^{-1}\)) & 0.41 \(\pm\) 0.09 \\ \hline \hline \end{tabular} The values listed are averages \(\pm\) standard deviation. \({}^{\text{a}}\) Estimated HB and HV values from the HB and HV precursors produced.  
\end{table} 
Table 5: EGSB performance in the last 10 days of operation (between days 20 and 29 of operation). EGSB operation at pH 4.5 was very close to the one obtained in the batch assays at pH 5 (0.49 \(\pm\) 0.09 and 0.50 \(\pm\) 0.06 gCOD-FP gCOD\({}^{-1}\) for OL of 10 and 20 gCOD L\({}^{-1}\), respectively). Regarding FP profile, HAc and HIsoval were the dominant fermentation products in the last 10 days of operation, with average values of ca. 22% and ca. 69% (COD basis), respectively. Small amounts of EtOH, HProp and HBut were also detected (4, 1.2 and 3% COD basis, respectively) (Table 5). This FP profile was different from the obtained on the batch assays, where HBut and HAc were the main acids produced (ca. 66 and 32% COD basis, respectively, at pH 5.0 and OL of 10 gCOD L\({}^{-1}\)). Even though both, the batch assays and the EGSB, were inoculated with the same biomass, it is difficult to compare the obtained results. In the batch assays the biomass was not acclimatized to the feedstock contrarily to the EGSB where the biomass is continuously adapting to the feedstock, being the presented results related to the last 10 days of operation. When raw BSG was used, HProp and HAc were the main acids produced in all the conditions tested, suggesting that microorganisms may use different metabolic pathways when a complex solid waste is used [8]. However, the monomeric HB:HV molar fractions of polymer, estimated from the HB and HV precursors (49:51) (Table 5) were in the same range of those (ca. 44-55% HV) reported by [PERSON] et al. [8]. Thus, the HB:HV precursors ratio is not affected by the type of substrate (solid vs hydrolyzed) nor by the reactor configuration (EGSB vs CSTR; continuous vs fed-batch), when PHA production is envisaged. The main difference in the estimated polymer precursors is in the batch tests, where the HB monomer was above 80%. The composition of the biogas produced along the reactor operation was analyzed. CO\({}_{2}\) was present during the whole experimental period and it was the most representative component of the biogas (52.1 \(\pm\) 3.5% mol). H\({}_{2}\) and N\({}_{2}\) were also present (40.7 \(\pm\) 3.5% mol and 5.90 \(\pm\) 0.1% mol, respectively). O\({}_{2}\) was detected in minor amounts (1.40 \(\pm\) 0.02% mol). No CH\({}_{4}\) was detected. Batch and EGSB have different gas dynamic behavior inside the reactors which might have also impacted the FP profile. Results indicate that is possible to operate an EGSB without pH control (Figure 4a), reducing the necessity of pH control by base or acid addition, resulting in costs reduction, and improving the economic feasibility of large-scale implementation. Sugars consumption with concomitant organic acids production demonstrated the potential to perform acidogenic fermentation of BSG hydrolysate using an EGSB reactor towards PHA production (Figure 4b). The two-stage process, where the acid hydrolysis was applied to the BSG as a pre-treatment, produced a liquor rich in fermentable sugars (BSG hydrolysate), reducing the necessity to operate the acidogenic step at high HRT as needed for the single stage process using raw BSG, leading to higher FP volumetric productivities.