## 3 Results
### Hydrogen Production Potential and Selection of the Best-Performing Combination  
Regarding the hydrogen production obtained for the different combinations of organic load and s/x tested in the experimental phase of the project, it was determined that, in general terms, the analyzed combinations had a hydrogen production period of around \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Value** & **Unit** \\ \hline TS & 20.80 & \% \\ VT & 15.07 & \% \\ VS on a dry basis & 72.47 & \% dry basis \\ DQO & 0.18 & g/L \\ NTK & 1.56 & \% dry basis \\ C & 48 & \% dry basis \\ H\({}_{2}\) & 6.4 & \% dry basis \\ O\({}_{2}\) & 37.6 & \% dry basis \\ N\({}_{2}\) & 2.6 & \% dry basis \\ S\({}_{2}\) & 0.4 & \% dry basis \\ Ashes & 5 & \% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical characterization of the substrate. 9 to 10 days before the methane concentration in the biogas increased to levels greater than or equal to 20%, a definitive indicator of the onset of the methanogenic phase, and consequently, the conclusion of the experiment. From the experimental phase, it was found that the combination with the best yields in terms of production was C6, with an operating time of 9 days, producing a total of 89,420 mL/gSV. The next combination was C3, which reported a production of 71,773 mL H\({}_{2}\)/gSV. On the other hand, the combinations with lower productions were C4 and C7, which presented a production of 11,547 mL H\({}_{2}\)/gSV and 24,173 mL H\({}_{2}\)/gSV, respectively. From Figure 1, it is identified that combinations C1, C2, C3, and C6 show the highest performance in biogas production. Excluding C6, the aforementioned combinations share the same value in organic load, 5 gVS/L. Taking this into account, it could be stated that low organic loads combined with low s/x ratios favor H\({}_{2}\) production. This situation has been evidenced in other studies that have concluded that an increase in organic load causes a decrease in hydrogen production and aspects such as COD and protein removal [46; 47; 48]. It is essential to consider the influence of s/x, as considerably low values may result in fermentative inhibition due to a lack of organic matter at low organic loads. Therefore, comparing the production obtained for each of the different combinations presented in Figure 1 reveals an influence of s/x on biogas production, where the highest volumes produced for each load were observed in combinations with s/x equal to 1. This result aligns with findings in other studies that demonstrated an increase in production with an increase in the s/x value [49; 50]. On the other hand, other research suggests possible acidogenic pathways taken by the process with high levels of propionic acid and VFAs during the tests, but with very low yields in H\({}_{2}\) production, emphasizing the importance of controlling the s/x variable [46; 51]. It is worth noting that studies conducted on FO using specific substrates have better hydrogen production yields than the present research. For example, those that have used substrates such as pig manure, cocoa mucilage, and coffee mucilage have organic matter determined by the percentage of volatile solids 20% higher than the current study [28; 52]. The low process efficiency could be a result of metabolic deficiency affecting the microorganisms present in the inoculum. This assertion gains strength when reviewing the Pareto diagram obtained, where the highest H\({}_{2}\) productions for each evaluated initial organic load concentrate in combinations with s/x equal to 1, meaning those with a higher substrate-to-inoculum ratio. Therefore, it is expected to have a lesser influence of metabolic deficiency on the process in such combinations. In Figure 2, it is evident that the percentages of biogas volume produced are distributed following a predetermined order that corresponds to the value of s/x. The higher the s/x, the greater the percentage of hydrogen produced by the combination.