## 3 Results and Discussion
### Hydrogen and Metabolites Evolution at pH 6.0 and 7.5  
Due to the fact that maximum H\({}_{2}\) production was reported at pH 6.0 and maximum VFAs production at pH 7.5, both runs are further discussed in this section. Concerning the experiment of run C, it lasted 6 days and was performed at pH 6.0 with an organic loading of 40 g COD/L. During the second day (after 40 h) of the experimentation period, a constant increase in H\({}_{2}\) production was observed until \(t=99\) h, reaching a total H\({}_{2}\) volume of 376 mL. According to other published works [21; 42], the maximum H\({}_{2}\) production observed under mesophilic biohydrogen potential tests or batch DF was 46 \(\pm\) 15 mL H\({}_{2}\)/g vs. and 33.8 mL H\({}_{2}\)/g vs., respectively, whereas in our work, the corresponding yield was calculated to be significantly lower (18.6 mL H\({}_{2}\)/g VS). The decreased hydrogen yield could be attributed to the OMW phenols presence. According to the literature, during the DF of glucose, the presence of phenolics decreased the H\({}_{2}\) production at an average of 15% [46]. However, such inhibitors are partially degraded during the process. The removal of the phenolic compounds at the end of the experiment in our work was calculated at 62%, a rate quite higher than 45%, as described by [PERSON] et al. [46]. An equally high yield was observed by [PERSON] and [PERSON] [7] during the high-rate thermophilic anaerobic digestion of OMW. Nevertheless, it should be always considered that such yields are strongly dependent on the individual phenolic components, which are included in the various tested OMW baches. Concerning the other metabolites' evolution, a simultaneous increase in acetate, ethanol, and butyrate was also presented during the second day. Due to the specific three produced metabolites, all the fermentation types that followed are characterized as H\({}_{2}\)-producing (acetate, ethanol and butyrate metabolic pathways) [47]. In [PERSON] et al. [21], during the third day of their experimentation period, where H\({}_{2}\) was detected, a completely different metabolites profile was observed. Even if acetate was still the dominant fermentation metabolite, lactate and formate were also produced, and butyrate was totally absent and appeared after the fifth day of their experiment. Despite the fact that lactate formation was significant in their work, this metabolite can be characterized as an intermediate product due to the fact that, during the eighth day of their experiment, lactate was totally converted to butyrate. In our study, lactate was also formed as an intermediate product (at a maximum concentration of 100 mg/L), as it was totally consumed during the 40 th hour of the test. For both total and soluble carbohydrates significant degradation was achieved, 66% for the former and 67% for the latter, respectively. Various substrates have exhibited optimal yields in terms of H2 production through batch DF, at pH 6.0 either under mesophilic or under thermophilic conditions. The most recent results according to other researchers are presented in Table 5.  
The final run during pH optimization was that of pH 7.5. Even if some metabolites appeared after the 20 th hour of the experimentation period, H2 production started after the 50 th hour. This was due to the metabolites type, as initially, not only ethanol but also lactate was produced. As ethanol is categorized as an H2-producing metabolite, and lactate as an H2-consuming one [44; 47], it was anticipated that we would observe no H2 until significant amounts of both acetate and ethanol were measured, indicating acetic and ethanol type fermentations [24; 44].