## 2 Materials and Methods  
### Experimental Set-Up and Procedure Description  
The experimental set-up comprised (a) a batch reactor, (b) a water bath, (c) a pH controller (HACH, SC200), (d) a NaOH solution inflow unit, and e) a biogas measuring unit. More specifically, the reactor was constructed by stainless steel (INOX 316) and was characterized by a total and a working volume of 1000 mL and 750 mL, respectively. Hot water, which came from the thermophilic water bath (\(55\pm 0.5\)\({}^{\circ}\)C), was recirculated through the double wall of the reactor in order to ensure thermophilic conditions. Additionally, a pH electrode was placed into the reactor and was connected to the pH controller for constant pH values monitoring. The pH controller had been adjusted to add certain amounts of NaOH solution (0.5 N) into the reactor through a peristaltic pump during the whole experimentation period, in case pH values fell below the tested limits. Continuous agitation was applied, and there were specific points for gas and liquid (reactor's content) sampling, respectively. The anaerobic conditions were ensured after sparging with N\({}_{2}\)-CO\({}_{2}\) (80-20\(\%\)\(v/v\)) for 5 min. Concerning the experimental set-up, more details can be seen in Figure 1, and regarding the biogas measuring unit, a detailed description can be found in Ref. [24].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Characteristics** & **Ref.** \\ \hline Membrane ultrafiltration & Purification of the substrate; however, some & \\ boundaries are presented usually due to & & \\ fouling problems. & & \\ Combination of ultrasound and & Reduction in organic content; however, the & \\ advanced oxidation processes & residual stream needs further treatment. & \\ Electro-coagulation & High organic content removal; however, the & \\  & residual stream needs further treatment. & \\  & High total organic carbon conversion and H\({}_{2}\) & \\ Steam reforming & yields; however the performance is strongly & \\  & depended on the catalyst. & \\ Hydrothermal carbonization & Production of hydrocarbons for & \\  & agricultural applications. & \\  & Biochar production, which can be applied as an & \\ Slow pyrolysis & organic fertilizer with promising results (e.g., K & \\  & bioavailability). & \\  & Biofuel (methane) production; however the & \\ Anaerobic digestion & inoculum is usually stressed by the & \\  & phenolics presence. & \\ Dark fermentation & Production of H\({}_{2}\) and conversion of the substrate & \\  & mainly to VFAs (and/or ethanol). & \\  & Good performance on organic compounds & \\ Aerobic process & treatment and polyphenols degradation, & \\  & however the organic load should be & \\  & relatively low. & \\  & Production of a nutrient-rich fertilizer; however, & \\  & reduction in moisture is mandatory. & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Presentation of several methods for OMW valorization/treatment according to the most recent literature and their characteristics.  
The main tested pH range was 6.0 \(\pm\) 1.0, as this range has been widely reported for efficient H\({}_{2}\) yields during the DF process [27], and the extreme values of 4.5 and 7.5 were also evaluated, as the authors had observed significant H\({}_{2}\) yields for both pH values in their previous works [24; 28]. Concerning the various tested runs, a detailed list of the pH values and the organic loading tests (on the optimum pH value) is presented in Table 2. Regarding especially the organic loading, a value for successful DF was selected between the optimum tested range of 20-50 g/L [25] and was achieved through substrate dilution. Subsequently, the increase in organic loading was further evaluated (by reducing the dilution). The experiments lasted from 107 to 172 h, until stable metabolites concentration in the fermented substrate. Each test was conducted in duplicate.