## 2 Materials and Methods
### Analytical Techniques  
For the physicochemical characterization of OMW, three samples (\(n=3\)) were measured, and for DF monitoring, biogas and liquid samples were taken every 3-6 h from the reactor, depending on the evolution of the experiment.  
The pH values measurement for the OMW physicochemical characterization was conducted off-line by an electrode (Thermo Scientific, Waltham, MA, USA, Orion ROSS Ultra Refillable pH/ATC Triode). Total solids, volatile solids, total suspended solids, total suspended solids, total COD, dissolved COD, total Kjeldahl nitrogen (TKN), ammonium nitrogen, alkalinity, and total and dissolved phosphorus were determined according to standard methods [38], and the methods are presented in detail by [PERSON] and [PERSON] [7]. The determination of total and dissolved carbohydrates as well as of phenolic compounds was accomplished according to Refs. [39; 40] with a Cary 50 UV-VIS spectrophotometer. In the case of carbohydrates, the reagents H\({}_{2}\)SO\({}_{4}\) and tryptophan, as well as glucose as a standard compound, were used, and the reagents of Folin-Ciocalteu, Na\({}_{2}\)CO\({}_{3}\), and syringe acid (as standard compound) were utilized, respectively, for total phenolics determination. Fats and oils were measured after hexane extraction of a known OMW amount by Soxhlet extractor (Velp Scientific, Usmate Velate, Italy, SER 148).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Average Value \(\pm\) SD** \\ \hline pH & - & 5.41 \(\pm\) 0.12 \\ Total COD & g\(\mathrm{o}\)2/L & 125.63 \(\pm\) 15.70 \\ Dissolved COD & g\(\mathrm{o}\)2/L & 40.21 \(\pm\) 0.04 \\ Total solids & g/L & 110.45 \(\pm\) 1.20 \\ Volatile solids & g/L & 95.28 \(\pm\) 1.22 \\ Total suspended solids & g/L & 22.67 \(\pm\) 0.20 \\ Volatile suspended solids & g/L & 22.43 \(\pm\) 0.24 \\ Total carbohydrates & gglucose/L & 15.36 \(\pm\) 0.93 \\ Dissolved carbohydrates & gglucose/L & 11.40 \(\pm\) 0.12 \\ Phenolic compounds & gsyngenic acid/L & 1.28 \(\pm\) 0.05 \\ TKN & g/L & 0.03 \(\pm\) 0.003 \\ Ammonium nitrogen & g/L & 0.59 \(\pm\) 0.05 \\ Fats and Oils & g/L & 2.23 \(\pm\) 0.45 \\ Total phosphorus & g/L & 0.48 \(\pm\) 0.01 \\ Dissolved phosphorus & g/L & 0.38 \(\pm\) 0.01 \\ Alkalinity & gCaCO\({}_{3}\)/L & 0.98 \(\pm\) 0.19 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Physicochemical characterization of the OMW.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Average Value \(\pm\) SD** \\ \hline pH & - & 8.89 \(\pm\) 0.36 \\ Total solids & g/L & 39.05 \(\pm\) 5.07 \\ Volatile solids & g/L & 33.26 \(\pm\) 3.79 \\ Total suspended solids & g/L & 30.95 \(\pm\) 4.70 \\ Volatile suspended solids & g/L & 27.13 \(\pm\) 3.15 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Main physicochemical characteristics of the inoculum.  
The volatile metabolites of DF (acetate, propionate, butyrate, isobutyrate, valerate, iso-valerate, caproate, and ethanol) were analyzed on a gas chromatograph (Agilent Technologies, Santa Clara, CA, USA, 7890A) with a flame ionization detector (FID) [41]. Additionally, lactic acid was determined by a DIONEX IC300 ion chromatography system, equipped with a thermostated (30 \({}^{\circ}\)C) Dionex IonPac analytical column (AS19 length 4 \(\times\) 250 mm and 7.5 mm I.D), a guard column (4 \(\times\) 50 mm length and 12 mm I.D), and an electron conductivity detector (Dionex, Sunnyvale, CA, USA). Finally, biogas composition analysis was performed by gas chromatography and a thermal conductivity detector (TCD), as described by [24]. The produced biogas was converted to standard conditions (i.e., STP = 0 \({}^{\circ}\)C and 1 atm).