## 2 Materials and Methods
### Analytical Methods  
All analytical determinations were performed according to the Standard Methods [31]. The following reference methods were used--Total Solids (TS) by 2540B, Volatile Solids (VS) by 2540E, total Chemical Oxygen Demand (tCOD), and soluble Chemical Oxygen Demand (sCOD) by 5220C,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Component** & **Units** & **ESBP** & **PM** & **Inoculum** \\ \hline pH & - & 6.22 \(\pm\) 0.03 & 8.26 \(\pm\) 0.10 & 7.96 \(\pm\) 0.08 \\ TS & g/kg & 857.7 \(\pm\) 0.04 & 276.3 \(\pm\) 0.27 & 55.67 \(\pm\) 0.15 \\ VS & g/kg & 758.6 \(\pm\) 0.06 & 156.8 \(\pm\) 0.30 & 28.43 \(\pm\) 0.11 \\ sCOD & g/kg & 21.0 \(\pm\) 0.42 & 7.0 \(\pm\) 0.28 & 3.4 \(\pm\) 0.39 \\ TVFA & g/kg & 1.99 \(\pm\) 0.31 & 1.2 \(\pm\) 0.02 & 0.72 \(\pm\) 0.10 \\ Alkalinity & g/kg & 2.19 \(\pm\) 0.10 & 33.1 \(\pm\) 0.50 & 11.8 \(\pm\) 0.77 \\ C/N ratio & - & 37.4 \(\pm\) 0.22 & 3.03 \(\pm\) 0.27 & - \\ Pectins & \% & 55.54 & - & 69.31 \\ Hemicellulose & \% & 22.52 & - & 11.29 \\ Cellulose & \% & 21.14 & - & 11.90 \\ Lignin & \% & 3.50 & - & 5.61 \\ \hline \hline \end{tabular}  
* All results were calculated in a wet basis. ESBP: Exhausted Sugar Beet Pulp, PM: Pig Manure, TS: Total Solids, VS: Volatile Solids, sCOD: Soluble Chemical Oxygen Demand, TVFA: Total Volatile Fatty Acid, -: No unit/Not analyzed.  
\end{table}
Table 1: Physicochemical characteristics of the substrates and inoculum.  
Figure 1: Equipment used for the biogas measurement and characterization. From left to right—infrared on-line sensors, the MilliGas counter, and the data interface module.  
Dissolved Organic Carbon (DOC) by 5310B, using a carbon/nitrogen analyzer (Analytic-Jena multi N/C 3100 with chemiluminescence detector, Endress+Hauser Company(r), Waldheim, Germany), alkalinity by 2320B and pH by 4500H\({}^{+}\). In addition, the concentration of the main individual volatile fatty acids (VFAs), from C2 to C7, were measured by using a gas chromatograph GC-2010 (Shimadzu Corporation(r), Kyoto, Japan) with flame ionization detector and a Nukol(r) capillary with a diameter of 0.25 mm and 30 m of length column (Merck KGaA(r), Darmstadt, Germany). The hydrogen was used as carrier gas with a flow of 50 mL/min and 75.5 kPa; synthetic air (400 mL/min and 50 kPa) and hydrogen (40 mL/min and 60 kPa) were used for the flame ignition, and nitrogen (30 mL/min and 75 kPa) was used as the make-up gas. All the gases were provided by Abello-Linde(r) (Barcelona, Spain).  
The pH was measured daily and directly into the reactor. The remaining parameters were analyzed three times per week. Prior to the analysis of sCOD, DOC, and VFA, the samples were centrifuged in a Consul-21 Ortoalresa(r) equipment (Madrid, Spain) at 3220\(\times\)\(g\) during 15 min and filtered with a 0.47 \(\upmu\)m glass microfiber filter (ref. GF52047, Hahnemuhle(r), Dassel, Germany). For the analysis of VFAs, an additional filtration was carried out through a 0.22 \(\upmu\)m PTFE filter manufactured by Rephile Bioscience(r) (Shanghai, China) [32].  
To determine the lignocellulosic fractions of the samples (cellulose, hemicellulose, and lignin), the FIBERTEC(tm) 8000 equipment (FOSS(r), Hilleroed, Denmark) was used by applying the [PERSON] method [33].