## 3 Results and Discussion
### Methane Production  
The reactors were maintained in operation until no significant methane production was detected. Figure 4 shows the specific methane production (SMP) obtained in all the digesters. It can be seen that the SMP was improved in all co-digestion mixtures, with respect to the individual digestion of ESBP. The following data were obtained for the different tests: 19.2; 108.1; 212.4; 141.4, and 47.9 mL CH\({}_{4}\)/gVS\({}_{\rm added}\) for the mixtures 0:100; 10:90; 25:75; 50:50, and 100:0, respectively.  
Figure 3: Evolution of total volatile fatty acidity (TVFA) concentration (**a**), the individual volatile fatty acids (VFAs) (**b**), and the ratio propionic (HPr)/ Acetic (HAc) acid (**c**) in all the ESBP:PM ratios.  
In terms of methane production, the best mixture was 25:75. In fact, the reactor with the mixture 25:75 had rapidly reached the maximum SMP of 212.4 mL CH\({}_{4}\)/gVS\({}_{\text{added}}\) with a very short lag phase (2 days). [PERSON] et al. [40] found an optimal ratio of 32:68 with an SMP of 494 mL CH\({}_{4}\)/gVS\({}_{\text{added}}\) at the mesophilic range. [PERSON] et al. [6] obtained an SMP of 377-422 mL CH\({}_{4}\)/gVS\({}_{\text{added}}\) for anaerobic co-digestion of cow manure with ensiled sugar beet tops at the proportion of 40%. The maximum SMP found for the mixture 25:75 in this work was approximately 11 times higher than for the mixture 0:100 (PM alone) and 5 times higher than 100:0 (ESBP alone), highlighting the synergy of the co-digestion of both substrates.  
Therefore, it could be admitted that the co-digestion of ESBP and PM in the thermophilic range showed a synergistic effect in the methane production and the rate of the organic matter degradation, with respect to the individual anaerobic digestion of PM and ESBP. The synergy observed was due to the fact that the mixture of both wastes generated more appropriate environmental conditions for the development of the process; that is, both the nutrient balance (C/N ratio) and the available alkalinity were improved. In addition, pig manure added microorganisms from its digestive tract which could enhance the hydrolysis of the lignocellulosic material present in the ESBP. These results were in line with what has been reported by several authors studying AD of agro-industrial wastes [45; 46].  
Nevertheless, it must be noted that the SMP obtained for the PM used in this study was clearly lower than that obtained in a previous study, conducted under mesophilic conditions [46], where a value of 468 mL/VS\({}_{\text{added}}\) was reached. Moreover, this effect seemed to extend to the assay with the ratio 10:90, due to its high PM proportion. This aspect could be due to the different characteristics of the manures used in both tests. Another possible explanation for this difference could be related to the presence of typical animal manure toxics (ammonia nitrogen, drugs, and antibiotics). The inhibition by ammonia nitrogen was especially relevant in the thermophilic anaerobic digestion [47] and several authors reported that animal manure were a reservoir of antibiotic-resistant genes, outlining that for PM, around 149 antibiotic-resistant genes were found [48; 49; 50]. However, to support this hypothesis, additional specific analytical determinations should be performed.