## 3 Results and Discussion
### Methane Production Yields and Process Efficiency
Moreover, in both co-digestion assays, above 60% of biomethanation degree was achievable at the different HRT studied, excepting at the critical HRT tested in each experiment: 5-day HRT in SBB + PM (BD was only 13%) and 12-day HRT in SBB + CM (BD was only 14%). The highest value of the BD was around 78% in the reactor of SBB+PM at the HRT of 12 days. These results demonstrated that co-digestion played an important factor in wastes degradation likely due to the synergistic effect created in digesters, being PM more suitable to improve the biodegradability of the digester content than CM. According to [PERSON] and [PERSON] [38] and from the results of their research using C\({}^{14}\) tracers in an AD study of sludge, authors observed that the 73% of the total methane produced in an anaerobic process comes from the acetoclastic pathway, while the 27% usually comes from the  
Figure 2: Biomethanation degree in the three SSTR digesters. hydrogen-utilizing pathway. Nevertheless, recent studies found that in the anaerobic digestion of nitrogen-rich substrates, the methanogenesis shift from acetoclastic pathways to the syntrophic acetate oxidation-hydrogenotrophic (SAO-HM) route [39; 40; 41]. In a recent study by [PERSON] et al. [41], the authors attempted to explain methanogenic pathways in anaerobic digestion of nitrogen-rich substrates. The authors studied acetate conversion by C\({}^{13}\) isotope under mesophilic and thermophilic conditions. Results showed that in both temperatures, the syntrophic acetate oxidation-hydrogenotrophic (SAO-HM) pathway was dominant and that methanogenic pathway shifting, induced by high ammonia levels, closely correlated to the process performance. In this sense, it has been deduced that the substrate characteristics condition the predominance of one pathway or another. In the present research, the combination of SBB as a high content carbonaceous material and manure as high content nitrogenous substrates led to balance the nutrients content of the anaerobic process and hence, the inhibition by ammonia was avoided. Aiming for a comprehensive comparison of the obtained methane productions in the present research, the criterion developed by [PERSON] and [PERSON] [38] has been applied to the results. Thus, specific methane productions (SMPs) for the three semi-continuous digesters are depicted in Figure 3. The highest SMP from SBB+PM (HRT from 18 to 8 days) and the highest SMP from SBB+CM (HRT of 15 days) have been selected as representatives of the optimal operation area (the blue box). The average value of the five data of SMPs was 331 LCH4/kgVS\({}_{\text{added}}\). The black discontinuous line in the figure corresponded to the SMP value of 27% of the average SMP obtained (89.5 LLCH4/kgVS\({}_{\text{added}}\)). The green line corresponded to the SMP value of 73% of the average SMP obtained (241.8 LCH4/kgVS\({}_{\text{added}}\)). As can be observed, the critical operational condition in each assay (lower HRT/Higher OLR) has shown SMP values slightly below the limit value of 89.5 LCH4/kgVS\({}_{\text{added}}\), indicating that acetoclastic methanogens were affected in failure periods. In critical HRTs, the accumulation of VFAs and the drop in pH to values ranged from 5.6 to 6.3 occurred indicating the inhibition of the systems. It has been reported that acetotrophic methanogens are strongly inhibited below a pH of 6.2 [41]. Figure 4 shows the average values of pH in each stage (HRTs) for the three SSTRs. Figure 3: Specific methane productions at the different hydraulic retention time in the three SSTR digesters. Greenline: the specific methane production (SMP) value of 73% of the average SMP obtained. Blackline: the value of 27% of the average SMP obtained. Blue box: the highest SMP values in anaerobic digesters. Figure 5 shows the organic matter removal expressed as the percentage of volatile solids removed from the systems. In the SBB digester, operation at a lower HRT (higher OLRs) of 18 days led to a decrease in the efficiency of the organic matter removal.