## 3 Results and Discussion  
During the first 50 days of fermentation, the biogas productivity was similar in both mesophilic (37 \({}^{\circ}\)C) and thermophilic conditions (55 \({}^{\circ}\)C): 3154-7803 mL/d and 3365-6874 mL/d, respectively, corresponding to yields of 0.26-0.65 L/g VS and 0.28-0.57 L/g VS, respectively. After 50 days, biogas productivity was higher in thermophilic conditions than in mesophilic conditions. In thermophilic conditions, the productivity first increased to about 12,000 mL/d (2000 mL/L), then stabilized at about 11,000 mL/d (0.92 L/g VS). In mesophilic conditions, the biogas productivity was about 5000 L/d (0.42 L/g VS). After initial fluctuations, in both variants, the production of biogas stabilized on about the 70 th day of the process. The resulting methane yields were 0.25-0.32 L CH\({}_{4}\)/g VS and 0.56-0.70 L CH\({}_{4}\)/g VS in mesophilic and thermophilic conditions, respectively (Figure 3). This difference in biogas productivity between mesophilic and thermophilic conditions is consistent with what has been reported in the literature [18]. Interestingly, however, the biogas productivity in the present study is higher than that in other studies. For example, [PERSON] et al. [19] reported that, during co-fermentation of waste activated sludge with biowaste, the specific biogas production increased from 0.34 to 0.49 L/g TVS when the reactor temperature was changed from mesophilic (37 \({}^{\circ}\)C) to thermophilic (55 \({}^{\circ}\)C). The final values of methane yield, however, seem to be higher than those reported in the literature. [PERSON] et al. [20] examined anaerobic digestion under thermophilic conditions (55 \({}^{\circ}\)C) of three source-separated organic fractions of waste: food waste, OFMSW and shredded OFMSW. The authors obtained methane yields of 0.18 L CH\({}_{4}\)/g VS, 0.05 L CH\({}_{4}\)/g VS and 0.08 L CH\({}_{4}\)/g VS, respectively. [PERSON] et al. [21], during batch anaerobic digestion of source-separated food wastes, obtained 0.43 L CH\({}_{4}\)/g VS. Interestingly, the authors used grinding and freezing as a method of pre-treatment and noted that the average methane content of biogas was 73%, similar to the present results. In the initial phase of fermentation, the methane content in the biogas increased to around 70% and 60% \(\
u\)/\(\
u\) at 37 \({}^{\circ}\)C and 55 \({}^{\circ}\)C, respectively. Then, regardless of the process temperature, methane synthesis slowed. At 37 \({}^{\circ}\)C, the slowdown took place between the 12 th and 44 th day; and at 55 \({}^{\circ}\)C, between the 12 th and 50 th day. In the stabilization phase, the methane content was about 75% \(\
u\)/\(\
u\) in both variants (Figure 4). In the present study, the short burst of intense methanogenic activity at the start of the process can be explained by the fact that anaerobic digester sludge was used as the inoculum. This type of sludge has been reported to be the best inoculum for a rapid start-up, as it has much higher numbers of aceticlastic methanogens than other start-up materials, such as cattle manure [22]. The subsequent slowdown in methane synthesis in both research variants may be related to the need for the microorganisms to adapt to the new substrate, as the methane share did not correlate with the other process parameters that were monitored. It is interesting to note, however, that not only was the proportion of methane high in comparison to other literature reports [23; 24], but the methane content was high in both variants despite substantial differences in the process parameters (pH, VFA concentration). The VFA concentrations differed in the two variants. At 37 \({}^{\circ}\)C, the VFA concentration first increased, then decreased and stabilized at about 600 mg VFA/L. At 55 \({}^{\circ}\)C, the VFA concentration increased from day 20 of the process, after which it ranged from 1500 to 2500 mg/L until it stabilized at 2400-2600 mg/L. In both reactors, the concentration of propionic acid was higher than that of acetic acid.