## 3 Results and Discussion
From day 20 of the process, the acetic acid concentration was around 5.5-times higher at 55 \({}^{\circ}\)C  
Figure 4: Changes in the biogas composition during the anaerobic digestion of ORF; (a) mesophilic—temperature 37 \({}^{\circ}\)C, (b) thermophilic—temperature 55 \({}^{\circ}\)C.  
Figure 3: Biogas production (a) and methane yield properties (b) obtained during the anaerobic digestion of ORF. than at 37 \({}^{\circ}\)C. At 55 \({}^{\circ}\)C, the concentrations of caproic and isobutyric acids were 147-184 mg/L and 84-101 mg/L, respectively. At 37 \({}^{\circ}\)C, in contrast, these acids were present only during the earlier stages of the process and their concentrations were relatively low (Figure 5). At both operational temperatures, propionate constituted the majority of the VFAs. Similar results were obtained by [PERSON] et al. [25] during co-digestion of anaerobic sludge and dog food in a CSTR reactor and by [PERSON] et al. [26] during co-digestion of cow manure and dog food in a CSADs reactor. [PERSON] et al. [26] attributed propionate accumulation to free energy limitations imposed by molecular hydrogen. According to the authors, thermophilic bacteria that oxidize LCFA have a higher PH2 threshold than their mesophilic counterparts (10-2.4 atm vs. 10-3 atm, respectively). Thus, LCFAs can be degraded to a greater extent at thermophilic temperatures, potentially producing additional H\({}_{2}\) and further inhibiting propionate oxidation. This, compounded by faster hydrolysis rates, can make thermophilic digesters more susceptible to propionate accumulation, and consequently process upsets. However, this is only speculation in the case of the present study, as the partial hydrogen pressure was not measured during the tests. Another interesting hypothesis concerns the effect of compounds formed during Maillard's reaction on the efficiency of methanogenesis. These compounds are produced by reactions between amino acids and carbohydrates at temperatures above 100 \({}^{\circ}\)C [27; 28], and they inhibit microbial activity [29; 30]. It can, therefore, be assumed that such compounds were formed during autoclaving of the municipal waste used in the present study. This may well have lowered the efficiency of anaerobic digestion in mesophilic conditions, as reported by [PERSON] et al. [12] and [PERSON] et al. [5]. In addition, the efficiency of anaerobic digestion under mesophilic conditions was not reduced in the present study, as also reported by [PERSON] et al. [31]. This suggests that, under thermophilic conditions, the microorganisms adapted to the presence of Maillard compounds. Considering that the methanogens' activity was not lowered, an explanation would be the inhibitory effect of Maillard compounds on the acetogenesis process at 37 \({}^{\circ}\)C, which would result in less biogas production with a higher methane content. However, it should be remembered that other inhibitors, such as furfural, hydroxymethylfurfural (HMF) or phenols could also be formed during the autoclaving process. Thus, further investigations would be necessary to indicate which inhibitors were present. During fermentation at 37 \({}^{\circ}\)C, the pH remained between 7.84 and 8.33. At 55 \({}^{\circ}\)C, it was higher and ranged from 8.03 to 8.51. Similar results were obtained by [PERSON] et al. [32] during single-stage mesophilic and thermophilic digestion of sewage sludge. Normally, increased gas production accompanies increasing pH because methanogenesis reduces VFA and produces alkalinity. In the presented studies, the changes in VFA did not affect the pH changes during fermentation. [PERSON] et al. [32] indicate that the increase in pH was caused by the degradation of nitrogenous compounds; nevertheless,  
Figure 5: Qualitative and quantitative changes of volatile fatty acids (VFA) during the anaerobic digestion of ORF after autoclaving; (**a**) mesophilic—temperature 37 \({}^{\circ}\)C, (**b**) thermophilic—temperature 55 \({}^{\circ}\)C.  
in the present studies, such an association was not found (Figure 6).