## 3 Results and Discussion
In both research variants, an increase in the concentration of dissolved organic compounds in the reactors was observed. At 37 \({}^{\circ}\)C, the increase occurred in the first 15 days and was on average 218 mg COD/L-d. After this time, the COD concentration stabilized at a level between 3200 and 3680 mg COD/L. At 55 \({}^{\circ}\)C, the concentration of organic compounds dissolved on day 70 of the process increased by an average of 52 mg/L-d. After this time, the concentration stabilized within the range of 4840-5000 mg/L. From day 70 of the process, COD concentration was about 1.4 times higher at 55 \({}^{\circ}\)C than at 37 \({}^{\circ}\)C (Figure 6). The course of changes in the concentration of ammonium nitrogen in both reactors was similar. After an initial increase, the ammonium nitrogen concentration dropped, then increased starting on day 30 and, finally, stabilized from the 70 th day at 169-207 mg/L and 262-304 mg/L at 37 \({}^{\circ}\)C and 55 \({}^{\circ}\)C, respectively. The initial concentration of orthophosphate was two-fold lower at 37 \({}^{\circ}\)C than at 55 \({}^{\circ}\)C, and it decreased at a rate two-times slower (0.63 mg/L-d). The final concentrations of orthophosphate were 19.96 mg/L and 28.43 mg/L at 37 \({}^{\circ}\)C and 55 \({}^{\circ}\)C, respectively (Figure 6). The initial increase in nitrogen concentration was related to the rapid biodegradation of the organic matter contained in the autoclaved municipal waste. The slowdown of the biodegradation process from the 30 th day of the process may have been due to the inhibitory effect of ammonium nitrogen on the fermentation process [33; 34], the effect of Maillard compounds, or the effect of other inhibitory compounds formed during the autoclaving process. From the 70 th day of the process, ammonium nitrogen and other process indicators (COD, phosphate, VS) stabilized, which confirms that the microorganisms had adapted to the process conditions, which is similar to the findings of [PERSON] [35]. The results of [PERSON]'s exact test (\(p=0.29609\)) did not show a significant effect of temperature on the efficiency of organic compounds removal. The efficiency of organic compounds removal in the initial stage was above 90%. After 70 days of anaerobic digestion, it dropped to 67-69% at 37 \({}^{\circ}\)C and 68-71% at 55 \({}^{\circ}\)C. Until the first complete replacement of the reactor volume (day 30), the average reduction in organic compound removal at 37 \({}^{\circ}\)C was 0.53%/d, after which it was 0.08%/d, although the technological conditions  
Figure 6: Changes of pH (**a**), dissolved organic matter (COD) (**b**), ammonium nitrogen (**c**) and orthophosphates (**d**) during the anaerobic digestion of ORF after autoclaving. remained the same. In the second variant, the changes were slower: 0.37%/d and 0.1%/d, respectively. The reduction in the use of organic compounds by microorganisms resulted in an increase in dry matter in the reactors. During the first 30 days, the changes took place more dynamically: at 37 \({}^{\circ}\)C, the organic dry matter content increased from 6.02 to 12.77 g/L; and at 55 \({}^{\circ}\)C, from 3.67 to 12.54 g/L. From the 70 th day of the fermentation process, stabilization of the organic dry matter content was observed in both reactors at 18.39-20.14 g/L and 17.67-19.34 g/L, respectively (Figure 7). The lack of a significant influence of temperature on the intensity of the transformation of organic compounds seems to be consistent with other reports. Similar results were obtained by [PERSON] et al. [26] during a single stage of mesophilic and thermophilic fermentation of sewage sludge. In the present study, ammonium nitrogen accumulated along with an increase in VS. This is particularly evident under thermophilic conditions, in which accumulation of VS and ammonium nitrogen occurred on the 30 th day of the process (correlation coefficient = 0.71). In mesophilic conditions, no correlation between these parameters was found.