## 3 Results and Discussion
### Effects of Ultrasonic Pretreatment on Biogas and Methane Production  
Generally, in this study, positive effects of UP on the efficiency of the AD of M-BGS and biogas and methane production were found. The biogas and methane yields in V1 without UP were, respectively, 506 \(\pm\) 38 cm\({}^{3}\)/g VS and 329 \(\pm\) 20 cm\({}^{3}\)/g VS, and the concentration of methane in the biogas reached 65.5 \(\pm\) 3.0% (Table 5, Figure 2a). The methane production rate (r) was at an average level of 43.4 cm\({}^{3}\)/g VS-d, while the process rate constant (k) was 0.13 (Figure 2a). A significant increase in the AD efficiency and biogas and methane production was observed with the use of ultrasound. In V2, 577 \(\pm\) 42 cm\({}^{3}\)/g VS of biogas and 382 \(\pm\) 29 cm\({}^{3}\)/g VS of CH\({}_{4}\) were obtained (Table 5, Figure 2b). In this variant, the AD kinetics increased significantly in relation to the CH\({}_{4}\) production (k = 64.9 cm\({}^{3}\)/g VS-d, r = 0.17), (Figure 2b). Extending the pretreatment time up to 100 s in V3 resulted in an increase of 479 \(\pm\) 22 cm\({}^{3}\)/g VS (r = 91.0 cm\({}^{3}\)/g VS-d, k = 0.19) in the methane yield and 697 \(\pm\) 37 cm\({}^{3}\)/g VS in the biogas yield (Table 5, Figure 2c), while the concentration of methane in the biogas achieved 68.7 \(\pm\) 2.1% (Table 5). The highest efficiency of methane and biogas production was obtained in V4 and V5 at the highest dose of ultrasound, which was connected with longer UP time from 150 s to 200 s. In V4, the methane production was 534 \(\pm\) 16 cm\({}^{3}\)/g VS, while in V5, it was 561 \(\pm\) 17 cm\({}^{3}\)/g VS (Table 5, Figure 2d,e). The methane concentration in the biogas amounted to 68.2 \(\pm\) 1.4% in V4 and 69.4 \(\pm\) 3.2% in V5, and the rates of CH\({}_{4}\) production were, respectively, 101.4 cm\({}^{3}\)/g VS-d and 106.6 cm\({}^{3}\)/g VS-d (Figure 2d,e, Table 5). The differences in the biogas and CH\({}_{4}\) yields in V4 and V5 were not statistically significant.  
In AD, hydrolysis is the rate-limiting step in the biodegradation of particle biomass and is carried out by extracellular enzymes [45; 65; 64]. Hydrolysis enhancement can be achieved via ultrasonic pretreatment, which disrupts the cell walls of the biomass and releases the intracellular components into the dissolved phase, making the organic matter more accessible to hydrolytic enzymes [48; 66; 67]. The ultrasonic solubilization of organic matter directly enhances the hydrolysis step but not acidogenesis; however, it promotes methanogenesis [68]. Thus, the effectiveness of ultrasonic biomass disintegration can also be assessed by the yields of biogas and methane [45; 48; 69]. According to [PERSON] and [PERSON] [70], the improvement in the methane and biogas production can be more than 75% and 50%, respectively, compared to the achievements without biomass sonication. In this study, UP enhanced the methane and biogas production by approx. 70.5% and 60%, respectively, at a sonication time of 200 s. Similar achievements were observed by [PERSON] et al. [71], who studied the effects of UP on fruit and vegetable market wastes' biogas yield in AD. They found the highest biogas yield of 396 mL/g VS\({}_{\text{in}}\) at 18 min of sonication, which was 80% higher than that without pretreatment. An ultrasonication of the filamentous alga _Hydrodictyon reticulatum_ prior to AD resulted in a methane production of 384 mL/g VS\({}_{\text{in}}\), which was 2.3 times higher than the production obtained for an untreated sample [72].