## 3 Results and Discussion
### Anaerobic Digestion
The use of CTP in S1 caused a significant increase in the CH\({}_{4}\) concentration in the biogas, which ranged from 67.9 \(\pm\) 3.1% (V3) to 70.4 \(\pm\) 2.7% (V1). The volume of biogas produced and the reaction kinetics were statistically comparable to those noted in S0 (Figure 3a). In turn, the study proved the effect of the initial OLR on the final outcomes of anaerobic digestion. The biogas yield reached 432 \(\pm\) 17 cm\({}^{3}\)/gCOD in V1 and 390 \(\pm\) 11 cm\({}^{3}\)/gCOD in V3 (Figure 3b). Similar, statistically comparable results were achieved in S2 (Figure 4a). In this stage of the experiment, the biogas yield ranged from 406 \(\pm\) 11 cm\({}^{3}\)/gCOD (V3) to 443 \(\pm\) 16 cm\({}^{3}\)/gCOD (V1) and was found to depend on the initial OLR (Figure 4b). The CH\({}_{4}\) content of the biogas fell within a narrow range from 67.2 \(\pm\) 3.1% to 69.1 \(\pm\) 2.4%, which allowed the yield to range from 272 \(\pm\) 13 cm\({}^{3}\)/gCOD to 306 \(\pm\) 12 cm\({}^{3}\)/gCOD (Table 4, Figure 4a)  
Higher AD efficiency was noted upon the use of UTP; however, the differences observed were significantly affected by the heating method and not by the heating temperature. This means that greater differences of the monitored indicators were revealed between S1 (CTP, 70 \({}^{\circ}\)C) and S3 (UTP, 70 \({}^{\circ}\)C) as well as between S2 (CTP, 90 \({}^{\circ}\)C) and S4 (UTP, 90 \({}^{\circ}\)C). In S3, the biogas yield ranged from 415 \(\pm\) 19 cm\({}^{3}\)/gCOD to 480 \(\pm\) 11 cm\({}^{3}\)/gCOD and was observed to decrease significantly along with an increasing initial OLR (Figure 5a,b). The CH\({}_{4}\) content of biogas was similar in all variants and ranged from 68.7 \(\pm\) 2.2% (V3) to 69.9 \(\pm\) 3.1% (V2). The rate of biogas production (r) ranged from 6.9 \(\pm\) 0.3 cm\({}^{3}\)/gCOD\(\cdot\) h to 8.8 \(\pm\) 0.2 cm\({}^{3}\)/gCOD\(\cdot\) h, whereas k constant values ranged from 0.10 \(\pm\) 0.01 to 0.11 \(\pm\) 0.01 (Table 4). The highest AD efficiency was determined in S4V1 (Figure 6a). The biogas production rate and biogas yield achieved in this variant reached 9.0 \(\pm\) 0.2 cm\({}^{3}\)/gCOD h and 492 \(\pm\) 10 cm\({}^{3}\)/gCOD, respectively (Figure 6b). Biogas produced contained 69.8 \(\pm\) 1.4% CH\({}_{4}\), enabling a biogas yield at 343 \(\pm\) 7 cm\({}^{3}\)CH\({}_{4}\)/gCOD, which approximated the maximal theoretical CH\({}_{4}\) production from 1.0 gCOD (350 cm\({}^{3}\)CH\({}_{4}\)/gCOD). The CH\({}_{4}\) yields noted in V2 and V3 were significantly lower and reached 325 \(\pm\) 11 cm\({}^{3}\)CH\({}_{4}\)/gCOD (70.6 \(\pm\) 2.1%) and 302 \(\pm\) 8 cm\({}^{3}\)CH\({}_{4}\)/gCOD (70  
The study conducted by [PERSON] et al. (2010) [35] aimed to evaluate the effect of low-frequency ultrasound pre-treatment (20 kHz) on the anaerobic biodegradation of meat processing effluent. Experiments were performed for 50 days for the effluents treated at three specific energy inputs, i.e., 2, 120, and 750 MJ/kgTS, and for the non-pre-treated effluent. Methane production ceased almost completely after 30 days in all variants. The highest methane yield, i.e., 376 cm\({}^{3}\), was determined in the variant with the specific energy input of 120 MJ/kg. In the case of the non-pre-treated effluent, the methane yield was barely 304 cm\({}^{3}\), indicating that the use of ultrasounds may ensure a 24% increase in methane production. The highest energy input tested, i.e., 750 MJ/kgTS, ensured a ca. 18.5% increase in the methane production yield, due to the diminished availability of organic matter in the dissolved phase [35]. Another study evaluated the effect of UD exposure time from 300 to 1200 s on the anaerobic digestion of dairy wastewater [36]. The volume of methane produced was observed to increase along with a UD extension from 300 to 900 s. The highest methane yield, i.e., 0.203 \(\pm\) 0.01 dm\({}^{3}\)CH\({}_{4}\)/gCOD, was achieved during 900-s sonication, after which the methane content of the biogas reached 70.9 \(\pm\) 2.8%.