## 3 Results and Discussion
### Stage 2  
#### 3.2.1 AD Efficiency  
The biogas yield in V1 (raw AGS) reached 319 \(\pm\) 22 cm\({}^{3}\)/gVS (Figure 3, Table 3) at r = 203.5 cm\({}^{3}\)/d. The CH\({}_{4}\) concentration in the biogas was 66 \(\pm\) 3%, which yielded 211 \(\pm\) 11 cm\({}^{3}\)CH\({}_{4}\)/gVS (Figure 3, Table 3). After disintegration with SCO\({}_{2}\), the biogas and CH\({}_{4}\) yields were observed to increase, reaching 482 \(\pm\) 21 cm\({}^{3}\)/gVS biogas at r = 457 cm\({}^{3}\)/d and 337 \(\pm\) 14 cm\({}^{3}\)CH\({}_{4}\)/gVS at r = 223.9 cm\({}^{3}\)/d in V4 (Figure 3, Table 3). The successive increase in the SCO\({}_{2}\) dose diminished the AD yield and significantly reduced the CH\({}_{4}\) concentration. In V5, the biogas production reached 442 \(\pm\) 21 cm\({}^{3}\)/gVS (r = 390.7), and the biogas produced contained 60 \(\pm\) 3% of CH\({}_{4}\), which yielded 265 \(\pm\) 11 cm\({}^{3}\)CH\({}_{4}\)/gVS (r = 40.7) (Figure 3, Table 3). In V6, the respective values were 436 \(\pm\) 22 cm\({}^{3}\)/gVS biogas (r = 380.2) and 253 \(\pm\) 11 cm\({}^{3}\)/gVS methane (58 \(\pm\) 1%) (r = 127.9) (Figure 3, Table 3).  
The pre-treatment is expected to boost the AD effectiveness [33]. The disintegration of complex biomass macromolecules and the effective transfer of particles to the dissolved phase increase substrate availability to the population of anaerobic microorganisms [34]. In our previous study, AGS disintegrated using SCO\({}_{2}\) was subjected to AD under mesophilic conditions at a temperature of 42 \({}^{\circ}\)C [21]. The highest yields of biogas and methane production, reaching 476 \(\pm\) 20 cm\({}^{3}\)/gVS and 341 \(\pm\) 13 cm\({}^{3}\)/gVS, respectively, were obtained at a SCO\({}_{2}\)/AGS volumetric ratio of 0.3. These findings are, thus, very similar to the results obtained in the present study under thermophilic conditions. Also, [PERSON] et al. [36] did not notice any significant differences in the volumes of biogas and CH\({}_{4}\) produced during SAS fermentation under mesophilic and thermophilic conditions. During AD performed at a temperature of 37 \({}^{\circ}\)C, they achieved 406 \(\pm\) 68 cm\({}^{3}\)/d biogas with a CH\({}_{4}\) content of 61.6 \(\pm\) 4.9%, whereas during AD performed at 55 \({}^{\circ}\)C, the respective values reached  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Indicator**} & \multirow{2}{*}{**Unit**} & \multicolumn{5}{c}{**V**} \\ \cline{3-8}  & & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline Biogas & [cm\({}^{3}\)/gVS] & 319 \(\pm\) 22 & 343 \(\pm\) 21 & 445 \(\pm\) 22 & 482 \(\pm\) 21 & 442 \(\pm\) 21 & 436 \(\pm\) 22 \\ r\({}_{\rm biogas}\) & [cm\({}^{3}\)/d]  
\(426\pm 56\) cm\({}^{3}\)/d and 60.2 \(\pm\) 5.7% [36]. In turn, the rate of fermentation conducted under thermophilic conditions is higher than that performed under mesophilic conditions. Also, other authors observed a beneficial effect of sewage sludge pre-treatment with SCO\({}_{2}\) on its AD. As reported by [PERSON] et al. (2014) [27], the use of SCO\({}_{2}\) for SAS pre-treatment prior to AD enabled a 49% increase in biogas production in the most effective variant compared to the crude SAS [27]. At a SCO\({}_{2}\)/SAS ratio of 0.55/1, Zawieja (2019) [33] obtained 0.62 dm\({}^{3}\)/gVS biogas with a CH\({}_{4}\) concentration reaching ca. 78% [33], while the respective values achieved for raw SAS were 0.43 dm\({}^{3}\)/gVS biogas and ca. 75% CH\({}_{4}\).  
Figure 3: The biogas and CH\({}_{4}\) production kinetics.