## 3 Results and Discussion
### Biogas and Methane  
Stage 2 (S2) performed significantly better in terms of biogas production, with the exact yields varying from variant to variant--from 20.7 \(\pm\) 0.6 dm\({}^{3}\)/day (V1) to 29.1 \(\pm\) 1.3 dm\({}^{3}\)/day (V2). In S1, the daily biogas yields peaked at 25.3 \(\pm\) 1.3 dm\({}^{3}\)/day (V2) (Figure 2a), with the other variants producing 18.8 \(\pm\) 0.9 m\({}^{3}\)/day (S1V1) and approx. 23.0 dm\({}^{3}\)/day (S1V3 and S1V4) (Figure 2a). Yields across all OLR values tested (V1-V4) in S2 averaged 25.1 \(\pm\) 1.0 dm\({}^{3}\)/day, which was significantly higher than the average determined for S1 (22.6 \(\pm\) 0.9 dm\({}^{3}\)/day) (2b). A similar pattern emerged for CH\({}_{4}\). The methane fraction content peaked at 70.1 \(\pm\) 1.0% in S2V2, exceeding the other experimental variants by a significant margin (Figure 3a). In S2, the CH\({}_{4}\) content of the biogas ranged from 59.1 \(\pm\) 2.2% (V4) to 65.9 \(\pm\) 0.6% (V1) (Figure 3a), whereas the range for S1 was 57.4 \(\pm\) 0.9% (V4) to 65.3 \(\pm\) 0.6% (V1). The SCO\({}_{2}\) pre-treatment significantly improved the biogas composition. Average CH\({}_{4}\) content across the OLRs tested in S2 was 64.9 \(\pm\) 1.4%, whereas in S1 it was at 61.8 \(\pm\) 0.8% (Figure 2a). Daily CH\({}_{4}\) production peaked at 20.4 \(\pm\) 1.1 dm\({}^{3}\) in S2V2, whereas the lowest yield was noted at 12.4 \(\pm\) 0.6 dm\({}^{3}\)/day for S1V1 (Figure 3b). Figure 3: CH\({}_{4}\) content of biogas (**a**) and average daily biogas and CH\({}_{4}\) production in particular experimental variants (**b**). Figure 2: Daily biogas and CH\({}_{4}\) production over time in S1 (**a**) and S2 (**b**). Daily biogas and CH4 yields per VSin and VSout significantly decreased as the OLR increased. In S1, biogas yields varied from between 0.47 \(\pm\) 0.02 dm\({}^{3}\)/gVS and 0.74 \(\pm\) 0.04 dm\({}^{3}\)/gVSremoved (V1) to 0.20 \(\pm\) 0.01 dm\({}^{3}\)/gVS and 0.45 \(\pm\) 0.01 cm\({}^{3}\)/gVSremoved (V4) (Figure 4a). The range for S2 was from 0.52 \(\pm\) 0.01 dm\({}^{3}\)/gVS and 0.19 \(\pm\) 0.01 cm\({}^{3}\)/gVSremoved (V1) to 0.77 \(\pm\) 0.02 dm\({}^{3}\)/gVS and 0.39 \(\pm\) 0.01 dm\({}^{3}\)/gVSremoved (V4) (Figure 4b). In terms of the CH4 fraction produced, the most pronounced differences were noted in V2, with 0.52 \(\pm\) 0.03 cm\({}^{3}\)/gVSremoved in S2V2 and 0.42 \(\pm\) 0.02 cm\({}^{3}\)/gVSremoved in S1V2 (Figure 4a,b). The average CH4 contents show that the pre-treatment successfully boosted performance in the whole stage. The average for S1 was 0.22 \(\pm\) 0.01 dm\({}^{3}\)/gVS and 0.37 \(\pm\) 0.02 cm\({}^{3}\)/gVSremoved, whereas S2 produced 0.25 \(\pm\) 0.01 dm\({}^{3}\)/gVS and 0.40 \(\pm\) 0.02 cm\({}^{3}\)/gVSremoved (Figure 4c). Other researchers have also reported that SCO2 pre-treatment significantly affects biogas production and CH4 content. The difference was the most pronounced within the OLR range of 3.0-4.0 gVS/dm\({}^{3}\).day and extended both to the output of gas metabolites and the CH4 fraction. The study has also shown OLR to have a significant effect on these values. The highest daily yields from AD were recorded at 3.0 gVS/dm\({}^{3}\).day (V2). S1 produced 17.7 \(\pm\) 0.8 dm\({}^{3}\) CH4/day, compared to 20.4 \(\pm\) 1.1 dm\({}^{3}\)CH 4/day determined in S2. In their review paper, [PERSON] and [PERSON] [20] presented a summary of the studies carried out so far to assess the efficiency of biogas production from sewage sludge pre-treated with SCO2. Based on their analysis, they identified areas requiring  
Figure 4: Biogas and CH4 production in relation to VS removed in S1 (**a**) and S2 (**b**), with mean values in relation to VS added (**c**). further research. The main one is the lack of research on anaerobic digestion in continuous reactors [20]. In literature, studies have only focused on the effect of SCO\({}_{2}\) pre-treatment of MSS on AD performed in batch reactors. As an example, [PERSON] et al. (2014) [21] managed to boost biogas production by 49% using this technology [21].