## 3 Results and Discussion
### Organic Matter and Digestion Coefficient  
Stage V1 and V2 produced the lowest levels of VS in the digestate. The levels for S1 were 64.6 \(\pm\) 0.9% TS and 64.5 \(\pm\) 0.8% TS, respectively. The S2 digestate contained \(63.6\pm 0.6\)% TS (V1) and 63.9 \(\pm\) 0.7% TS (V2) (Figure 5a). Increasing OLR to 5.0 gVS/dm\({}^{3}\)-day led to significantly higher rates of VS in the digestate, i.e., 70.2 \(\pm\) 1.2% TS in S1 and 67.6 \(\pm\) 1.4% TS in S2 (Figure 5a). There was a positive linear relationship between the OLR and VS removed. The biodegradation rate in S1V1 was 25.4 \(\pm\) 0.2 gVS/day, which corresponded to 63.3% of the initial load (Figure 5b). Similarly, the mineralization rate in S2V1 was 67.1%, which translated to 26.8 \(\pm\) 0.4 gVS removed per day (Figure 5b). At an OLR of 5.0 gVS/dm\({}^{3}\)-d (V4), the removal rates were 54.6 \(\pm\) 1.2 gVS/day (43.4%) in S1 and 58.7 \(\pm\) 1.3 gVS/day (47.9%) (Figure 5b). The VS mineralization rate was somewhat affected by the pre-treatment with SCO\({}_{2}\), but OLR was the main determinant.  
The digestion coefficient/portion digested (n\({}_{\rm IF}\)) decreased as the OLR increased in successive experimental variants (Table 3). In S1, n\({}_{\rm IF}\) peaked in V1 and V2 at 64.21 \(\pm\) 1.2% and 64.57 \(\pm\) 0.8%, respectively (Table 3). Subsequent variants of this stage showed a notable decline in n\({}_{\rm IF}\), with rates of 58.25 \(\pm\) 1.0% in V3 and only 44.88 \(\pm\) 0.8% in V4 (Table 3). On the other hand, S2 was characterized by higher n\({}_{\rm IF}\) across all variants, ranging from 65.85 \(\pm\) 1.3% in V3 to 67.75 \(\pm\) 1.1% in V1 (Table 3). V4, however, marked a significant decrease in n\({}_{\rm IF}\), which fell to 50.87 \(\pm\) 0.9% (Table 3).  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Type of** & **Pre-Treatment Method** & \begin{tabular}{c} **Condition** \\ **Pre-Treatment** \\ \end{tabular} & **AD Method** & **Effects** & **Ref.** \\ \hline MSS & Alkaline treatment & 0.1 mol NaOH/dm\({}^{3}\) & AD batch (BMP), 21-day & +1.5\% biogas production & [43] \\ \hline \begin{tabular}{c} Dairy sewage \\ sludge \\ \end{tabular} & Pretreatment with SCO\({}_{2}\) & \begin{tabular}{c} SCO\({}_{2}\)/sludge: 0.1–0.5, \\ 4.5 \(\pm\) 0.3–22.7 \(\pm\) 1.3 Wh \\ \end{tabular} & \begin{tabular}{c} AD respirometers, 42 \({}^{\circ}\)C, \\ 25-day, OLR 5 gVS/dm\({}^{3}\), \\ HRT 21-day \\ \end{tabular} & \begin{tabular}{c} +20 \(\pm\) 2.1–43 \(\pm\) 3.2\% \\ biogas production \\ \end{tabular} & [14] \\ \hline MSS & Pretreatment with SCO\({}_{2}\) & SCO\({}_{2}\)/sludge: 0.55/1 & \begin{tabular}{c} AD respirometers, 37 \({}^{\circ}\)C, \\ 28-day \\ \end{tabular} & \begin{tabular}{c} +44\% biogas \\ production \\ \end{tabular} & [22] \\ \hline MSS & \begin{tabular}{c} Pretreatment with SCO\({}_{2}\) \\ and alkaline \\ \end{tabular} & \begin{tabular}{c} SCO\({}_{2}\)/sludge: 0.75/1 \\ +2 M NaOH \\ \end{tabular} & \begin{tabular}{c} AD respirometers, 35 \({}^{\circ}\)C, \\ 21-day \\ \end{tabular} & \begin{tabular}{c} +15\% biogas \\ production \\ \end{tabular} & [44] \\ \hline MSS & \begin{tabular}{c} Pretreatment with SCO\({}_{2}\) \\ and hydrodynamic \\ cavitation \\ \end{tabular} & SCO\({}_{2}\)/sludge: 1/1 & \
\begin{tabular}{c} AD respirometers, 35 \({}^{\circ}\)C, \\ 25-day \\ \end{tabular} &
\begin{tabular}{c} +62\% biogas \\ production \\ \end{tabular} & [45] \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 5: VS percentage in dry mass (**a**) and VS removal (**b**) across experimental variants.  
The digestion coefficient--i.e., the organic removal rate and the final levels of organics in the digestate--is mainly shaped by the process parameters [46]. The stability of the digestate is primarily determined by OLR, HRT, and the process temperature [47]. Hydrolysis support and the pre-treatment methods also play a role [48]. A high digestion coefficient is desirable in order to reduce the environmental impact of the digestate (including putresibility and odor emission) [49].