## 3 Results and Discussion  
### Stage 1  
In VI, the COD concentration in the supernatant was 150 \(\pm\) 11 mgO\({}_{2}\)/dm\({}^{3}\), whereas the concentrations of the nutrients were 82 \(\pm\) 4 mg N-NH\({}_{4}\)\({}^{+}\)/dm\({}^{3}\) and 63 \(\pm\) 3 mg P-PO\({}_{4}\)\({}^{3-}\)/dm\({}^{3}\) (Table 2). The concentrations of COD, N-NH\({}_{4}\)\({}^{+}\), and P-PO\({}_{4}\)\({}^{3-}\) in the supernatant were observed to significantly increase in V2-V4. The COD concentrations ranged from 330 \(\pm\) 13 mgO\({}_{2}\)/dm\({}^{3}\) in V2 to 435 \(\pm\) 13 mgO\({}_{2}\)/dm\({}^{3}\) in V4. In V2, the N-NH\({}_{4}\)\({}^{+}\) concentration reached 157 \(\pm\) 7 mg/dm\({}^{3}\) and that of P-PO\({}_{4}\)\({}^{3-}\) reached 68 \(\pm\) 4 mg/dm\({}^{3}\) (Table 2). In V4, the concentrations of both these indicators increased ultimately to 273 \(\pm\) 10 mg/dm\({}^{3}\) and 77 \(\pm\) 4 mg/dm\({}^{3}\), respectively (Table 2). The increase in the COD content is due to the destruction of the SAS structures by the disintegration of the microorganisms' cells [29]. This process also leads to the distribution of the enzymes from their protoplas, whose biodegradation activity results in transformations of organic and biogenic compounds. This, in turn, contributes to increased concentrations of the mineral forms of nitrogen and phosphorus in the dissolved phase [30]. Likewise, [PERSON] (2018) [28] determined an increase in the COD concentration in the SAS supernatant. The SCO\({}_{2}\)/SAS increase from 0.05/1.0 to 0.75/1.0 resulted in a COD concentration increase from 119 mgO\({}_{2}\)/dm\({}^{3}\) to 296 mgO\({}_{2}\)/dm\({}^{3}\)[28]. In another study by [PERSON] (2019) [33], the N-NH\({}_{4}\)\({}^{+}\) concentration in the raw SAS supernatant was approximately 43 mg/dm\({}^{3}\) and was observed to increase along with an increasing SCO\({}_{2}\) dose, reaching ca. 102 mg/dm\({}^{3}\) at a SCO\({}_{2}\)/SAS volumetric ratio of 0.75/1.0 [33]. In another study [29] addressing the impact of pre-treatment with SCO\({}_{2}\) on dairy SAS, the COD concentration in the raw SAS supernatant reached 400.5 \(\pm\) 23.8 mg/dm\({}^{3}\). The highest COD values, from 490.6 \(\pm\) 12.9 to 510.5 \(\pm\) 28.5 mg/dm\({}^{3}\), were noted at SCO\({}_{2}\)/SAS ratios ranging from 0.3 to 0.5. Disintegration with SCO\({}_{2}\) caused an increase in the N-NH\({}_{4}\)\({}^{+}\) and P-PO\({}_{4}\)\({}^{3-}\) concentrations in the supernatant from 155.2 \(\pm\) 10.2 mg/dm\({}^{3}\) and 198.5 \(\pm\) 23.1 mg/dm\({}^{3}\) in the raw sludge to 185.9 \(\pm\) 11.1 mg/dm\({}^{3}\) and 300.6 \(\pm\) 35.9 mg/dm\({}^{3}\), respectively, at a SCO\({}_{2}\)/SAS volumetric ratio of 0.5 [29]. In experiments performed by [PERSON] et al. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Indicator**} & \multirow{2}{*}{**Unit**} & \multicolumn{6}{c}{**V**} \\ \cline{3-6}  & & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline COD & [mgO\({}_{2}\)/dm\({}^{3}\)] & 150 \(\pm\) 11 & 330 \(\pm\) 16 & 412 \(\pm\) 17 & 435 \(\pm\) 15 & 440 \(\pm\) 14 & 445 \(\pm\) 15 \\ N-NH\({}_{4}\)\({}^{+}\) & [mg/dm\({}^{3}\)] & 82 \(\pm\) 4 & 157 \(\pm\) 7(2019) [34], the COD content in the dissolved phase increased to 205 mgO\({}_{2}\)/dm\({}^{3}\) at a SCO\({}_{2}\)/SAS volumetric ratio of 0.25 and peaked at 889 mgO\({}_{2}\)/dm\({}^{3}\) at a SCO\({}_{2}\)/SAS ratio of 1.0, whereas it reached 63 mgO\({}_{2}\)/dm\({}^{3}\) in the non-pre-treated sludge. A rapid decrease in temperature directly affects the external and internal cellular structures of microorganisms. Frozen microorganisms are mechanically destroyed, resulting in positive phenomena for sludge management, including the dissociation of cellular lipoproteins and other organic substances, the dissolution of intracellular substances, and the release of bound water into the dissolved phase [35]. In the next stage of the SCO\({}_{2}\) pre-treatment, the medium and the sediment are gradually heated, which increases the volume of water from the previously formed ice crystals.