## 2 Materials and Methods
### Physico-Chemical Analysis  
The TS and VS of pig slurry and peat moss were analyzed using APHA standard methods [40]. The humic acid, fulvic acid, and humin of pig slurry and peat moss were analyzed. About 5 g of sample (peat moss and pig slurry) was taken in a 50 mL centrifuge tube, 40 mL of CHCl\({}_{3}\)/MeOH (3:1, \(v/v\)) was added, and the mixture was allowed to react for 24 h to extract free lipids, followed by centrifugation (Mega 17R, Hanil, Daejeon, Republic of Korea). After centrifugation, the supernatant (humic acid + fulvic acid) and the precipitate (humin) were separated, and 40 mL of sodium pyrophosphate alkali solution (0.5 M NaOH and 0.1 M Na\({}_{4}\)P\({}_{2}\)O\({}_{7}\)) was added to the remaining residue except for the supernatant, followed by shaking for about 1 h.  
All the extracted supernatants were combined, 6 N HCl was added little by little to adjust the pH of the solution to 1.0, and it was allowed to stand for about 6 h so that the humic acid precipitated to the bottom, followed by centrifugation. A total of 1 M NaOH solution was added to the precipitate, ultrasonic extraction was performed for about 1 h, left overnight, centrifuged, and the supernatant was filtered through a dry filter (Whatman No. 4) and diluted to 100 mL with distilled water. Humic and fulvic acid were measured using a UV/VIS spectrophotometer (UV-2450, Shimadzu Co., Kyoto, Japan). Humin was analyzed as a precipitate generated using sodium pyrophosphate alkali solution and 200 mL of 10% hydrofluoric acid. Table 2 presents the quantification of HS in pig slurry and peat moss.  
A pig slurry supernatant mixed with 25% metaphosphoric acid was injected into the gas chromatography-flame ionization detector (GC-FID) system (iGC 7200, DS Science, Daejeon, Republic of Korea) to analyze total volatile fatty acid (TVFA). The injector and detector temperatures were set to 280 \({}^{\circ}\)C as the default setting of the GC-FID. The GC carrier gas (N\({}_{2}\)), detector H\({}_{2}\) gas, and airflow were set to 1.0, 30.0, and 300 mL\(\cdot\)min\({}^{-1}\), respectively. The split ratio was 10.  
\begin{table}
\begin{tabular}{c c c} \hline
**Parameters** & **CTL** & **PFS** \\ \hline pH & \(7.8\pm 0.0\)\({}^{\text{n.s.}}\) & \(8.1\pm 0.0\)\({}^{\text{n.s.}}\) \\ \hline Total solid (TS, g\(\cdot\)kg\({}^{-1}\) slurry) & \(127.5\pm 1.7\)\({}^{\text{n.s.}}\) & \(133.5\pm 3.8\)\({}^{\text{n.s.}}\) \\ \hline Volatile solid (VS, g\(\cdot\)kg\({}^{-1}\) slurry) & \(103.4\pm 1.7\)\({}^{\text{n.s.}}\) & \(105.5\pm 3.9\)\({}^{\text{n.s.}}\) \\ \hline Carbon (C, g\(\cdot\)kg\({}^{-1}\) slurry) & \(57.4\pm 0.9\)\({}^{\text{n.s.}}\) & \(58.6\pm 2.2\)\({}^{\text{n.s.}}\) \\ \hline Total volatile fatty acid (TVFA, g kg\({}^{-1}\) slurry) & \(24.0\pm 0.6\)\({}^{\text{n.s.}}\) & \(13.8\pm 3.5\)\({}^{\text{n.s.}}\) \\ \hline \end{tabular} Data (means \(\pm\) S.E., n = 3) with superscript n.s. indicates no significant difference (\(p>0.05\)) between CTL and PFS. Carbon was calculated by dividing VS by a factor of 1.8 [39].  
\end{table}
Table 1: Physico-chemical properties of raw pig slurry obtained from CTL and PFS.  
\begin{table}
\begin{tabular}{c c c} \hline
**Humic Substances** & **Rank Pig Slurry** & **Peat Moss** \\ \hline
**CTL** & **PFS** & **Peat Moss** \\ \hline
**g\(\cdot\)kg\({}^{-1}\) slurry** & **g\(\cdot\)kg\({}^{-1}\) slurry** & **\% TS** \\ \hline Humic acid & \(0.08\pm 0.0\)\({}^{\text{a}}\) & \(0.18\pm 0.0\)\({}^{\text{b}}\) & 6.43 \\ \hline Fulvic acid & \(7.8\pm 0.1\)\({}^{\text{a}}\) & \(14.4\pm 0.2\)\({}^{\text{b}}\) & 8.03 \\ \hline Humin & \(54.3\pm 0.9\)\({}^{\text{a}}\) & \(60.8\pm 1.0\)\({}^{\text{b}}\) & 26.5 \\ \hline \end{tabular} Data (means \(\pm\) S.E., n = 3) with different letters in the same row are significantly different (\(p<0.05\)).  
\end{table}
Table 2: Humic substances (HS) concentration of raw pig slurry and peat moss.