## 4 Results and Discussion
### Cumulative Biogas Production and Methane Content  
The investigation revealed varying biogas production yields across different inoculum-to-substrate ratios in mono-digestion and co-digestion experiments with a known amount of inoculum. In the case of mono-digestion, the highest biogas production was observed in a 1:2 ratio of CD:FW, i.e., 970 \(\pm\) 14.14 mL/g VS followed by CD:FW (2:1), CD:FW (1:1), CD:WH (1:1), CD:WH (2:1), and CD:WH (1:2). In the initial experimental phase (day 6), the mono-digestion ratio of CD:FW (2:1) exhibited the highest biogas production, measuring 598.5 \(\pm\) 20.50 mL/g VS. Conversely, the CD:WH (2:1) ratio demonstrated the lowest biogas production at 167.5 \(\pm\) 24.74 mL/g VS. Among the co-digestion combinations of WH:FW, \begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{3}{c}{**(a)**} \\ \hline  & **Inoculum (g)** & **Substrate (g)** & **Ratio** & **Total (g)** \\ \hline CD:WH (2:1) & 226.74 & 173.25 & 2 & 400 \\ \hline CD:FW (2:1) & 354.39 & 45.60 & 2 & 400 \\ \hline CD:WH (1:2) & 98.61 & 301.38 & 0.5 & 400 \\ \hline CD:FW (1:2) & 264.08 & 135.91 & 0.5 & 400 \\ \hline CD:WH (1:1) & 158.21 & 241.78 & 1 & 400 \\ \hline CD:FW (1:1) & 318.13 & 81.86 & 1 & 400 \\ \hline Blank & 400 & - & - & 400 \\ \hline  & **WH (g)** & **FW(g)** & **Ratio** & **Total (g)** & **CD (g)** & **Total (g)** \\ \hline WH:FW (1:1) & 256.76 & 43.23 & 1 & 300 & 100 & 400 \\ \hline WH:FW (2:1) & 276.70 & 23.29 & 2 & 300 & 100 & 400 \\ \hline WH:FW (1:2) & 224.41 & 75.58 & 0.5 & 300 & 100 & 400 \\ \hline \end{tabular}
\end{table} 
Table 1: (a) Optimization of WH and FW mono-digestion experiments using CD as an inoculum. (b) Effect of WH/FW ratio on co-digestion experiments using CD as an inoculum. In the case of the co-digestion of WH and FW with fixed inoculum (CD), the highest biogas production was observed to be 1655 \(\pm\) 91.92 mL/g VS in a 1:1 (WH:FW) ratio and 1400 \(\pm\) 56.56 mL/g VS in 1:2 (WH:FW) ratio. The co-digestion ratio of 2:1 (WH:FW) also exhibited a considerable biogas production of 1140 \(\pm\) 169.70 mL/g VS, which is also greater than mono-digestion (Figure 2a). The biogas production increased by approximately 70.62% when moving from the highest yield in mono-digestion to the highest yield in co-digestion. The observed increase in biogas production during co-digestion compared to mono-digestion can be attributed to several synergistic factors that enhance the overall anaerobic digestion process. Co-digestion, involving the combination of substrates such as WH and FW, results in a more balanced nutrient profile, particularly in terms of the C/N ratio which is crucial for optimal microbial activity. In mono-digestion, the highest methane yield was observed in the CD and FW at a 1:1 ratio, measuring 630 \(\pm\) 113.13 mL CH\({}_{4}\)/g VS, followed by CD:FW (1:2), CD:FW (2:1), CD:WH (2:1), CD:WH (1:1), and CD:WH (1:2), respectively. During the co-digestion, the average methane production exhibited higher values in the 1:1 and 1:2 mixtures of WH and FW, i.e., 890 \(\pm\) 70.71 mL CH\({}_{4}\)/g VS and 775 \(\pm\) 49.49 mL CH\({}_{4}\)/g VS, respectively, followed by the 2:1 ratio (585 \(\pm\) 49.49 mL CH\({}_{4}\)/g VS) (Figure 2b). In mono-digestion, the cumulative biogas and methane production collectively indicated that FW yielded a higher biogas production than WH. Moreover, the co-digestion of two substrates (WH and FW) notably enhanced gas and methane production compared to mono-digestion. FW might be more readily digestible, providing an immediate source of easily degradable organic matter. Analyses of the variation in the production of biogas between co-digestion, mono-digestion, and overall (both co-digestion and mono-digestion) digestion are presented in Supplementary Table S1. Analysis showed that in the case of co-digestion and mono-digestion, the models were non-significant (\(p\) < 0.310 and \(p\) < 0.824, respectively), whereas the overall biogas production from various substrates showed a significant result (\(p\) < 0.024).