## 4 Results and Discussion
### Reduction in Volatile Fatty Acids  
Figure 5 shows the VFA concentrations for both co-digestion and mono-digestion of WH, FW, and CD in different ratios. The highest VFA concentrations were consistently observed in certain mixtures. The WH:FW (1:2) co-digestion had the highest concentrations for most VFAs, with acetic acid at 1069.97 mg/L, followed by propionic acid at 644.63 mg/L and butyric acid at 170.95 mg/L. The CD:FW (1:2) mono-digestion also showed elevated levels, particularly of acetic acid (986.09 mg/L) and butyric acid (135.80 mg/L). Lower concentrations were generally found in CD:WH combinations for all VFAs, while the blank sample consistently had the lowest VFA levels, serving as a reference for comparison. The higher VFA production observed in samples containing FW can be attributed to its rich and easily degradable organic content. FW is composed of significant amounts of carbohydrates, proteins, and lipids that are quickly broken down by anaerobic microorganisms during digestion, forming VFAs such as acetic acid, propionic acid, and butyric acid. Carbohydrates in FW are rapidly converted into simple sugars and subsequently fermented into VFAs by acidogenic bacteria.  
Additionally, proteins and lipids in FW contribute further to VFA production through their breakdown into amino and long-chain fatty acids, respectively. This rapid degradation of FW contrasts with substrates like WH, which contains more fibrous material that requires longer digestion time, resulting in lower VFA production. Consequently, the high VFA concentrations observed in FW samples underscore the substrate's suitability for co-digestion, enhancing biogas production due to its readily fermentable organic matter.  
Elevated levels of VFA were prominently noted on days 6, 9, 12, and 15, coinciding with periods of notably lower biogas production which increased steadily. Diminished VFA concentrations were observed on days 18, 21, and 24, registering notably reduced VFA levels, suggesting a temporal decline in VFA concentrations over the course of the observation period. The reduction in volatile fatty acids along with the biogas production is described in Figure 6.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Reactors**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{2}{c}{**Grouping**} & \multicolumn{1}{c}{**COD**} & \multicolumn{1}{c}{**Mean**} & \multicolumn{1}{c}{**Grouping**} \\ \hline \multirow{6}{*}{Biogas production} & WH:FW (1:1) & 1339.3 & A & & & COD 3 & 6488.6 & A \\ \cline{2-8}  & WH:FW (1:2) & 1139.0 & A & B & & COD 6 & 5013.4 & B \\ \cline{2-8}  & WH:FW (2:1) & 943.4 & & B & C & COD 9 & 4123.8 & C \\ \cline{2-8}  & CD:FW (2:1) & 861.5 & & B & C & COD 12 & 3248.5 & D \\ \cline{2-8}  & CD:FW (1:2) & 857.5 & & B & C & COD 15 & 1936.3 & E \\ \cline{2-8}  & CD:FW (1:1) & 793.6 & & B & C & COD 18 & 1414.0 & E & F \\ \cline{2-8}  & CD:WH (1:2) & 742.3 & & & C & COD 21 & 1001.4 & & F \\ \cline{2-8}  & CD:WH (1:1) & 709.8 & & & C & & & \\ \cline{2-8}  & CD:WH (2:1) & 709.5 & & & C & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fisher’s Least Significant Difference test of biogas production and COD reduction.