## 3 Results and Discussions  
### Anaerobic Digestion Experiments  
The values of the ratio of VS\({}_{\text{WHDS}}\) to VS\({}_{\text{SY}}\) of the different substrate mixtures are listed in Table 3, where the cumulative biomethane potentials obtained after 20 days are also listed. The daily biomethane potential in mL/gVS\({}_{\text{added}}\) and cumulative biomethane potential in mL/gVS\({}_{\text{added}}\) from the anaerobic degradation of the different substrate mixtures of Mix-1 to Mix-4, for WHDSonly and SYonly, are shown in Figures 3 and 4, respectively. The daily biomethane potential curves presented in Figure 3 show that initially, a rapid anaerobic degradation occurred for most of the substrate mixtures, with a high daily biomethane production occurring within one day of the experiments. This observation may be explained by acknowledging that the microbial population (the inoculum) was harvested from an environment similar to the environment in which the AD process was undertaken in this study. This led to shorter adaptation times. Some of these environmental similarities are in terms of the substrate concentrations, temperature and pH conditions. Furthermore, the displacement of oxygen gas in the bioreactor headspace using nitrogen gas, minimized unwanted oxygen-related inhibitions, thus further reducing the microbial adaptation time. \begin{table}
\begin{tabular}{c c c c} \hline \hline \(\mathbf{VS_{WHDs}}\): \(\mathbf{VS_{SY}}\) & **Designation** & **Biomethane Potential (mL/gVS\({}_{added}\)) 1** & **Biomethane Content (vol.\%) 1** \\ \hline
1:0 & WHDs only & 65.43 & 56.5 \\
4:1 & Mix-1 & 148.58 & 58.8 \\
3:2 & Mix-2 & 179.03 & 61.1 \\
2:3 & Mix-3 & 170.53 & 61.9 \\
1:4 & Mix-4 & 264.13 & 61.2 \\
0:1 & SY only & 169.64 & 61.5 \\ \hline \hline \end{tabular} \({}^{1}\) Mean value.  
\end{table} 
Table 3: Biomethane potential and biomethane contents of biogas generated from the degradation of the different substrate mixtures. Figure 4: The variation of cumulative biomethane potential with time from the different substrate mixtures. The mass ratios of the volatile solids of WHDs to the volatile solids of SY for the different substrate mixtures are listed in Table 3.  
Figure 3: Daily biomethane potential from the different co-substrate mixtures investigated. The mass ratios of the volatile solids of WHDs to the volatile solids of SY for the different substrate mixtures are listed in Table 3.  
The values of the biomethane potential of the different co-digestion substrate mix ratios (BMP) are listed in Table 3. The cumulative biomethane potential curves are subsequently presented in Figure 4. Table 3 and Figure 4 show that the substrates designated as Mix-4 and WHDS only were characterized with the highest biomethane potential of 264.13 mL/gVS\({}_{\text{added}}\) and the lowest biomethane potential of 65.43 mL/gVS\({}_{\text{added}}\), respectively. The poor cumulative biomethane potential generated from the degradation of the WHDS only substrate may be a reflection of the presence of some residual reagents, such as non-polar hexane used in fatty acid recovery from the WHDS [3] that may be toxic to the microbial population present. The results also suggest that the VS\({}_{\text{WHDS}}\) to VS\({}_{\text{SY}}\) mass mixing ratio of 1:4 (wet mass mix ratio of SY to WHDS of 1.029:1), with a moisture content of the substrate mixture being 7.26 times the mass of dry solids, provided the best balance of nutrients for enhanced biodiversity and microbial activity for the anaerobic degradation process. The results presented in Table 3 and Figure 4 show that the introduction of the SY waste as a co-digestion substrate may serve to improve the biomethane content of the biogas generated from the degradation of the WHDS only substrate by diluting any residual toxic chemical present. Slight variations in the biomethane content were measured for all other substrate mixes investigated, with the biomethane content of the biogas generated kept approximately close to 61 vol.% and carbon dioxide as a residual biogas component.