## 4 Results and Discussion
### Cumulative Biogas Production and Methane Content
It may be concluded that production efficiency greatly varies among the different varieties of substrates, and future studies should focus on the specific substrates and different ratios. Paired sample t-tests between different co-digestions are presented in Supplementary Table S2. The table shows that the differences between WH:FW (1:1) and WH:FW (2:1) and WH:FW (2:1) and WH:FW (1:2) were highly significant (\(p\) < 0.004; \(p\) < 0.003, respectively). The difference between WH:FW (1:1) and WH:FW (1:2) also showed significant results but less significant than the other two co-digestions. In the comparison of biogas production during the co-digestion experiments, the ANOVA values did not indicate significant variations among them. On the other hand, the ANOVA results for mono-digestion revealed significant variations in biogas production. Figure 2: Cumulative average production (**(a)** biogas production and (**b**) methane yield). [PERSON] et al. [56] conducted a similar investigation on biogas production utilizing FW and WH, where the highest methane content of 68.15% was observed in a substrate ratio of 25:75 g (WH:FW). [PERSON] and [PERSON] [76] reported the highest biogas production from the mixture of WH and FW in a ratio of 1:2, producing biogas of 0.55 L, with a corresponding methane content of 73.3%. [PERSON] [67] explored the biogas production potential of FW and seagrass combined with macroalgae using 2000 mL of plastic bottles, achieving a maximum cumulative biogas production of 8410 mL with a mixture of 60% FW and 40% seagrass/microalgae. [PERSON] et al. [77] investigated the anaerobic batch co-digestion of sisal pulp and FW, identifying the highest methane yield of 0.62 CH\({}_{4}\) m\({}^{3}\)/kg VS (i.e., 620 CH\({}_{4}\) L/kg VS) when FW and sisal pulp were mixed in a ratio of 33:67. [PERSON] et al. [68] examined anaerobic digestion with various concentrations of TS in FW and obtained the maximum biochemical methane potential (BMP) of 464.55 mL CH\({}_{4}\)/g VS from 1% TS. [PERSON] [69] conducted anaerobic digestion of FW with different substrates from a fish-oil refinery, achieving a maximum biogas production of 2089 mL from a mixture of FW and light ethyl monoester, with a methane production of 72%. [PERSON] et al. [71] investigated anaerobic digestion using FW, bagasse collected from a sugar mill as a substrate, and municipal sludge as an inoculum, observed a maximum biogas yield of 409.5 mL/g VS with a 73.34% methane content from the mono-digestion of FW, compared to co-digestion with bagasse producing biogas with a 67.8% methane content. [PERSON] and [PERSON] [73] reported a biogas yield of 757 mL/g VS from FW after 60 days of digestion, with a methane yield of 554 mL/g VS\({}^{-1}\) and a methane content of 73%. [PERSON] et al. [78] investigated anaerobic digestion using FW and fish crude oil waste, obtaining methane contents of 540.5 CH\({}_{4}\) mL/g VS and 426.36 mL/g VS, respectively, using anaerobic microbiota from a biogas plant sludge mixture as the inoculum. It can be noted that the highest biogas yield was observed in the present study when using WH and FW in co-digestion and mono-digestion experiments, when compared to other studies. Different studies conducted by various researchers on mono-digestion and co-digestion using FW and WH as substrates with different inoculum are summarized in Table 3. The results from these studies indicate that biogas production from mono-digestion is lower compared to co-digestion, which aligns with the present study's findings. Our results confirm this trend, showing a significant increase in biogas production when WH and FW are co-digested, underscoring the advantages of substrate synergy in anaerobic digestion.