## 3 Results
### Biogas Yield and Composition  
Studies from around the world have tested the potential of co-digesting low-C/N microalgal biomass with carbon-rich energy crops. [PERSON] et al. [51] co-digested algal biomass with corn straw, achieving a biogas yield of around 687.3 cm\({}^{3}\)/gVS with 63.3% methane. [PERSON] et al. [52] observed that the co-digestion of algae and corn straw at a C/N ratio of 20/1 increased methane yield by 62% compared with the digestion of microalgae alone, reaching 325 cm\({}^{3}\)/gVS. Researchers have reported that 20/1 is the optimal C/N ratio for the feedstock mix. A study by [PERSON] et al. [53] has shown that even small admixtures of microalgal biomass in corn silage (1 to 6 ratio) can produce a 9% increase in the biogas yield, reaching 660 m\({}^{3}\)/kgTS. The superior biogas output from the co-digestion of algal biomass and other organic feedstocks has been attributed to the synergistic effects occurring during anaerobic degradation of mixed feedstocks. Microalgal biomass can serve as a source of the nitrogen and micronutrients needed for the optimal growth of anaerobic microbes. [PERSON] et al. [54] have demonstrated that using a combination of carefully selected feedstocks can directly and significantly improve the efficiency of anaerobic degradation. [PERSON] and [PERSON] [55] operated a pilot-scale system where _Laminaria_ sp. and _Ulva_ sp. macroalgae were mixed with organic waste. Their experiment indicated that the stable operation of anaerobic reactors can be maintained if aquatic plants are mixed with the co-feedstock at appropriate ratios. In Variant 1 (MA100/MS0), the biogas yields averaged 384.72 \(\pm\) 22.11 cm\({}^{3}\)/gTS and 438.73 \(\pm\) 25.21 cm\({}^{3}\)/gVS, and the biogas contained 65.12 \(\pm\) 1.94% methane (Table 3). The total biogas output was around 3509.82 \(\pm\) 201.71 cm\({}^{3}\)/d, of which 2285.59 \(\pm\) 199.46 cm\({}^{3}\)/d was methane (Table 3). Variants MA80/MS20, MA60/MS40 and MA40/MS60 showed markedly improved digestion efficiency. We noted a steady improvement in biogas yields, ranging from 453.06 \(\pm\) 26.05 cm\({}^{3}\)/gTS in Variant 2 to 573.83 \(\pm\) 12.54 cm\({}^{3}\)/gTS in Variant 4 (Table 3). The methane content in the biogas ranged from 63.92 \(\pm\) 3.02% in Variant MA80/MS20 to 60.62 \(\pm\) 4.13% in Variant MA40/MS60 (Table 3). The total daily production of methane was: 2605.51 \(\pm\) 272.94 cm\({}^{3}\) CH\({}_{4}\)/d (MA80/MS20), 2817.10 \(\pm\) 256.62 cm\({}^{3}\) CH\({}_{4}\)/d (MA60/MS40) and 3045.56 \(\pm\) 274.06 cm\({}^{3}\) CH\({}_{4}\)/d (MA40/MS60) (Table 3). Variant MA20/MS80 produced 612.42 \(\pm\) 19.07 cm\({}^{3}\)/gVS and 567.11 \(\pm\) 17.66 cm\({}^{3}\)/gTS biogas (Table 3). The methane fraction in the biogas averaged 57.38 \(\pm\) 3.69%, which translated to a methane production rate of 351.40 \(\pm\) 33.54 cm\({}^{3}\)/gVS and 325.41 \(\pm\) 31.06 cm\({}^{3}\)/gTS (Table 3). Biogas and methane output was 4899.33 \(\pm\) 152.57 cm\({}^{3}\)/d and 2811.24 \(\pm\) 268.35 cm\({}^{3}\)/d, respectively (Table 3). The MS-only variant performed significantly worse, with biogas yields of only 569.42 \(\pm\) 8.25 cm\({}^{3}\)/gVS and 534.29 \(\pm\) 7.74 cm\({}^{3}\)/gTS (Table 3). The methane content in the biogas was around 51.35 \(\pm\) 2.88% (Table 3). The low methane concentration was offset by high biogas yields, meaning that the nominal biogas production was 292.40 \(\pm\) 20.64 cm\({}^{3}\)/gVS and 274.36 \(\pm\) 19.36 cm\({}^{3}\)/gTS (Table 3). Nevertheless, these values were still much lower than those noted for Variants MA60/MS40 and MA20/MS80. The daily biogas and methane yields averaged 4555.39 \(\pm\) 65.99 cm\({}^{3}\)/d and 2339.19 \(\pm\) 165.09 cm\({}^{3}\)/d, respectively (Table 3). In our study, we noted that the digestion of \"pure\" substrates (microalgal biomass or giant miscanthus silage) performed the worst, with biorethane production rates of 2285 cm\({}^{3}\) CH\({}_{4}\)/d and 2339 cm\({}^{3}\) CH\({}_{4}\)/d, respectively.