## 3 Results  
### Feedstock Properties and Parameters  
It is widely acknowledged that the composition of anaerobic reactor feedstock is a major determinant of the growth rate of anaerobic bacteria and the resulting biogas yields. According to [PERSON] et al. [42], insufficient trace elements and alkalinity in methanegested plant biomass can disrupt the methane production process. Data in the literature indicate that co-digestion of various substrates leads to more balanced physicochemical parameters of the fermented mixture and thus to improved methane output [34; 35]. Many authors have also noted that co-digestion has additional technological, commercial and environmental benefits [43]. Research to date has indicated that there are several factors which limit the efficiency of methane digestion and can effectively limit or stop biogas production in microalgae-mediated processes [44; 45]. Such limiting factors include cellulose or hemicellulose in the cell walls (which are resistant to anaerobic digestion), formation of compounds that are toxic to anaerobic bacteria by certain algae strains and an insufficient C/N ratio in the digested feedstock [46].  
The tested microalgal biomass had an organic load rate of 87.7 \(\pm\) 1.1% (Table 1). The TC and TOC contents were 464 \(\pm\) 25 mg/gTS and 437 \(\pm\) 20 mg/gTS, respectively. TN content averaged 46.0 \(\pm\) 3.9 mg/gTS, while TP content approximated 4.4 \(\pm\) 0.9 mg/gTS. The biomass C/N ratio, a key parameter for methane fermentation, was very poor at 9.5 \(\pm\) 0.4. The fractions of total protein, lipids and carbohydrates in the mixed microalgal culture were 28.7 \(\pm\) 2.5% TS, 20.0 \(\pm\) 1.4% TS and 15.8 \(\pm\) 2.6% TS, respectively.  
The giant miscanthus silage had higher rates of organic matter, with the carbon in dry matter at 93.8 \(\pm\) 0.2% TS, an average TC of 460 \(\pm\) 13 mg/gTS and TOC of around 441 \(\pm\) 15 mg/gTS (Table 1). The nitrogen and phosphorus contents in the silage were found to be significantly lower than in the microalgal biomass, with values of 11.1 \(\pm\) 0.9 mg/gTS and 2.4 \(\pm\) 0.3 mg/gTS, respectively. The C/N ratio averaged 39.6 \(\pm\) 1.7. The respective contents of the total protein fraction, the lipid fraction and reducing sugars were 9.2 \(\pm\) 0.6% TS, 2.2 \(\pm\) 0.5% TS, and 60.4 \(\pm\) 1.0% TS.  
Mixing giant miscanthus silage with microalgal biomass in different proportions had a direct effect on the key feedstock parameters for methane digestion, particularly the C/N ratio (Table 1), which ranged from 9.5 \(\pm\) 0.4 to 39.6 \(\pm\) 1.7, depending on the experimental variant. The best C/N ratios were obtained when the proportion of microalgal biomass was between 20% and 40% dry organic matter: 24.3 \(\pm\) 1.4 and 17.5 \(\pm\) 1.2, respectively (Table 1). Different feedstock combinations also affected the other tested parameters, including dry organic matter, and total phosphorus, protein, lipids and sugars (Table 1). However, these variations were less significant than in the case of the C/N ratios.  
The C/N ratio in the digester feed should be within the 20/1-30/1 range, whereas the ratio of most algae-based feedstocks is below 10, inhibiting methane generation [47; 48]. This limiting effect of low C/N may stem from high concentrations of ammonia nitrogen and volatile fatty acids in the digesters, which are detrimental to methane production [49]. One way to counteract this hindrance to anaerobic processing of algal biomass is to improve the C/N ratio by adding biomass that is rich in organic carbon to the feedstock mix [50].