## 3 Results and Discussion
### Anaerobic Digestion  
[PERSON] et al. [33] claim that few metric tons of aquatic biomass are removed annually from water reservoirs in Germany to keep them suitable for ship and boat traffic and for recreational purposes. They proved that specific methane yields from individual aquatic plants ranged from 142 to 372 \(\mathrm{dm^{3}CH_{4}}\)/kgVS. The results of their study demonstrate that aquatic macrophytes can be used, with no restrictions, as substrates in installations for anaerobic digestion. In the present study, a higher biogas yield was achieved while fermenting the biomass of hydrophytes harvested by moving the banks of water reservoirs (S1). Regardless of study variant, the fermentation processes spanned 20 days. In V1, the biogas yield was at 341 \(\pm\) 18 \(\mathrm{cm^{3}/gVS}\), and methane production was 174 \(\pm\) 9 \(\mathrm{cm^{3}/gVS}\) (Figure 5a). The methane content in this biogas reached 51 \(\pm\) 3%. The rate of biogas production in this part of the study reached 61.4 \(\pm\) 3.2 \(\mathrm{cm^{3}/gVS}\)-day, whereas that of methane was 29.6 \(\pm\) 1.5 \(\mathrm{cm^{3}/gVS}\)-day (Table 6). Significantly better effects were achieved in V2 with the silage of submerged hydrophytes used as the substrate. In this variant, biogas yield reached 371 \(\pm\) 9 \(\mathrm{cm^{3}/gVS}\), and methane content in the biogas was 59 \(\pm\) 1% (Figure 5b). The rates of biogas and methane production reached 77.9 \(\pm\) 1.8 \(\mathrm{cm^{3}/gVS}\)-day and 34.8 \(\pm\) 0.9 \(\mathrm{cm^{3}/gVS}\)-day, respectively (Table 6). Figure 5: The course of anaerobic digestion in S1 depending on experimental variant ((**a**) variant 1; (**b**) variant 2). [PERSON] et al. [37] studied the efficiency of biogas production for four types of grasses, including a species characteristic of wetlands, namely reed canary grass (_Phularis arundinaceae_). This species was also abundant in the biomass of mown hydrophytes in the present study. The biomethane production efficiency obtained by [PERSON] et al. [37] for this substrate averaged 296 dm\({}^{3}\)/kg VS. This study assumed no pre-treatment of the substrate, and it shows that the amount of biogas produced was about 35% higher for the biomass harvested in June compared to that harvested in August. This was most likely due to the higher content of hardly degradable lignins in plant tissues at the end of the growing season. Analyses conducted in the present study did not confirm this phenomenon. [PERSON] and [PERSON] [38] investigated the fermentation of biomass from broadleaf cattail (_Typha latifolia_). They recorded an approximately 66% decrease in the VS content of the tested substrate, and they showed that the biogas produced was mainly composed of methane and small amounts of hydrogen. In another research, [PERSON] and [PERSON] [39] achieved biogas yields ranging from 200 to 280 dm\({}^{3}\)/kgVS, depending on the fragmentation extent of the biomass used, the content of nitrogen compounds, and, thus, the C/N ratio. They used water hyacinth (_Eichhornia cassipes_) as a substrate and carried out anaerobic digestion at 35 \({}^{\circ}\)C. In turn, [PERSON] and [PERSON] [35] investigated the possibility of using higher vegetation from the water and wetland environment in the process of methane fermentation. They performed respirometric measurements to analyze the methane fermentation of common reed (_Pharginities australis_ (Cav.) Trin. ex Steud. ), broadleaf cattail (_Typha latifolia_ L.), and sweet flag (_Acorus calamus_ L.). The obtained biogas yields ranged from 132 cm\({}^{3}\)/gVS to 344 cm\({}^{3}\)/gVS, and the methane content of the biogas ranged from 51.1% to 61.7%, depending on the species tested and OLR applied. These authors recorded the highest biogas yields for common reed, whereas the highest percentage content of methane in biogas was recorded for hornwort fermentation. Their highest-unit biogas production was OLR = 1.0 gVS/dm\({}^{3}\).