## 3 Results and Discussion
### Anaerobic Digestion
In the present study, a significantly lower rate of effectiveness of anaerobic digestion was recorded in S2, in which respirometers were fed with hydrophytes harvested by hacking the bottoms of water reservoirs. The fermentation of fresh biomass (V1) spanned for 25 days (Figure 6a). The biogas yield obtained was 266 \(\pm\) 8 cm\({}^{3}\)/gVS (r = 31.9 \(\pm\) 1.0 cm\({}^{3}\)/gVS-day), whereas its methane content was 149 \(\pm\) 5 cm\({}^{3}\)/gVS (r = 12.0 \(\pm\) 0.4 cm\({}^{3}\)/gVS-day) (Table 6). When converted per TS, the subsequent values were 234 \(\pm\) 6 cm\({}^{3}\)/gTS and 131 \(\pm\) 4 cm\({}^{3}\)/gTS. The methane content was 56 \(\pm\) 2% (Figure 6a). In V2, the ultimate volume of biogas produced was achieved after 22 days of fermentation in respirometers. At this stage of the study, the biogas yield was 283 \(\pm\) 12 cm\({}^{3}\)/gVS, the mean production rate was 48.1 \(\pm\) 2.1 cm\({}^{3}\)/gVS-day, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Indicator**} & \multirow{2}{*}{**Unit**} & \multicolumn{4}{c}{**Value**} \\ \cline{3-6}  & & \multicolumn{2}{c}{**Stage 1**} & \multicolumn{2}{c}{**Stage 2**} \\ \cline{3-6}  & & **Variant 1** & **Variant 2** & **Variant 1** & **Variant 2** \\ \hline \multicolumn{6}{c}{Fermentation time} & days & 20 & 20 & 25 & 22 \\ \hline \multirow{2}{*}{Biogas yield} & cm\({}^{3}\)/gVS & 341 \(\pm\) 18 & 371 \(\pm\) 9 & 266 \(\pm\) 8 & 283 \(\pm\) 12 \\ \cline{2-6}  & cm\({}^{3}\)/gTS & 288 \(\pm\) 15 & 310 \(\pm\) 8 & 234 \(\pm\) 6 & 244 \(\pm\) 10 \\ \hline \multirow{2}{*}{Biogas production rate (r)} & cm\({}^{3}\)/gVS-day & 61.4 \(\pm\) 3.2 & 77.9 \(\pm\) 1.8 & 31.9 \(\pm\) 1.0 & 48.1 \(\pm\) 2.1 \\ \hline \multirow{2}{*}{Biogas production rate constant (k)} & 1/day & 0.18 & 0.21 & 0.12 & 0.17 \\ \hline \multicolumn{6}{c}{Methane content} & \% & 51 \(\pm\) 3 & 59 \(\pm\) 1 & 56 \(\pm\) 2 & 60 \(\pm\) 1 \\ \hline \multirow{2}{*}{Methane yield} & cm\({}^{3}\)/gVS & 174 \(\pm\) 9 & 219 \(\pm\) 5 & 149 \(\pm\) 5 & 170 \(\pm\) 7 \\ \cline{2-6}  & cm\({}^{3}\)/gTS & 146 \(\pm\) 7 & 183 \(\pm\) 4 & 131 \(\pm\) 4 & 147 \(\pm\) 5 \\ \hline \multicolumn{6}{c}{Methane production rate (r)} & cm\({}^{3}\)/gVS-day & 29.6 \(\pm\) 1.5 & 34.8 \(\pm\) 0.9 & 12.0 \(\pm\) 0.4 & 23.8 \(\pm\) 1.0 \\ \hline \multicolumn{6}{c}{Methane production rate constant (k)} & 1/day & 0.17 & 0.19 & 0.08 & 0.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Indicators characterizing the course of methane fermentation of hydroophyte biomass. and the biogas contained 60 \(\pm\) 1% methane (Figure 6b). The methane production rate was 23.8 \(\pm\) 1.0 cm\({}^{3}\)/gVS-day, which allowed the production of 170 \(\pm\) 7 cm\({}^{3}\)/gVS and 147 \(\pm\) 5 cm\({}^{3}\)/gTS (Figure 6b). According to data in the literature, the fermentation of Western waterweed (_Elodea nuttalli_) biomass harvested from five different lakes in Germany allowed the production of 415-520 dm\({}^{3}\) biogas/kgVS [40]. A study conducted by [PERSON] et al. [41] proved the feasibility of harnessing _Phragmites australis_ for the anaerobic digestion performed after pre-treatment with the steam explosion method. The biogas yield was up to 677 dm\({}^{3}\) biogas/kgVS. Indian studies demonstrated the feasibility of the anaerobic biodegradation of duckweed (_Lemma_ sp.). [PERSON] et al. [42] co-fermented duckweed with cattle dung in various ratios. They achieved the highest cumulative biogas production, reaching 12 dm\({}^{3}\), at a cattle dung to duckweed ratio of 1:1 (\(\
u\)/\(\
u\)), which showed its viability as a co-substrate in anaerobic digestion. In turn, [PERSON] et al. [43] analyzed the impact of pre-treating gibbous duckweed (_Lemma gibba_) on its specific methane yield. They discovered that it was feasible to achieve 24 times higher biogas production by autoclaving the biomass at 120 \({}^{\circ}\)C (1 bar) for 30 min. The total CH\({}_{4}\) yield (cm\({}^{3}\)CH\({}_{4}\)/gVS) ranged from 60 to 468 for pre-treated biomass and from 9 to 76 for non-pre-treated biomass.