## 2 Materials and Methods
### Station for Anaerobic Respirometric Measurements  
The effectiveness of the anaerobic digestion of the tested hydrohypte biomass was determined with the volumetric gas production method performed in sequencing batch respirometric reactors (AMPTS II, BPC Instruments AB, Lund, Sweden). Fermentation was performed at a temperature of 38 \(\pm\) 1 \({}^{\circ}\)C. bioreactors were equipped in a vertical \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Unit**} & \multicolumn{2}{c}{**Value**} \\ \cline{3-4}  & & **Variant 1** & **Variant 2** \\ \hline Total solids (TS) & (\%FM) & 27.2 \(\pm\) 2.9 & 26.7 \(\pm\) 3.2 \\ \hline Volatile solids (VS) & (\%TS) & 84.3 \(\pm\) 3.9 & 83.7 \(\pm\) 4.2 \\ \hline Total nitrogen (TN) & (mg/gTS) & 31.7 \(\pm\) 6.3 & 32.3 \(\pm\) 7.2 \\ \hline Total phosphorus (TP) & (mg/gTS) & 12.1 \(\pm\) 2.7 & 13.9 \(\pm\) 4.3 \\ \hline Total carbon (TC) & (mg/gTS) & 486 \(\pm\) 94 & 471 \(\pm\) 54 \\ \hline Total organic carbon (TOC) & (mg/gTS) & 409 \(\pm\) 77 & 392 \(\pm\) 63 \\ \hline C:N ratio & - & 12.9 \(\pm\) 3.2 & 12.3 \(\pm\) 2.4 \\ \hline pH & - & 7.12 \(\pm\) 0.11 & 6.34 \(\pm\) 0.09 \\ \hline Protein & (\%TS) & 19.8 \(\pm\) 4.1 & 20.2 \(\pm\) 4.5 \\ \hline Lipids & (\%TS) & 12.9 \(\pm\) 3.8 & 13.6 \(\pm\) 2.4 \\ \hline \multicolumn{1}{c}{Saccharides} & (\%TS) & 21.3 \(\pm\) 6.6 & 20.9 \(\pm\) 3.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of the biomass of hydrohypytes harvested by hacking the bottoms of water reservoirs in stage 2 of the study. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Parameter**} & **Unit** & **Value** \\ \hline Total solids (TS) & (\%FM) & 4.7 \(\pm\) 1.3 \\ \hline Volatile solids (VS) & (\%TS) & 70.9 \(\pm\) 2.5 \\ \hline Total nitrogen (TN) & (mg/gTS) & 45.3 \(\pm\) 3.1 \\ \hline Total phosphorus (TP) & (mg/gTS) & 4.0 \(\pm\) 1.0 \\ \hline Total carbon (TC) & (mg/gTS) & 384 \(\pm\) 29 \\ \hline Total organic carbon (TOC) & (mg/gTS) & 316 \(\pm\) 30 \\ \hline C:N ratio & - & 6.9 \(\pm\) 0.2 \\ \hline pH & - & 6.7 \(\pm\) 0.2 \\ \hline Protein & (\%TS) & 28.3 \(\pm\) 1.9 \\ \hline Lipids & (\%TS) & 6.1 \(\pm\) 0.8 \\ \hline Saccharides & (\%TS) & 1.8 \(\pm\) 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristics of anaerobic sludge used as the inoculum in respirometric tanks. stirrer operating at 100 rpm for 30 s every 10 min. The active volume of respirometers was 500 cm\({}^{3}\). The initial organic load rate (OLR) was 5.0 gVS/dm\({}^{3}\). To ensure anaerobic conditions inside the digesters, before the measurements, their contents were purged with pure nitrogen for 5 min with an efficiency of 150 dm\({}^{3}\)/h. Fermentation was continued for 40 days. Biogas composition was monitored chromatographically, and the measurement system was equipped in an ex situ CO\({}_{2}\) adsorption unit, in which 3 M NaOH was used to enable CO\({}_{2}\) fixation from biogas. A biomethane production report was issued every 24 h using software which generated results for the volume of normalized biogas (standard atmospheric pressure 101.3 kPa at 0 \({}^{\circ}\)C and null humidity). Endogenous biogas generated by the anaerobic sludge was excluded from computations. Figure 3 presents the scheme of the research station. Resiprometric studies also included determination of the biogas production rate (r) depending on the experimental variant. Reaction rate constants (k) were determined based on the obtained experimental data with the non-linear regression method and by using Statistica 13.1. An iterative method was used in which the function is replaced by a linear differential with respect to the determined parameters in each iterative step. A consistency coefficient (\(\varphi^{2}\)) was adopted as a measure of curve fit (at the determined parameters) to the experimental data.