## 3 Results and Discussion
### Energy Balance  
The mass of biomass of aquatic and swamp-water vegetation, which is harvested annually only as a result of planned and necessary cleaning and maintenance work, is approximately 38,073 Mg FM/year, of which vegetation from moving accounts for nearly 36,806 Mg FM/year, whereas biomass obtained by hacking reaches 1267 Mg FM/year (Table 4). Each year, cleaning works cover a total of nearly 2240 ha. The total VS mass of emerging hydrophytes is 8425 MgVS/year, whereas the mass of harvested submerged vegetation is approximately 210 MgVS/year. The calculations took into account the average volumes of biogas and methane obtained during experimental research. The average volume of biogas produced reached 350 m\({}^{3}\)/MgVS, and 55% methane content was taken into account in calculations made for stage 1. In turn, in stage 2, the efficiency of biogas production and biogas content were assumed to be 270 m\({}^{3}\)/MgVS and 58%, respectively. The assumed values are averages for the efficiency of methane fermentation of fresh biomass and biomass preserved via ensilaging. The energy balance shows that the annual volume of biogas derived from a facility fed by this type of waste biomass can produce approximately 1,654,560 m\({}^{3}\)CH\({}_{4}\)/year, including 1,621,707 m\({}^{3}\)CH\({}_{4}\)/year (S1) and 32,853 m\({}^{3}\)CH\({}_{4}\)/year (S2). This makes it theoretically possible to achieve 866 kW of power from the facility. After taking into account the assumed efficiency of the co-generation system (81%) and the expected heat and electric energy consumption for process purposes, the net electric power of the facility would be 636 kWe (Table 7). The contribution of biomass from moving in this energy effect was about 84%, whereas that of the submerged vegetation obtained by hacking the bottoms was 16%. The mass balance of the harvested hydrophyte biomass was carried out by [PERSON] et al. [34]; however, it was based only on theoretical assumptions. They made an attempt to estimate the amount of aquatic plant biomass derived from all stagnant waters \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Indicator**} & \multirow{2}{*}{**Unit**} & \multicolumn{3}{c}{**Biomass Type**} & \multirow{2}{*}{**Total**} \\ \cline{3-4}  & & **Mowing/S11** & & **Hacking/S2** \\ \hline Annual methane production & m\({}^{3}\)CH\({}_{4}\)/year & 1,621,707 & 32,853 & 1,654,560 \\ \hline Potential energy production & kWh/year & 14,871,050 & 301,263 & 15,172,313 \\ \hline Theoretical thermal power & kW & 730 & 136 & 866 \\ \hline Theoretical electric power & kW & 645 & 120 & 765 \\ \hline Gross heat production & GJ/year & 21,023,136 & 3,905,568 & 24,928,704 \\ \hline Heat consumption for process purposes & GJ/year & 6,306,941 & 1,171,670 & 7,478,611 \\ \hline Net heat production & GJ/year & 14,716,195 & 2,733,898 & 17,450,093 \\ \hline Gross electric energy production & kWe/year & 5,160,720 & 958,720 & 6,119,440 \\ \hline Electric energy consumption for process purposes & kWe/year & 464,465 & 86,285 & 550,750 \\ \hline Net electric energy production & kWe/year & 4,696,255 & 872,435 & 5,568,690 \\ \hline
**Net power of biogas plant** & **kWe** & & **636** \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Energy balance of methane fermentation of hydrophytes harvested in Warmia–Mazury Province. in Germany that is potentially useful for biogas production. To this end, they assumed that the percentage of standing water surface area with a water depth of 0-2 and 0-5 m defined for the Mecklenburg-Vorpommern region was accurate enough to estimate the biomass potential for the whole country. Based on these authors' observations, 25% of the shore surface was covered with aquatic vegetation, which constitutes a trophogenic stratum available for solar radiation in the entire vertical cross-section of the water column.