###### Abstract  
One of the identified obstacles to the development of biogas plants is the limited sources of organic substrates. Hydrophytes varying in morphology, chemical composition, and origin proved to be viable alternatives. This study's aim was to determine the feasibility of deploying biomass from the cleansing of water reservoirs for anaerobic digestion. A mass and energy balance was estimated for Warmia-Mazury Province (Poland), the so-called Thousand Lake District. The effectiveness of anaerobic digestion was determined in anaerobic batch respirometric reactors. The biomass of hydrophytes harvested from this area approximated 38/070 Mg FM/year. The biogas yield from emergent plants approximated 350 m\({}^{3}\)/MgVS, and the average CH\({}_{4}\) content of biogas reached 55%, whereas the respective values obtained from submerged biomass reached 270 m\({}^{3}\)/MgVS and 58% CH\({}_{4}\). The total CH\({}_{4}\) volume achievable from the facility fed with waste biomass of this type allows producing ca. 1,654,560 m\({}^{3}\)CH\({}_{4}\)/year, which enables reaching 866 kW theoretical thermal power and 636 kWe net electrical power with the facility. The contribution of biomass from moving in this energy effect was about 84%, whereas that of the submerged vegetation obtained by hacking the bottom was 16%. hydrolytes; cleansing of water reservoirs; organic substrate; anaerobic digestion; methane; energy balance +
Footnote †: journal: Applied Sciences  
eutrophicationed, and degraded water reservoirs seem to be very prospective sources of hydrophyte biomass [10]. Available reports on biogas production from aquatic vegetation harvested from natural aquifers are mainly based on bench-scale studies or concern theoretical considerations, estimates, or calculations of the application potential of this substrate [11; 12; 13]. Nevertheless, harvesting algae biomass from areas located in the moderate climatic zone is hampered by multiple limitations. Variable climatic conditions have a direct impact on the achievable amount of biomass as well as on the taxonomic structure of the algae and, ultimately, the characteristics and properties of the organic substrate obtained in this way [14]. Due to decreased vegetation rates, winter periods feature negligible amounts of algal biomass in natural water reservoirs that are often covered with ice, which makes biomass harvesting ineffective from both technological and economic perspectives [15]. Different is the case with vascular hydrophytes, whose biomass is obtained systemically during the periodic maintenance and maintenance of hydrotechnical facilities [16]. It was proven that this substrate can be effectively preserved with the use of commonly used commercial ensilaging methods and then stored for a long time [17]. This ensures the right amount of organic substrate throughout the season. However, it should be emphasized that, due to the small amounts harvested, the biomass of vascular hydrophytes from natural reservoirs should rather be considered as one of the elements of a more varied substrate composition [18]. The application of vascular aquatic vegetation usually entails harvesting its biomass from reservoirs featuring higher trophic levels and sites in their vicinity [19]. In addition, biomass produced in hydrophyte-based wastewater treatment systems can be converted for energetic purposes [20]. Basically, these environments are conducive to the growth of biomass which, when used for energy purposes, significantly contributes to reducing the eutrophication rate of water reservoirs [21]. In many cases, hydrophytes represent a naturally growing raw material which reduces the financial outlays related to their cultivation in contrast to those of dedicated facilities. The costs incurred for sowing, fertilization, and proper maintenance on a plantation through the use of, e.g., plant protection agents are vital for the economic effectiveness of terrestrial energy crops [22].