Evaluation of the Potential of Eutrophic Urban Water Reservoirs as a Source of Biomass for Anaerobic Digestion Using the Example of the City of Olsztyn, Poland ^†

Olsztyn is a city with 170,000 inhabitants in the north-east of Poland in the Masurian Lake District with an area of 1732 km², of which 486 km² are lakes. There are currently 16 lakes within the administrative boundaries of Olsztyn, 14 of which are larger than 1 ha. In total, all lakes cover an area of 725 ha, which is 8.25% of the area of the entire city. In the western part of Olsztyn, the proportion of lakes is 40% and 8% in the eastern part. Two rivers also flow through the city, namely the Łyna and its left tributary, the Kortówka. The waters within the city limits are mostly heavily polluted, often eutrophic, and degraded. Therefore, they are characterized by considerable primary production and can be a source of biomass for biogas production.

The biomass of vascular plants, phytoplankton, and bottom sediments, which are rich in organic matter, is characterized by considerable energy potential. It should be emphasized that the removal of organic matter from the biomass pool of the lake, on the one hand, provides a substrate useful for further energy conversion and, on the other hand, achieves an environmental effect related to the reclamation and improvement of the trophic state of rivers and lakes. The aim of this work was to analyze the feasibility of using the biomass of aquatic plants, phytoplankton, and bottom sediments resulting from regular mowing of the banks and cleaning of the bottom of water reservoirs, as well as targeted remediation measures for energy purposes. The work focused on the efficiency of biogas production from fresh biomass and biomass preserved by pickling, and it also created a mass–energy balance for the city of Olsztyn. In addition, the efficiency of co-digestion of biomass from eutrophic urban water reservoirs with typical biodegradable organic waste characteristic of the city’s municipal economy, namely sewage sludge and the organic fraction of municipal waste, was evaluated.

The research was conducted from April to October and focused on the amount of primary production in the monitored period of the growing season and the potential for its utilization in the methane fermentation process. Experiments to evaluate the efficiency of biogas production were carried out on a laboratory scale using respirometric measurements. The kinetics of the anaerobic biodegradation process of the tested organic substrates were determined and the quantity and qualitative composition of the gaseous products of anaerobic bacterial metabolism were evaluated.