The application and treatment of freshwater macrophytes as potential biogas base materials: A review

Working towards a carbon-neutral future requires countries to decrease fossil fuel utilisation which is closely related to decreasing the natural gas dependence of countries. The utilisation of energy sources within the countries is preferable, for example by increasing biogas production and utilising materials used to a smaller extent. Such bioresources may be various freshwater macrophytes, especially such specimens that are quick to spread due to their invasive nature. If the atmospheric carbon dioxide bound by these macrophytes is not turned into methane via anaerobic digestion but the plants are used as base material, it could be a significant step to achieve a carbon-neutral future. In the article, various biogas production experiments with these plants are reviewed, focusing on the gas and methane yield from the mono- and co-digestion of the different plants, including the possible pre-treatment methods, and the disadvantages of their use. The biogas yield of freshwater macrophytes is generally between 200 and 400 Lgas/kgVS which can be increased with co-digestion in most cases. The use of aquatic plants after phytoremediation has not been fully explored yet, but the results so far indicate that biogas production is not hindered by the bound elements. The co-digestion experiments revealed that the addition of aquatic plants in 25–50 % to the base material produces the best gas and methane yield values. Various pre-treatment methods can be used to process aquatic plants before anaerobic digestion. Comminution and thermal treatment can increase gas yield but the effect of chemical pre-treatment depends on the type of chemical used. The utilisation of freshwater macrophytes in biogas production could be successfully up-scaled, as the pilot-sized experiments showed promising results.