Anaerobic digestion of mixed silage of waterweed biomass and wheat straw in a long-term semi-continuous biogas production process

Background: As part of the maintenance of lakes and rivers, large quantities of aquatic biomass are produced every year. Neophytes, such as Elodea nuttallii, compete with native species and cause a disturbance to the ecosystem, which can manifest itself in the form of eutrophication. Aquatic macrophytes are not in competition with the production of food and animal feed and thus can be used in anaerobic digestion plants to replace a fraction of commonly used renewables such as maize or grass silage. Mixed silage of Elodea biomass with wheat straw was recently developed to allow for the conservation of mowed aquatic biomass. In this paper, the digestibility of this silage was tested in a long-term semi-continuous fermentation experiment with a duration of 212 days. Methods: A continous-stirred tank reactor with a working volume of 31 L was used for the semi-continuous fermentation experiment. The substrate Elodea-straw silage was supplied to the fermenter once a day. The samples of digestates were analyzed directly after sampling twice a week for total solids, volatile solids, VFA/TAC, NH4-N, acetate, propionate, butyrate, and water-solutle elements. The biogas composition was determined by means of gas chromatograph once a week. Results: The results show that mono-fermentation of Elodea straw silage was stable for a period of at least 6 months. A stable anaerobic digestion process was achieved at an organic loading rate (OLR) of up to 2 gVS/(L*d), when the highest specific biogas yield of 642 mLN/(gVS*d) was reached. The process turned unstable when the OLR was increased to 3 gVS/(L*d) due to the accumulation of volatile fatty acids. A nitrogen source, urea, was added in order to support the anaerobic digestion process. No addition of any further supplements was needed. Throughout the whole experiment, the specific biogas yield reached 647 ± 71 mLN/(gVS*d) with a mean methane content of 51%. Conclusions: The results of this semi-continuous fermentation experiment confirm the possibility of using Elodea straw silage as a mono-substrate in the anaerobic digestion process for the production of biogas being the aim of this Short Communication.