Background

Waterweeds (Elodea nuttallii and Elodea canadensis) are invasive neophytes, which have been proliferating at a phenomenal rate during the last decades in German waterways. In case of overgrowth, the strong covering of vegetation can cause problems in hydroelectric power plants and leads to limitations in ship and boat traffic as well as in use for bathing and fishing activities. After vegetation period, dead plants can accumulate and then negatively influence flood protection and water engineering works. For this reason, the aquatic biomass has been periodically removed and disposed without further use. In order to enable the energetic use of this water-containing substrate, the aim of the present study was the optimization of storage methods for an aquatic plant-based feedstock for biogas production. In climatic cold regions, substrate conservation is necessary in order to guarantee a year-round substrate availability. With waterweed (Elodea) taken as an example, the ensiling of aquatic plants was studied. The main focus was to develop practical methods for biomass conservation while producing high biogas yields.

Methods

Elodea was harvested in the river Parthe in Leipzig-Schönefeld in October 2015. Silage mixtures of Elodea and wheat straw were tested after 180 days of storage for pH, volatile fermentation products, and methane potentials. The effect of different silage moisture contents and straw particle sizes on the substrate quality was studied.

Results

Results show that waterweeds can be stored by ensiling and can achieve considerable biogas yields. However, with a water content of about 95%, the storability of the material is challenging. Mixed silage of waterweeds and wheat straw were suitable for storage in clamp silos. The pH values were between 4.9 and 6.5, and the volatile fatty acid content as lactic acid ranged from 0.0 to 1.9% total solid. The mixed silages achieved methane potentials between 166 and 228 mL g^− 1 volatile solid (VS), which is equivalent to 52 and 72% of maize silage. Considering a methane potential of 228 mL g^− 1 VS and costs for material pretreatment, the most promising silage variant was mixed silage from waterweeds and ground straw with 30% total solid content.

Conclusions

Long storability of waterweeds could be possible by ensiling fermentation. Mixed silages from Elodea and wheat straw show suitable substrate characteristics for biogas production and can achieve high biogas yields.