Using aquatic plant biomass from de-weeding in biogas processes-an economically viable option?

Background: Landscape maintenance in Germany today requires regular and extensive de-weeding of waterways, mostly to ensure water runoff and provide flood protection. The costs for this maintenance are high, and the harvested biomass goes to waste. Methods: We evaluated the economic feasibility of using water plant biomass as a substrate in biogas generation. We set up a plausible supply chain, used it to calculate the costs of using aquatic water biomass as a seasonal feedstock to generate biogas, and compared it against maize silage, a standard biogas substrate. We also calculated the costs of using the aquatic biomass mixed with straw silage. Results: Although subject to estimation errors, our results do show that it is economically feasible to use water plants as a seasonal feedstock in a biogas plant, even in markets where their disposal yields only moderate gate fees. Ensiling water plants with straw, however, incurs the added high price of straw and thus only yields a positive financial result if gate fees for water plant disposal are very high. Conclusions: Water plant biomass need not remain an unwelcome by-product of de-weeding waterways. The funds for its costly disposal can be redirected to the biomass supply chain and support the profitable use of aquatic biomass as a seasonal feedstock in biogas plants. However, the legal status of material from de-weeding needs to be clarified before biogas operators can act. Further development of technology for harvesting aquatic biomass is also called for.