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Abstract Background Utilization of energy crops for biogas production has been controversially discussed in Germany due to negative environmental effects and the “food vs. fuel” debate. This led to a search for alternative substrates focusing on material from landscape management measures. Aquatic biomass is harvested during water body management, yet it has not been considered for energy generation. Methods The information where and which amount of biomass is collected by aquatic de-weeding operations in rivers and lakes was gathered via a nationwide survey. In addition to that, the amount of aquatic plant biomass potentially available in water bodies was estimated exemplarily for the flowing waters of Baden-Württemberg—by using data from the European Water Framework Directive surveys. Results The survey revealed 172 locations of de-weeding operations in flowing waters and 93 in standing waters. These locations are concentrated in lowland rivers of the North German Plain as well as the Upper Rhine Plain. Standing water de-weeding operations were reported mainly from the foothills of the Alps. The overall amount of biomass harvested per year is 36,244 t of fresh biomass. Taking into account missing data, a maximum of 100,000 t of fresh biomass per year can be estimated for Germany. The case study on plant biomass de-weeded from waters in Germany revealed that only a small part of the total aquatic plant biomass is actually harvested. Conclusions The amount of biomass harvested and removed from water bodies in Germany is considerably lower than the harvest of other substrates from landscape management measures such as mowing of meadows or trimming of trees and hedges. However, larger amounts are accumulating locally, concentrated in some regions or at specific water bodies, e.g., reservoirs, for which regional value chains could be established. In order to make the exploitation of these local potentials economically viable, changes regarding the economic and technological framework are required.

Abstract Background A major problem with farming systems is the deficiencies in phosphorus (P) due to fixation in soils, erosion and run-off, and exports of herbal and animal products. P resources for the compensation of these losses will sooner or later be depleted. For this reason, innovative ideas for phosphorus recycling are highly relevant. The P excess from farming systems mostly ends up in surface waters, leads to eutrophication, and promotes the growth of aquatic plants. Particularly invasive neophytes such as western waterweed (Elodea nuttallii) can rapidly generate high levels of biomass in waters with good nutrient supply and bind relevant amounts of phosphorus. Methods In the renatured open-pit mine Goitzsche (Saxony-Anhalt, Germany), biomass from E. nuttallii was harvested (2005–2008) and the biomass dry matter and the P concentration were determined. The phosphorus recovery potential from this plant biomass was calculated by extrapolation based on the phosphorus analyses and the area potentially populated by E. nuttallii. One analysis of E. nuttallii was conducted to evaluate the content of potentially toxic elements (PTEs). Results The results showed that with 0.5 to 6.3 kg P/Mg of total solid (dry matter), E. nuttallii could have a high potential to recycle phosphorus, e.g., by anaerobic digestion and digestate fertilization. Lake Goitzsche offered an annual recovery potential from 0.5 to 1.7 Mg phosphorus in the investigation period. This could meet the needs of 114-ha organic farming land based on a 7 kg/(ha*year) regional phosphorus deficit. The digestate of E. nuttallii is very well suited as a fertilizer due to its high phosphorus concentration. The concentrations of PTEs in the current digestate (related to an individual case) are sufficient for legal admission in Germany. In this study, nickel was above the threshold values for Germany. The elevated nickel levels in the Elodea biomass correspond to the geogenic high nickel concentrations in the sediment of this lake. Conclusions Aquatic macrophytes have a significant potential for recovering phosphorus from waters and sediments of relevant phosphorus concentrations. Further studies of surface water zones, particularly with regard to the aquatic plant biomass and phosphorus concentration of sediments, are needed to assess future exploration.

Abstract 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.

Abstract 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.

Abstract Background Aquatic plants are an important component of aquatic ecosystems. They are valuable for the oxygen and carbon dioxide household and generate habitats especially for small fish and other small organisms. However, problems for the maintenance of water bodies can result from mass occurrences of these plants. Invasive neophytes - such as members of the Elodea genus - are particularly problematic in this regard. Aquatic plants need to be harvested regularly to ensure that water bodies remain usable and to safeguard flood protection for flowing water bodies. Energy can be produced from the harvested material by anaerobic digestion in biogas plants. Therefore, it is beneficial to know the best time for harvesting in this context. Methods To identify the best time for harvesting, samples of the Elodea stock in the river Parthe in Leipzig-Schönefeld were taken each week over the course of the two hydrological years 2015 and 2016. The composition of these samples was analyzed in the laboratory. In the second hydrological year, three samples from surface areas of 1 m2 were also harvested once each month in order to determine the biomass yield per unit area. Results The best harvesting time for energy production from Elodea biomass in Germany is in the summer months (June to September). During this period, the specific yield of 0.5–0.7 kg VS/m2 is relatively high and the Elodea biomass contains the highest fractions of volatile solids (80.1 ± 2.3%), high contents of plant nutrients (N 35.9 ± 4.0 g/kg TS; P 6.1 ± 1.4 g/kg TS; and K 47.7 ± 8.0 g/kg TS), and low concentrations of heavy metals (Cr ≤8.9 mg/kg TS, Cd ≤0.9 mg/kg TS, Cu ≤120 mg/kg TS, Ni ≤30 mg/kg TS, Pb ≤8.6 mg/kg TS, and Zn ≤439 mg/kg TS). Conclusions Energy production from Elodea biomass is feasible. This biomass also provides the nutrients and trace elements necessary for the digestion in the anaerobic process.

The occurrence of persistent foaming is observed in many anaerobic digesters that have sugar beet as their feedstock. The formation of foam entails a significant risk of damage to biogas plants, as gas pipes can become blocked. For this reason, foaming tests have been conducted to investigate which measures lead to reductions in foam development. It was found that generally available fertilizers such as urea, ammonium nitrate and calcium cyanamide have a foam-reducing effect. However, batch fermentation tests showed inhibition of biogas production at higher concentrations of these substances, which means that they should be used with care. Calcium cyanamide was found to be very unsuitable, as this substance inhibited biogas production even at low concentrations and caused the fermentation process to come to a complete stop at higher concentrations.

Several thousand metric tonnes of aquatic biomass are removed from water bodies every year, so that these waters can continue to be used for ship and boat traffic and for leisure activities. The mowed material is generally disposed off without any further use. Therefore, the crop properties of samples from 39 weed control measures all over Germany were examined to assess the suitability of aquatic plant biomass as a substrate for anaerobic digestion systems. Analysis of the crop samples consisted of the identification of plant species and the evaluation of sediment contents and concentrations of macroelements. The methane yield was determined for selected aquatic plants. Analysis revealed a carbon/nitrogen ratio (C/N) of between 10 and 20 in 74% of samples. The concentrations of nitrogen and phosphorous in the samples were comparable to grass silages. With regard to heavy metal concentrations, the threshold values for biowaste for nickel, zinc, and cadmium were exceeded in three samples. There were no significant seasonal differences in substrate characteristics and qualities. The specific methane yields of individual aquatic plants were between 142 and 372 LCH4/kg volatile solids (VS). The results of this study showed that aquatic macrophytes can be used as substrates in anaerobic digestion plants without any restrictions.