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BTU
The input of biowaste material in biogas plants has increased in the recent years. With that comes a certain amount of residues that can potentially be applied as organic soil amendment in order to recycle nutrients and to maintain and improve soil functions. The aim of this article is to give an impression of preliminary results of our study on characteristics of solid fermentation residues from different German biogas plants using organic waste material as main input substrate. A fundamental analysis shall reveal information such as material composition, nutrient and heavy metal concentration and impurity content. These characteristics are compared to existing data on composts and residues from organic waste treatment and will be evaluated in the context of organic amendment application.
Mit der Novellierung des Erneuerbare-Energien-Gesetzes (EEG) im Jahr 2014 und der flächendeckenden Einführung der getrennten Erfassung der Bioabfälle seit 2015 (Kreislaufwirtschaftsgesetz 2012) wurden deutliche Anreize für die Verwendung von organischen Reststoffen zur Biogaserzeugung gesetzt. Ein verstärkter Einsatz von Bioabfällen als Gärsubstrat ist zu erwarten. Die festen Gärrückstände sollen im Sinne der Kreislaufwirtschaft als organischer Bodenverbesserer auf landwirtschaftliche Flächen rückgeführt werden. Die vorgestellte Forschungsarbeit dient der Untersuchung von Aufbereitungsmöglichkeiten der festen Gärrückstände aus der Bioabfallvergärung und soll insbesondere die Zusammenhänge zwischen Vergärung, Aufbereitung und anschließender Bodenwirkung klarstellen.
The structural degradation of agricultural soils is an increasing problem of our times caused by intense land use. The treatment of soils with organic amendments shall maintain and improve long-term soil fertility. Solid residues from anaerobic digestion of municipal organic waste are rich in nutrients and organic matter and have a promising potential to be used as soil amendment. Less is known about characteristics of solid residues from organic waste digestion as composting of organic waste materials is still common. Before digestion residues can be applied to soils they need to be conditioned as they have a high impurity content (especially plastic bags and packaging materials) and need to meet the requirements of German regulation on biowaste (Bioabfallverordnung). Our research is investigating on conditioning methods for solid residues of municipal organic waste digestion with regard to the potential effect of products on soil structure, soil water regime and nutrient supply. Investigations have been made with different digestion residues from lab scale, semi-industrial and industrial scale biogas plants. Conditioning methods have been tested using our in-lab treatment facilities consisting of a drying, composting, sieving and pelletizing unit. Preliminary results show that drying and pelletizing measures influence hydrophobicity of digestion residues. As water repellency directly affects water flow and soil moisture this is a determining parameter regarding the amendment performance of conditioned products.
Influence of soil amendments made from digestate on soil physics and the growth of spring wheat
(2016)
Every year 13 million tons of organic wastes accumulate in Germany. These wastes are a potential alternative for the production of energy in biogas plants, especially because the financial subventions for the cultivation of renewable resources for energy production were omitted in 2014. The production of energy from biomass and organic wastes in biogas plants results in the accumulation of digestate and therefore causes the need for a sustainable strategy of the utilization of these residues. Within the scope of the BMBF-funded project ‘VeNGA - Investigations for recovery and nutrient use as well as soil and plant-related effects of digestate from waste fermentation’ the application of processed digestate as soil amendments is examined. Therefore we tested four different mechanical treatment processes (rolled pellets, pressed pellets, shredded compost and sieved compost) to produce soil amendments from digestate with regard to their impact on soil physics, soil chemistry and the interactions between plants and soil. Pot experiments with soil amendments were performed in the greenhouse experiment with spring wheat and in field trials with millet, mustard and forage rye.
After the first year of the experiment, preliminary results indicate a positive effect of the sieved compost and the rolled pellets on biomass yield of spring wheat as compared to the other variations. First results from the
Investigation on soil physics show that rolled pellets have a positive effect on the soil properties by influencing size and distribution of pores resulting in an increased water holding capacity. Further ongoing enhancements of the physical and chemical properties of the soil amendments indicate promising results regarding the ecological effects by increased root growth of spring wheat.
Mit der Einführung der flächendeckenden Getrenntsammlung von Bioabfällen und im Zuge der Förderpolitikentwicklung unter dem EEG wird ein verstärkter Einsatz von Bioabfällen in Biogasanlagen erwartet. Die Gärreste sollen im Sinne der Kreislaufwirtschaft als organischer Bodenverbesserer eingesetzt werden. Neben inhaltlichen Anforderungen schreibt die BioAbfV eine Hygienisierung der Gärreste vor. Dies kann im Anschluss an die Vergärung durch Trocknung oder Kompostierung realisiert werden. Die vorgestellte Forschungsarbeit befasst sich mit Frage wie sich das gewählte Hygienisierungsverfahren auf die Düngeeigenschaften der Gärreste auswirkt. Für die Untersuchung wurden 3 Gärrestchargen aus einer 2-stufigen Biogasanlage (GICON-Biogastechnikum Cottbus) getrocknet (BTU Labor) und kompostiert (Kompostwerk Becker Umweltdienste). Die Ergebnisse zeigen, dass die getrockneten Gärreste einen höheren Anteil an sehr schnell verfügbaren, eluierbaren Nährstoffen und Schwermetallen aufweisen. Der Nmin-Gehalt des getrockneten Gärrestes ist ebenfalls höher. Durch die Kompostierung kommt es zur Aufkonzentration von Elementen im Gärrest. Diese weisen aber eine geringere Eluierbarkeit auf. Der Anteil an pflanzenverfügbarem P ist im kompostierten Gärrest höher. Auf die Verfügbarkeit von K hat das Aufbereitungsverfahren keinen signifikanten Einfluss. Beide Aufbereitungsverfahren bieten Vor- und Nachteile im Hinblick auf die potentielle Wirkung des Gärrestes auf Boden- und Pflanzenentwicklung. Eine eindeutige Empfehlung für ein Aufbereitungsverfahren kann daher nicht gegeben werden.
The application of organic soil amendments is a common measure to prevent structural degradation of agricultural soils and to maintain and improve long-term soil fertility. Solid residues from anaerobic digestion of municipal organic waste (MOW) are rich in nutrients and organic matter and have a promising potential to be used as soil amendment. However, no study has related amendment properties of MOW digestate of one origin to different treatment procedures. We therefore investigated the impact of drying, composting and sieving on final digestate properties and specifically nutrient availability and heavy metal and carbon elution.
Samples were provided by a semi-industrial two-stage biogas plant with dry fermentation of MOW. Results confirm that in comparison to drying, composting of MOW digestates leads to a significant increase of K, P, Mg, Ca, Cd and Cr in the digestates. Sieving of composted digestates showed that heavy metals are not evenly distributed and that heavy metal concentration in the digestate increases with decreasing mesh sizes (highest concentrations in the fractions <1 mm). Although the element concentration is higher in composted batches, the water-extractability of nutrients, heavy metals and carbon is significantly lower from composted over dried digestates. A significant correlation was found between the dissolution of Zn, Ni, Ca and Mg and pH of eluate as well as dissolved organic carbon (DOC) release (R > 0.7, p < 0.05). Results confirm that the extent of carbon elution depends on the degradation rate of digestates. DOC may therefore be a good measure to evaluate digestate stability and to decide on treatment measures.
The biogas technology is a promising approach for the recovery of energy and fertilizer from municipal organic waste (MOW). However, only scarce information on the development of initial nutrient and heavy metal loads during processing is available. Therefore, this study investigates properties of source-separated MOW during treatment in a semi-industrial scale two-stage biogas plant and subsequent digestate composting including impurities removal. Data from 15 batch experiments was investigated by material and substance flow analysis. Results of this study have shown that about 40% of nutrients contained in the MOW inflow are mineralized during anaerobic and subsequent aerobic treatment. A higher nutrient release was observed during the anaerobic treatment step. Additionally, impurities removal causes a significant reduction of final nutrient content. Heavy metal analysis confirmed a high heterogeneity of contamination levels. However, digestion and composting do not seem to significantly impact on total heavy metal loads in the substrate flow.
The provision of nutrients and organic matter to arable soils is critical to facilitate an intensive agriculture and secure long-term soil functionality. Municipal organic waste (MOW) is rich in nutrients and organic matter and its recycling onto agricultural lands presents a promising alternative to conventional fertilizers. In comparison to common aerobic treatment, the application of the biogas technology enables the recovery of both energy and soil amendments.
The introduction of a mandatory separate collection in Germany in 2015 reflects the political will to increase MOW recovery rates and facilitates its utilization as feedstock in biogas plants. However, MOW is a challenging feedstock as its composition varies and it is often contaminated with impurities such as plastics and metals. The two-stage anaerobic digestion process with dry fermentation is very robust and offers possibilities for process control so that no extensive MOW pretreatment is required. To close nutrient circles, remaining digestates shall be processed to soil amendments, which can be redistributed to arable land. However, less is known about digestate properties from two-stage digestion of MOW and how they are influenced during the treatment process. Furthermore, only scarce information on nutrient recovery rates and the accumulation of elements during processing is available. Therefore, this thesis investigates the development of digestate properties during anaerobic and subsequent aerobic treatment at laboratory and semi-industrial scale.
During a first experiment, changes in nutrient and heavy metal concentration in the solid digestate were monitored during anaerobic treatment of MOW in a two-stage laboratory biogas plant. A second investigation related amendment properties of MOW digestate of one origin to different post-treatment procedures. The impact of drying, composting and sieving on final digestate properties and specifically nutrient availability and heavy metal and carbon elution was evaluated. A third experimental approach investigated total material and substance flows during treatment of source-separated MOW in a semi-industrial scale two-stage biogas plant and subsequent digestate composting including impurities removal.
Results show that MOW composition is rather heterogenic, which is reflected by a high variance of biogas yields and degradability as well as digestate properties. The heterogeneity of nutrient composition is much lower than the heterogeneity of heavy metal contents. A positive correlation was found between impurity contents and the concentration of Cd, Pb and Zn in the samples. The impurity content of investigated MOW samples is up to 12%. A few of the investigates MOW samples already exceed thresholds of BioAbfV for Cd, Pb and Zn. Results underline the influence of feedstock quality on final digestate characteristics. Organic matter degradation during biological treatment leads to further accumulation of unwanted substances. Subsequently, high impurities contamination of feedstocks and heavy metal redistribution during digestion and post-treatment become an exclusion criteria for soil amendments. Therefore, a low feedstock contamination needs to be striven to guarantee the production of marketable, high quality MOW-based soil amendments.
Results from the laboratory experiments show that nutrient and heavy metal concentration in the solid digestate is increasing during digestion except for N, P and Mg. The deficit of N, P and Mg amounts to the same extent of up to 45% and a struvite precipitation can be expected. There was also a deficit of Ca, Cd, Cr Ni, Pb and Zn in the solid digestate. This emphasizes that element retention in the digester system has a decisive impact on nutrient contents of digestates. Regular percolation transports mineralized elements from the location of release into storage tanks and the fixed-bed digester, where they may deposit. This distinguishes element distribution in two-stage systems from one-stage systems. In contrast, K is the only investigated element, which was completely rediscovered in the solid digestate during laboratory experiments. K+ is assumed to adsorb to free exchange sites of existing molecular structures of the solid matter of digestate.
The total element mineralization was lower during semi-technical scale digestion, caused by shorter retention of MOW, which also resulted in a lower VS (volatile solids) degradation. Release of Mg did not occur to the same extent as for N and P. Consequently, as Mg is the limiting element for struvite precipitation, this effect is assumed to be rather low during semi-technical scale experiments. Furthermore, no K enrichment in the solid digestate was discovered during semi-technical scale experiments. Therefore, it can be concluded that K adsorption occurs only at longer retention times, which determines the rate of degradation, the complexity of the structure of the remaining organic matter and the number of free exchange sites. In comparison to nutrients, total heavy metal loads in the substrate flow were not significantly affected by semi-technical scale treatment. Therefore, the overall share of heavy metals in MOW is assumed to be present in stable compounds. However, results of laboratory digestion with longer retention time revealed a potential heavy metal release from MOW of up to 15% for Cd, Cr, Pb and Zn. The mobility of heavy metals may be promoted by dissolved organic carbon (DOC). When applicated to soil, organometallic complexes can easily be taken up by plants. Consequently, the binding of heavy metals onto soluble organic matter during digestion may lead to an increased heavy metal toxicity in soil after digestate application. To prevent this, impurities need to be removed from MOW in advance of anaerobic treatment.
Investigations on different treatment procedures confirm that in comparison to drying, composting of MOW digestates leads to a significant accumulation of K, P, Mg, Ca, Cd and Cr in the digestate. Although the element concentration is higher in composted batches, the water-extractability of nutrients, heavy metals and carbon is significantly lower than from dried batched. A significant correlation was found between the release of Zn, Ni, Ca, Mg and the pH as well as the DOC content of eluates. Results confirm that organic matter stability is a key factor regarding the potential performance of digestates in soil. The elution of DOC may be a good measure to evaluate digestate quality and to decide on treatment measures. Investigated digestates were characterized by a high residual degradation potential. Subsequently, for these digestates composting seems to be the better treatment option as it leads to an improved organic matter stability and higher nutrient concentrations. However, heavy metal accumulation is problematic. Sieving of composted digestates showed that heavy metals are not evenly distributed and that highest heavy metal concentration can be found in the fractions below 1 mm. Consequently, sieving leads to further heavy metal enrichment in the digestate, as fractions with lower contamination are refused in the sieve overflow. Experiments also show that sieving below 10 mm is needed to ensure the legal threshold of 0.5 % impurity content. However, this is related to a mass loss of up to 50%. Again, this emphasizes the importance of feedstock purity.
Investigations of semi-industrial scale MOW treatment show that about 60% of nutrients contained in the MOW inflow can be retained in the process outflow during two-stage digestion and subsequent aerobic post-treatment. A higher nutrient release was observed during digestion forced by percolation. Additionally, impurities removal causes a significant reduction of the final nutrient outflow. Based on the results of substance flow analysis, soil amendments from two-stage digestion and subsequent composting of 9 million tons of MOW could provide 1.5% of annual demand for N, 4.1% of annual demand for P and 3.3% of annual demand for K to agriculture. However, investigations have shown that N availability is very low in composted digestates. Consequently, the final output of investigated MOW treatment is most suitable to complement P and K fertilization.
Future research is needed to investigate the recovery of nutrients, which are mineralized during anaerobic treatment. Furthermore, the extent of heavy metal mobilisation remains unclear. A deeper understanding of how contaminants behave during biological treatment is needed to estimate the ecotoxicological potential of digestates when applied as soil amendments.
The structural degradation of agricultural soils is an increasing problem of our times caused by intense land use. The treatment of soils with organic amendments shall maintain and improve long-term soil fertility. Solid residues from anaerobic digestion of municipal organic waste (MOW) are rich in nutrients and organic matter and have a promising potential to be used as soil amendment. Our research investigates the characteristics of differently treated amendments produced from MOW digestate. We want to know how long these amendments are present in the soil and which amendment type contributes most to nutrient provision after application. Results were obtained using four different MOW digestate amendments of one origin (loose, pelletized and agglomerated) during a 24-hours shaker-experiment and a bag-experiment on grassland, which was subsequently sampled over 21 months. Results show that short-term element leaching is higher from loose amendment and small agglomerates, probably caused by higher surface area und lower stability of these amendments. Biggest degradation effects can be seen 7 months after application to soil. However, all four amendments are still present after 21 months. Pelletized amendments are more degradation resistant than loose amendment, which may be advantageous regarding long-term effects e.g. on water holding capacity. In turn, loose material delivers a higher share of small organic particles to soil and may therefore have a higher contribution to humus development.