Browsing by Subject "Biogasgärreste"

Now showing 1 - 2 of 2

Characterization of phosphate fertilizers recycled from biogas digestates and their influenceon plant-soil fertility indicators
(2022) Bach, Inga-Mareike; Müller, Torsten
Phosphor (P) supply to plants is a key production factor for quantity and quality of food in agriculture. P consumption in modern agriculture has increased with raising world population. Mineral P fertilizer derived from Phosphate Rock (PR) mines is a limited resource on earth and large amounts of P used in agriculture are diluted by distribution into the environment, causing unwanted environmental side effects. Future oriented use of P therefore has to be based on technologies for P-recycling from the main anthropogenic product streams. In this thesis, P recycling products from a pilot plant were investigated for their biological efficien-cy as fertilizer in comparison to a conventional mineral fertilizer triple superphosphate (TSP). Investigations were part of two research projects (BioEcosim & GOBi) that had the goal to develop scalable technology for a sustainable P recycling in agriculture. Inputs into the pilot plant were unprocessed pig manure, and on the other hand a biogas co-digestate from cow manure and maize. Outputs were salt precipitates (P-Salt) from the separated liquid fractions with high P content, and solid fractions dried by pyrolysis, air-drying or steam-drying with moderate P content and high organic carbon. The objective of the work described here was the biological and agronomical investigation of the recycled fertilizer fractions for their potential to substitute a mineral fertilizer. In a first step, the obtained fractions were chemically characterized for basic characteristics. Based on the P content of the recycled fertilizers, greenhouse pot experiments were set up to compare equivalent P concentrations of single doses and combinations, in different crops and soils, with TSP and an unfertilized control as reference. Fertilizers were applied once before the beginning of the vegetation phase at recommended field rates. Variables investigated were above ground plant biomass production, concentration, and content of P in shoots, and plant-available P in soil. The characterization of the precipitated P rich fractions revealed that the composition of the P bound minerals was a mixture of magnesium ammonium phosphate (struvite) and calcium phosphates. Their total P content (circa 110 g/kg DM) was slightly lower than TSP (190 g/kg). The organic solids contained lower (circa 20 g/kg) but still significant amounts of P. All fractions dis-played a slightly alkaline pH in CaCl2, between 7 and 8.5. In all experiments, single dosing with the recycled P-Salt fractions resulted in fertilizer effects on biomass growth similar or higher than the reference TSP. This result was found in all soils and crops investigated, indicating that the recycled P-Salt was an effective substitute for TSP. Under the conditions tested, three of the investigated crops, namely marigold, Chinese cabbage and ryegrass, did not develop P induced biomass increase at all, probably because the relevant growth phases were not covered or because the initial P concentrations in soil were already equal or above the optimum P concentration in soil. Highest effects were found in maize, a typical input crop for biogas plants. The single dosing of the isolated solid fractions in two acidic soils, using maize and sunflower, resulted in an even higher biomass increase compared to TSP and P-Salt, whereas effects were generally lower in neutral soils. Steam-dried solids showed a tendency to be superior to air-dried and pyrolyzed solids. When some combinations of solids with P-Salt were applied, biomass increased to an extent equal or higher than P-Salt or TSP alone. Effects were partly synergistic or additive, but never antagonistic. Different mixing techniques investigated resulted in only small differences in biomass increase. A fertilizer induced increase of P concentration or content in the above ground plant biomass, dependent on the plant growth rate, was found in almost every tested crop. The results indicate that uptake of P from soil treated with recycled fertilizers occurred to the same extent than with TSP, independent from the individual growth rate. Plant available P in soil, detected as CAL-P, was increased by all fertilizer fractions compared to untreated controls. This suggests that the chemical composition of the recycled P fertilizers was favorable for a high release of plant available P in soil and underlines the high technical quality of the established manufacturing processes. Overall, the results indicate that P fertilizers recycled from unprocessed manure or biogas plant digestates can be used as an adequate substitute for mineral P fertilizer in a range of different crops and soils. Confirmation of the results in the field and adoption to actual crop-soil-climate situations will be needed for practical use in agriculture. A detailed sustainability evaluation, taking into account all input and output parameters, will help to assess the practical use, applicability and value of the described recycling process.
Influence of biogas-digestate processing on composition, N partitioning, and N₂O emissions after soil application
(2023) Petrova, Ioana; Pekrun, Carola
The ever-growing need for agricultural products represents a global issue, particularly with a view to the limited availability of cultivable land. According to the latest estimates, the arable land per capita decreases and, in 2050, is expected to account for about 60% less than in the 1960s. In order to meet the demand, agriculture has evolved into industrial-like structures. This development often goes along with nutrient surpluses (e.g., excess of nitrogen and phosphorus) and increased emissions, caused by mismanagement and inappropriate agricultural practices (e.g., over-fertilization). Biogas plants offer a possibility to valorize organic residues and wastes, but potentially aggravate this problem since additional organic residues (referred to as digestates) with considerable nutrient contents are generated as by-products. A simple approach to adjust nutrient levels in the affected regions is the transfer of manures and digestates. However, to make this feasible, a reduction of water content (and consequently of total mass/volume) of digestates is required. Up to now, various techniques for digestate downstream processing are available. Previous research mainly addressed single processing stages or differences between feedstock mixtures. Only limited information was found about the influence of a completed downstream processing on total mass reduction and nitrogen concentration in digestate. Studies about the (gaseous) N losses that occur after the application of the respective intermediate and final products to soils were equally scarce. Therefore, the aims of the current doctoral thesis were to determine (i) the mass reduction achieved by the gradual removal of water within competing processing chains, (ii) the nitrogen partitioning after every single processing step and its recovery in the end products, and (iii) the amount of greenhouse gases (especially N₂O) released after the application of intermediate and end products to soils in comparison to untreated, raw digestate. For that purpose, two commercial, full-scale biogas plants were examined, which completely processed either the solid or the liquid fraction after mechanical screwpress separation of raw digestate. The separated solid fraction was subsequently dried and pelletized, while the liquid fraction was treated by vacuum evaporation with partial NH₃ scrubbing. As final products, digestate pellets and N-enriched ammonium sulfate solution were generated. Calculation of a mass flow balance served as the basis for determining (total) mass reduction, the partitioning of fresh mass and nitrogen during digestate processing, and the recovery of initial N in the products. Additionally, the environmental impact of utilizing digestate as an organic fertilizer was studied by measuring the N₂O release after application to soil under field and laboratory conditions. A further in-depth analysis was performed to observe the main factors influencing the production and release of climate-relevant N₂O from digestate pellets. It was found that the mass reduction caused by water removal during subsequent processing accounted for 6% (solid chain) and 31% (liquid chain) of the total mass of raw digestate. Liquid processing required 40% less thermal energy per ton of water evaporated than solid processing. At the end of the downstream processing, the recovery of initial nitrogen in pellets was 33% lower than in ammonium sulfate solution. Regarding the environmental impact of digestate application to soil, mechanical solidliquid separation showed the potential to reduce N₂O emissions. Contrary to expectations, pelletizing of dry solid boosted the emissions, which was linked to the properties and composition of the pellet. Here, indigenous microbial activity triggered N₂O production and release from denitrification immediately after wetting. Overall, the present work has shown that the subsequent processing of separated solid or liquid digestate generates different products with individual benefits and challenges. Solid digestates are characterized by a high share of recalcitrant organic compounds and therefore can serve, e.g., as soil improver. After processing to pellets, they can be easily transported, stored, and commercialized. However, it is questionable whether the pelletizing process is advisable, since pellets emitted a considerable amount of GHGs during utilization. Liquid processing produces ammonium sulfate solution, which can be utilized as a valuable inorganic fertilizer rich in plant-available N. Besides the discussed advantages, a final decision for or against digestate processing always depends on individual factors, such as local situation and financial means. Smart decision-making must include fertilizer properties, technological performance, and economic feasibility. With a view to future research, additional aspects were identified, such as returning to a laboratory-scale biogas plant for more accurate digestate sampling and analysis, consideration of digestate storage and transport, and economic evaluation of the entire digestate value chain including the assessment of digestate fertilizer value (expressed as e.g., N use efficiency or N fertilizer replacement value).