Browsing by Subject "Biogas digestate"

Now showing 1 - 1 of 1

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.