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Landesanstalt für Agrartechnik und Bioenergie

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Browsing Landesanstalt für Agrartechnik und Bioenergie by Person "Dinkler, Konstantin"

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    Phosphate turnover during anaerobic digestion of chicken, pig and dairy manure
    (2023) Dinkler, Konstantin; Müller, Joachim
    Phosphate (P) is used extensively in agriculture. This has led to a reliance on P imports. Meanwhile, the framework for fertilization with digestate and manure in the European Union has become more stringent in recent years. Therefore, nutrients should be recovered as fertilizer to reduce dependencies, redistribute nutrient and amplify the product portfolio of biogas plants. Current nutrient recovery processes have in common that they are post digestion treatments of digestate, which neglect the phosphate behavior during digestion. It is necessary to closely evaluate P behavior during AD to optimize post digestion treatments of digestate by using digestion as a pretreatment for digestate. Therefore, it was the overall objective of this work to evaluate the turnover of P during anaerobic digestion in laboratory scale batch and continuous digestion systems. In laboratory experiments with batch reactor systems three different manures, namely pig, dairy and chicken manure were digested. Activated sludge served as inoculum. A set of 120 mL batch digesters were filled and individual bottles were opened after defined times and discarded afterwards until the last reactors were opened on day 30. The results showed that H2O-P and NaHCO3-P decreased over the digestion period by up to 40.1 %. Meanwhile, NaOH-P increased. Overall, it could be concluded that anaerobic digestion leads to a mineralization of P. The mineralization was especially profound during the first few days after the substrate was mixed with the inoculum, concluding that the ions in the inoculum played a significant role in this mineralization. In effect, AD reduces immediate plant availability but increases slow-release fertilization effects. During the batch experiments it was found that for a defined measurement wavelength for digestate the absorbance spectrum of digestate extracts needed to be analyzed and a drying temperature needed to be determined for sample treatment. For the evaluation of these two aspects samples were dried at 50°C and at 105°C and freeze dried. These samples and undried digestate were extracted by Hedley fractionation. The coloring agent was added to the extracts and the spectra between 600 nm and 1100 nm were measured. The spectral lines showed two peaks (709 nm and 889 nm). The lower wavelength proved to be more stable at low absorbance, making this the better wavelength for analysis. The analysis of the Hedley extracts showed that drying increases the H2O-P and NaHCO3-P fraction by up to 70 %. The samples were rinsed with preceding solvent to increase accuracy. Overall, the adapted method achieved higher accuracy for H2O-P, NaHCO3-P than the former method. The adapted fractionation was used for the analysis of samples during experiments in continuously stirred tank reactors. Chicken and dairy manure were each co-digested with straw and the parameters OLR and temperature were varied. The results showed that OLR had a negative correlation with H2O-P, which decreased by up to 50.49 %. Meanwhile, HCl-P increased significantly in chicken manure digestate, showing a positive correlation with OLR. It was proven that temperature has a minor effect on P transformation with a slightly higher mineralization of P under thermophilic conditions. Especially the high calcium concentration in chicken manure dominated the P turnover during the digestion, which can also be seen in the positive correlation of OLR with HCl-P as well as a high Pearson correlation coefficient above 0.85 for calcium and phosphate in chicken manure digestion. The results of this work have proven that P changes its chemical composition significantly during anaerobic digestion. The parameters of the digestion process had a decisive effect on the final composition with OLR and substrate composition being the major drivers. The results further showed that gas production and high P solubility are in conflict because for increased H2O-P OLR needs to be reduced. Future work should focus specifically on the combination of this anaerobic digestion and post-digestion treatments for cost effective recovery. This can play a key role for future profitability of biogas projects.