Browsing by Person "Uppuluri, Naga Sai Tejaswi"

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Effect of reactive and non-reactive additive treatment on the recovery of phosphorus from biogas digestate
(2023) Uppuluri, Naga Sai Tejaswi; Dinkler, Konstantin; Ran, Xueling; Guo, Jianbin; Müller, Joachim; Oechsner, Hans
The annual phosphate (PO43−) utilization has increased, leading to a depletion of existing sources of phosphorus (P). To overcome this, digestate as a source to recover P is being investigated. Due to the abundance of nutrients, the digestate from an agricultural biogas plant is used as fertilizer for crops. The separation of solids and liquids from the digestate by a screw press is the simplest form of concentrating, therefore, recovering PO43−. This is the most commonly employed method in existing biogas plants. However, the separation is not very efficient as only 20–30% of P is recovered in the solid phase. The goal of this study is to increase the separation efficiency and recover more P into the solid phase, in order to improve the transportability. For this, separation trials at a laboratory scale were performed for five experimental groups, with biochar and straw flour as non-reactive additives and kieserite as a reactive additive. In addition, untreated digestate was studied as a control. The control and the treatment with biochar and straw flour were carried out at 25 °C, while the treatment with kieserite was performed at 25 °C and 50 °C. The separation trails were performed at treatment times of 0 h, 1 h, 2 h, 8 h, and 20 h. The results showed that the treatment with additives had a beneficial effect on the recovery of P. It was noted that kieserite treatment at 25 °C and 50 °C bound about 61% of the total P present in the digestate to the solid phase. A sequential extraction was performed to study the effect of additives on the recovery of different P species. The results concluded that, compared to biochar and straw flour, kieserite was efficient in recovering the non-labile fractions (NaOH-P and HCl-P) of P, which act as slow-release fertilizers. This study shows that the use of additives, especially kieserite, has a positive influence on recovering P from digestate, and further research to optimize the recovery process would be beneficial.
Investigation of phosphorus recovery from biogas digestate: low-technology approaches for enhanced solid-liquid separation
(2025) Uppuluri, Naga Sai Tejaswi; Müller, Joachim
Phosphorus (P) is a critical nutrient for agriculture, essential for plant growth and food security. However, the global dependence on finite P reserves, primarily located in northern Africa and China, presents significant challenges, including supply chain vulnerabilities and rising fertilizer costs. Additionally, excessive P runoff into water bodies contributes to environmental issues, such as eutrophication. Addressing these challenges requires sustainable P management strategies, including P recovery from alternative sources like biogas digestate. To tackle these challenges, regulatory measures and innovative recovery techniques are being explored to improve P sustainability. In Germany, regulations like the Fertilizer Ordinance and the Sewage Sludge Ordinance have introduced strict limits on P application and mandated the recovery of P from waste streams, promoting environmentally friendly nutrient management practices. These regulations aim to reduce environmental impacts while ensuring the efficient use of P resources. A primary objective of this research is to develop a cost-effective, practically feasible method for P recovery that can be easily integrated into existing biogas plants without requiring substantial infrastructure upgrades. This study investigated the recovery of P from biogas digestate using additives such as kieserite (MgSO₄∙H₂O), straw flour, and biochar to improve separation efficiency and nutrient recovery. The additives enhanced the P content in the solid phase, making it easier to recycle P back into agricultural systems. The laboratory-scale separation trials were conducted to evaluate the effectiveness of three additives categorized into reactive (kieserite) and non-reactive (straw flour and biochar) groups. The separations were carried out using a hydraulic tincture press, with a pressure of 5 MPa applied for 120 seconds. The trials tested five different treatment times: 0 h, 1 h, 2 h, 8 h, and 20 h. Kieserite was tested at 25 °C and 50 °C, while straw flour and biochar were tested only at 25 °C. The results revealed that longer treatment durations with kieserite were more effective, with 61% of P shifting into the solid phase after 20 h. In contrast, treatment duration had little impact on the effectiveness of straw flour and biochar. Kieserite treatment increased NaOH-P and HCl-P, indicating the formation of more stable, non-labile P fractions due to the interaction between (Mg²⁺) ions from kieserite and phosphate (PO₄³⁻) ions. These findings concluded that kieserite, as a reactive additive, is more effective at enhancing P recovery by converting labile P into more stable, non-labile forms. Based on the conclusions from laboratory-scale separation trials, practical scale experiments were conducted at the research biogas facility ‘Unterer Lindenhof’ at the University of Hohenheim. In these trials, straw flour and kieserite were used as additives, with treatment durations of 4 h and 22 h. The shorter duration represented same-day processing, while the longer duration simulated overnight treatment. Results showed that extending the treatment time with kieserite significantly improved P removal efficiency (PRE), reaching 67% of P shifted to the solid phase after 22 h. Straw flour, on the other hand, achieved a 52% PRE at the same duration, with most of the P remaining in labile fractions regardless of treatment time. Kieserite treatment resulted in notable changes in the distribution of P fractions, shifting from NaHCO₃-P to more stable NaOH-P and HCl-P fractions as the treatment duration increased. These experiments provide a technical proof-of-concept for the use of additives in biogas plants for digestate treatment to enhance P recovery into the solid phase, supporting more sustainable nutrient management practices. Biochar modified with kieserite and calcium chloride (CaCl₂) was evaluated as an additive for P recovery, along with the metal salts used independently. The modification aimed to load Mg²⁺ and Ca²⁺ ions onto the biochar surface, enhancing its effectiveness. Initial separation trials established that using 5 gadditive/Ldigestate with a 22 h treatment time provided optimal conditions for solid-liquid separation. The separation trials were made in the laboratory using a hydraulic tincture press. The modification significantly increased the Mg content in kieserite-modified biochar (KIS-B) and the Ca content in CaCl₂-modified biochar (Ca-B). Both modified biochars and the metal salts increased P transfer to the solid phase, with the metal salts alone demonstrated higher PRE. KIS-B and Ca-B shifted P to non-labile fractions, while kieserite and CaCl₂ alone resulted in an even higher proportion of non-labile P fractions. Although the modified biochar showed slightly lower PRE compared to metal salts, its potential benefits in agricultural applications are noteworthy. Future studies should include a comprehensive cost-benefit analysis to evaluate the long-term financial sustainability of implementing these recovery techniques at full scale. Reducing the reliance on expensive, finite phosphorus reserves through local recovery can lower input costs for farmers, making the process economically attractive. Additionally, plant pot trials will be essential in assessing the agronomic efficiency of P recovered from biogas digestate. These trials can help determine how effectively the recovered P promotes plant growth and nutrient uptake compared to conventional fertilizers. Ultimately, refining these recovery techniques and assessing their impact on both economic viability and agricultural productivity will play a key role in advancing sustainable nutrient management practices globally.