Phosphorrückgewinnung aus Nassschlamm - Eine Optionsanalyse im Kontext der Anforderungen der Klärschlammverordnung

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This paper draws on theoretical assumptions and laboratory scale studies to identify constraints in processes that make recovering phosphorus from wet sludge appear a feasible pathway for creating regional value in the context of the requirements of the revised Sewage Sludge Ordinance. In particular, it examines the effects of different phosphorus inflow loads, varying levels of enhanced biological phosphorus removal and different phosphorus extraction grades by solid-liquid separation. Boundary conditions are derived and evaluated with regard to the provisions of the Sewage Sludge Ordinance. Removing phosphorus from wet sludge at municipal wastewater treatment plants can offer a different way to handle sewage sludge if the constraints are adhered to.

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Phosphorus (P) is a key nutrient for agriculture and global food security, but the remaining phosphate rock reserves are finite and contain increasing toxic impurities. Thus, phosphorus recovery from secondary waste sources, such as sewage sludge, became a strategic goal for the European Union to reduce its import dependency and stimulate circular economy. Previous studies focused on phosphorus recovery predominantly from primary, activated and anaerobically digested sludge but not from post-precipitated (tertiary) chemical sludge. In this research, leaching tests were performed with aluminum and iron containing tertiary sludge from two different wastewater treatment plants. The feasibility to re-dissolve the phosphorus bound into the sludges was investigated systematically by applying acidic or alkaline treatment. The shift in pH causes the dissolution of phosphorus which makes it readily available for further recovery as a high-value fertilizer product. For the aluminum containing sludge, the phosphorus re-dissolution efficiency after acidic leaching was ∼95% at pH 2.0. For the iron containing sludge, ∼80% efficiency was achieved only after significant acidification to pH 1.5. Alkaline leaching at pH 13 showed considerably lower potential with ∼70% phosphorus re-dissolution for the aluminum containing and ∼40% for the iron containing tertiary sludge. Additionally, the co-dissolution of the associated aluminum and iron metals was also analyzed and all experimental results were compared against data obtained from the leaching of anaerobically digested sewage sludge and synthetically precipitated sludge. In conclusion, the optimal pH for phosphorus dissolution depended strongly on the type of metal-phosphate compounds in the sludge.
Phosphorus (P) is a nonrenewable mineral resource that is critical to achieving food security for the growing world population. However, nearly half of total annual mined P is lost through nonpoint particulate runoff streams. Previously, we demonstrated inorganic phosphate extraction from runoff particulate mineral phases using P-solubilizing fungi (PSF) generating the organic acids citrate and oxalate. The objective of the current study was to investigate the effect of extraction process conditions on P extraction efficiency and kinetics from impounded urban storm water runoff sediments using citrate as the sole P solubilization agent. Abiotic extraction experiments were conducted according to a one-factor-at-a-time approach. High P removal and solubilization were favored at low sediment loading (5 g/100 mL), low initial pH (3), and high initial organic acid loading (75 mM), with P recovery efficiencies of 75% to 100% and 240 ppm solubilized phosphate levels achieved under these conditions. Initial and equilibrium extractant pH levels between citrate pKa1 (3.08) and pKa2 (4.74) appeared conducive for citrate ligand-induced particulate P release, possibly due to optimal balance of protonated and dissociated carboxylic groups. The results suggest that operating the extraction under fed-batch or semi-continuous modes with pH control can help maximize runoff particulate P recovery efficiency in PSF-mediated bioextractions wherein citric acid is the major fungal organic acid product.
The alkaline solubilization of excess mixed sludge was investigated and subsequently the released phosphorus was recovered as magnesium ammonium phosphate (MAP). Considerable and rapid release of glycogen and protein was encountered after alkaline addition into the sludge. Only 45.0% of sludge cells were destructed after 240 min treatment in R1 while the corresponding ratio was 96.1% in R3 according to the release of DNA. Non-apatite inorganic phosphorus (NAIP) in the alkaline treated sludge decreased due to the dissolution of Al(OH)3 and AlPO4, which was the dominant reason for phosphorus release. Soluble orthophosphate (SOP) in the supernatant of alkaline treated sludge could be recovered quickly with the recovery efficiency of 84.6% within 5 min and about 53-55% of P participated in MAP reaction, producing large amounts of acicular crystals.
Phosphorus (P) leaching characteristics of polyaluminium chlorides (PAC) treated wastewater sludge was investigated by wet chemicals (acid and alkali). Sludge fractionation showed non-apatite inorganic P was the dominant P (90.9% of TP) while apatite P only accounted for 3.7%. After 2 h extraction with 1 N NaOH or 2 N HCl, 80.5% and 77.9% of total P was leached, while sludge dissolution reached 72.7% and 75.6%, respectively. Kinetic study with HCl and NaOH showed that P release and sludge dissolution follow first order reaction with rate constants of 0.50 and 0.35 min−1 (P release) and 0.47 × 10−2 and 0.15 × 10−2 min−1 (sludge dissolution), respectively. Sequential extraction by NaOH/HCl leached 91.7% of the total P. This study will help in understanding the P release behavior of the PAC treated wastewater sludge.
Selective release characteristics of phosphorus and nitrogen from waste activated sludge (WAS) were investigated during combined thermal and alkali treatment. Alkali (0.001-1.0N NaOH) treatment and combined thermal-alkali treatment were applied to WAS for releasing total P(T-P) and total nitrogen(T-N). Combined thermal-alkali treatment released 94%, 76%, and 49% of T-P, T-N, and COD, respectively. Release rate was positively associated with NaOH concentration, while temperature gave insignificant effect. The ratio of T-N and COD to T-P that released with alkali treatment ranged 0.74-0.80 and 0.39-0.50, respectively, while combined thermal-alkali treatment gave 0.60-0.90 and 0.20-0.60, respectively. Selective release of T-P and T-N was negatively associated with NaOH. High NaOH concentration created cavities on the surface of WAS, and these cavities accelerated the release rate, but reduced selectivity. Selective release of P and N from sludge has a beneficial effect on nutrient recovery with crystallization processes and it can also enhance methane production. Copyright © 2015 Elsevier Ltd. All rights reserved.
A sustainable way to recover phosphorus (P) in swine wastewater involves a preliminary step of P dissolution followed by the separation of particulate organic matter. The next two steps are firstly the precipitation of struvite crystals done by adding a crystallization reagent (magnesia) and secondly the filtration of the crystals. A design of experiments with five process parameters was set up to optimize the size of the struvite crystals in a synthetic swine wastewater. More than 90% of P was recovered as large crystals of struvite in optimal conditions which were: low Mg:Ca ratio (2.25:1), the leading parameter, high N:P ratio (3:1), moderate stirring rate (between 45 and 90rpm) and low temperature (below 20°C).These results were obtained despite the presence of a large amount of calcium and using a cheap reactant (MgO). The composition of the precipitates was identified by Raman analysis and solid dissolution. Results showed that amorphous calcium phosphate (ACP) co-precipitated with struvite and that carbonates were incorporated with solid fractions.
Different modes of preparation of a by-product of a magnesite calcination process for magnesium oxide production (BMP) were compared for phosphorus removal to struvite at a molar Mg:P ratio of 1.6 mol Mg/mol P from liquor of a full-scale sewage treatment plant. Five experimental runs were performed—Run 1: raw BMP; Run 2: screened to particle size < 0.04 mm; Run 3: milled to particle size < 0.04 mm; Run 4: suspension at a concentration of 3 g of BMP/l of tap water; Run 5: supernatant after settling for 4 h after 28 h aeration. Temperature and pH were determined in liquid samples and analyses of phosphorus (P) and magnesium (Mg) were also performed in liquid and solid samples. pH increased with the reaction time (t) in Runs 1–4. However, in Run 5 there was an initial increase from 8.28 to 8.41 only to decrease to 8.34 at the end of the experiment. Phosphorus concentration decreased with t, but also with the mode of BMP preparation where the concentration of phosphorus in relation to the reaction time was lower in Runs 2 and 3 than in the other runs, the minimum being in Run 5. It was found that the smaller the particle size, the lower the concentration remained in solution. This fact was confirmed by the values obtained in the equilibrium concentrations which were: 14.7, 9.1, 13.2, 17.4 and 7.3 mg/l for Runs 1–5, respectively. The amount of struvite obtained was improved by the increase of t, the maximum being in Run 5 at 0.5 h (450 mg/l). In Runs 2 and 3, values of struvite production of 400 and 350 mg/l were achieved at t higher than 2.5 h. It was found that a first-order kinetic model fitted in well with the experimental data obtained. The values of the reaction constants were found to be: 1.2, 1.5, 1.4, 1.3 and 2.9 h−1 for Runs 1–5, respectively. The proposed model accurately predicted the performance of the process, showing deviations lower than 5% between the experimental and theoretical data of struvite production.
The aim of this work was to examine the influence of mineral elements on phosphorus (P) release from heated waste sludges. Energy dispersive X-ray microanalysis suggested that P was associated with Al, Ca, and Mg on the surface of waste sludge biomass obtained from six wastewater treatment plants. The extent of P release decreased with increasing the total concentrations of Al, Mg, and Ca in waste sludges. The addition of Al2(SO4)3, Ca(OH)2, CaCl2, MgSO4, or NaAlO2 to activated sludges, which were taken from a bench-scale EBPR process, reduced the P release significantly.
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DWA-M 368: Biologische Stabilisierung von Klärschlamm, DWA, Hennef, 2014
Phosphorus Recovery on a Wastewater Treatment Plant by Alkaline Sludge Treatment
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Abou Hassan, M.: Phosphorus Recovery on a Wastewater Treatment Plant by Alkaline Sludge Treatment, Master Thesis, KIT, Karlsruhe, 2021
Promoting Phosphorus Recovery from Sewage Sludge by Thermal Hydrolysis Processes
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Azam, M.: Promoting Phosphorus Recovery from Sewage Sludge by Thermal Hydrolysis Processes, Master Thesis, KIT, Karlsruhe, 2019
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Baumann, P., Jedele, K.: Phosphorelimination -Optimierung auf Kläranlagen, Praxisleitfaden für den Betrieb von Kläranlagen, 1. Aufl., Praxisleitfaden, Heft 14, DWA-Landesverband Baden-Württemberg, Stuttgart, 2019
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Ping, Q., Lu, X., Li, Y., Mannina, G.: Effect of complexing agents on phosphorus release from chemical-enhanced phosphorus removal sludge during anaerobic fermentation, Bioresour. Technol. 2020, 301, 122745, doi:10.1016/j.biortech.2020.122745