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Im Rahmen des EU-Projekts P-REX werden unter anderem die zwei thermischen Verfahren ASH DEC und Mephrec zur technischen Rückgewinnung von Phosphor aus Klärschlamm und Klärschlammaschen für die P-Düngemittelproduktion betrachtet. Die Produkte haben geringe Konzentrationen an Schwermetallen und eine bessere P-Löslichkeit als die nicht behandelten Klärschlammaschen. Die Klärschlammasche wird im ASH DEC-Verfahren mit Natriumverbindungen unter reduzierenden Bedingungen im Drehrohrofen behandelt. Das Verfahren wurde erfolgreich bei einem Demonstrationsversuch getestet. Das Mephrec-Verfahren ist ein metallurgisches Schmelzverfahren und wird unter reduzierenden Bedingungen im Schachtofen durchgeführt. Es ist eine Pilotanlage in Nürnberg geplant.
Phosphorus (P) is essential for all forms of life and cannot be substituted. It is one of the macro nutrients applied in form of mineral or organic fertilizers in agricultural crop production. Mineral P fertilizers are generally based on phosphate rock, a limited resource. Western Europe completely depends on imports as it has no own relevant phosphate mines. The most abundant phosphate deposits are located only in a few countries such as Morocco, China, South Africa and the United States of America. Furthermore, phosphate rock and as a consequence mineral P fertilizers are often contaminated with the toxic elements cadmium and uranium. In Order to substitute phosphate rock P-bearing waste streams are investigated for their potential to be recyded as fertilizers. Wastewater is one of the important P-bearing waste streams that were in the focus of research in the last 10 years. German wastewater is e.g. a carrier of approx. 70,000 t of phosphorus per year.
This report gives technological information about processes for the recovery of phosphorus from sewage sludge ash including mass- and energy balance, process conditions and product quality.
The information in this report aim at giving an overview on different concepts for the recovery of phosphorus from sewage sludge ash and are intended to allow a general qualitative comparison between the different technical approaches. This report also describes the technological backgrounds of the processes that were furthermore investigated by life cycle assessment (LCA) and life cycle cost (LCC) analysis. However, most process data stated in this report are site specific data. The LCA/LCC study includes the definition of a common base line for all processes, detailed process modeling and plausibility assessments. Results of the LCA/LCC-studies are reported in deliverables D9.2 und D10.1.
Klärschlammaschen weisen hohe Gehalte an Phosphor auf, dieser ist allerdings schlecht für Pflanzen verfügbar. Des Weiteren enthalten sie Schwermetalle die vor dem Einsatz als Düngemittel entfernt werden müssen. Vor diesem Hintergrund wurden in den letzten Jahren nasschemische und thermochemische Verfahren entwickelt und zum Teil bereits in Demonstrationsanlagen getestet. Ein thermochemischer Verfahrensansatz wird in diesem Beitrag näher vorgestellt. Klärschlammaschen werden mit Klärschlamm als Reduktionsmittel und Natriumsulfat als Aufschlussmittel im Drehrohrofen thermisch behandelt. Bei diesem Prozess wird pflanzenverfügbares NaCaPÜ4 gebildet, unerwünschte Elemente wie As, Cd, Hg und Pb werden zum Teil über die Gasphase separiert. Die Ergebnisse eines Demonstrationsversuchs werden vorgestellt.
Phosphorus fertilizer from sewage sludge ashes by thermochemical treatment - Benefits and challenges
(2015)
Mineral phosphorus (P) fertilizers are solely produced from phosphate rock. Europe completely depends on imports since there are no relevant phosphate rock deposits on the continent. Furthermore, phosphate rock is contaminated with heavy metals such as Cd and U that pollute the farmlands and pose environmental risks. Sewage sludge ash (SSA) might be a promising source for recycling fertilizer since it contains large amounts of P (up to 13 %). However, fertilizer from SSA has to comply with the respective ordinances, particularly the heavy metal limit values stated in the fertilizer ordinance, and requires sufficient P bioavailability. Thus, we conducted a complete survey of SSA from German mono-incineration facilities and developed a thermochemical treatment for SSA to reduce toxic elements and increase P bioavailability.
Aufgrund der natürlichen und vom Menschen verursachten Heterogenität von Böden erfordert der Erhalt und die Verbesserung ihrer Fruchtbarkeit durch landwirtschaftliche Bewirtschaftungsmaßnahmen sorgfältig geplante Ent-scheidungen, die auf einer detaillierten Erfassung der Bodeneigenschaften und einem vertieften Verständnis der Bodenprozesse beruhen. Es ist bekannt, dass die konventionelle, flächen-einheitliche Bewirtschaftung Ertragsver-luste durch zu geringe Bewirtschaftungsintensität (z. B. Düngung) auf einen Teil der Fläche verursacht, während andere Teile des Feldes zu hohe Dosen erhalten und es dadurch zu Verschwendung von Ressourcen und zu Um-weltbelastungen kommt. Trotz der Verfügbarkeit von Technologien für ortsspezifische Düngung ist die Akzeptanz ortsspezifischer Bewirtschaftung (Precision Agriculture) in der Praxis noch gering. Ein wesentlicher Grund dafür ist das Fehlen von kostengünstigen Methoden zur Erfassung der bewirtschaftungsrelevanten Bodenmerkmale. So-wohl für die elementanalytische Untersuchung von Böden als auch für die Untersuchung von Ausgangsmaterialien (z.B. Klärschlammaschen) für ihre Verwendung als Dünger ist eine zuverlässige Analytik und Prozessüberwachung erforderlich.
Sewage sludge incineration is extensively practiced in some European countries such as the Netherlands, Switzerland, Austria and Germany. A survey of German sewage sludge ash showed that the recovery potential is high, approx. 19,000 t of phosphorus per year. However, the survey also discovered that the bioavailability of phosphorus in the sewage sludge ash is poor and that more than half of the ashes cannot be used as fertilizers due to high heavy metal content. A new thermochemical process for sewage sludge ash treatment was developed that transforms the ash into marketable fertilizer products. Sewage sludge ash was thermochemically treated with sodium and potassium additives under reducing conditions, whereby the phosphate-bearing mineral phases were transformed into plant available phosphates. High P-bioavailability was achieved with a molar Na/P ratio > 1.75 in the starting materials. Sodium sulfate, carbonate and hydroxide performed comparably as additives for this calcination process. Potassium carbonate and -hydroxide have to be added in a molar K/P ratio > 2.5 to achieve comparable P-solubility. The findings of the laboratory scale investigations were confirmed by an industrial demonstration trial for an ash treatment with sodium sulfate. Simultaneously, the volatile transition metal arsenic (61% removal) as well as volatile heavy metals such as cadmium (80%), mercury (68%), lead (39%) and zinc (9%) were removed via the off-gas treatment system. The product of the demonstration trial is characterized by high bioavailability and a toxic trace element mass fraction below the limit values of the German fertilizer ordinance, thus fulfilling the quality parameters for a P-fertilizer.
Phosphorus-fertilizers from secondary resources such as sewage sludge ash (SSA) will become more important in the future as they could substitute conventional fertilizers based on the nonrenewable resource phosphate rock. Thermochemical approaches were developed which remove heavy metals from SSA prior to its fertilizer application on farmlands. We analyzed the chemical state of mercury and selenium in SSA before and after thermochemical treatment under different conditions for P-fertilizer production by X-ray absorption near edge structure (XANES) spectroscopy. In some incineration plants the mercury loaded carbon adsorber from off-gas cleaning was collected together with the SSA for waste disposal. SSAs from those plants contained mercury mainly bound to carbon/organic material. The other SSAs contained inorganic mercury compounds which are most probably stabilized in the SSA matrix and were thus not evaporated during incineration. During thermochemical treatment, carbon-bound mercury was removed quantitatively. In contrast, a certain immobile fraction of inorganic mercury compounds remained in thermochemically treated SSA, which were not clearly identified. HgSe might be one of the inorganic compounds, which is supported by results of Se K-edge XANES spectroscopy. Furthermore, the chemical state of selenium in the SSAs was very sensitive to the conditions of the thermochemical treatment.
An environmentally friendly and cost efficient way for the management of municipal solid waste incineration (MSWI) fly ash represents its thermal co-treatment together with combustible waste. However, the safe introduction and storage of MSWI fly ash in the waste bunker is challenging and associated with severe problems (e.g. dust emissions, generation of undefined lumps and heat in case of moistened MSWI fly ash). Therefore, the aim of this study is to investigate the suitability of pelletisation as a pretreatment of MSWI fly ash. In particular, MSWI fly ash was characterised after sampling, pelletisation and thermal treatment and the transfer of constituents to secondary fly ash and flue gas was investigated. For this purpose, MSWI fly ash pellets with a water content of about 0.15 kg/kg and a diameter of about 8 mm have been produced by disc pelletiser and treated in an electrically heated pilot-scale rotary kiln at different temperatures, ranging from 450°C to 1050°C. The total contents of selected elements in the MSWI fly ash before and after thermal treatment and in the generated secondary fly ash have been analysed in order to understand the fate of each element. Furthermore, leachable contents of selected elements and total content of persistent organic pollutants of the thermally treated MSWI fly ash were determined. Due to the low total content of Hg (0.7 mg/kg) and the low leachate content of Pb (<0.36 mg/kg), even at the lowest treatment temperature of 450°C, thermally treated MSWI fly ash pellets can be classified as nonhazardous waste. However, temperatures of at least 650°C are necessary to decrease the toxic equivalency of PCDD/F and DL-PCB. The removal of toxic heavy metals like Cd and Pb is significantly improved at temperatures of 850°C, 950°C or even 1050°C. The observed metal removal led to relatively high contents of e.g. Cu (up to 11,000 mg/kg), Pb (up to 91,000 mg/kg) and Zn (up to 21,000 mg/kg) in the secondary fly ash. This metal enriched secondary fly ash might represent a potential raw material for metal recovery (e.g. via acidic leaching). Due to the high content of total dissolved solids observed in the leachate of thermally treated MSWI fly ash pellets, a wet extraction procedure is suggested to enable its safe disposal at non-hazardous waste landfills.
Herstellung neuer Düngemittelausgangsstoffe durch thermochemische Behandlung von Klärschlammaschen
(2018)
Klärschlammasche kann viel Phosphor enthalten (bis zu 12%). Aus diesem Grund ist diese Asche ein gutes Ausgangsmaterial für Phosphordünger. Aufgrund der geringen Pflanzenverfügbarkeit des Phosphors in der Asche ist eine Aufbereitung notwendig.
In diesem Buchbeitrag wird das thermochemische Verfahren AshDec vorgestellt, welches von dem Fachbereich 4.4 mit der Firma Outotec entwickelt worden ist. Es werden die Hintergründe diskutiert, die Phasenreaktionen bei diesem Prozess besprochen, Pflanzenversuche ausgewertet, verschiedene Extraktionen diskutiert und ein Konzept für eine mögliche Pilotanlage dargestellt.