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Über den derzeitigen und zukünftigen Umgang mit Klärschlamm wird in Deutschland seit Jahren kontrovers diskutiert. Zum einen enthalten kommunale Klärschlamme Nährstoffe, die sinnvoller Weise wieder in die landwirtschaftliche Nahrungsmittelproduktion eingebracht werden sollten. In diesem Zusammenhang steht Phosphor im Vordergrund, da diese Ressource begrenzt ist und deren zukünftig zu erwartende Knappheit neben der geringen Anzahl an Exportländern für Rohphosphate zu unberechenbaren Preissprüngen am Weltmarkt führen können. Zum anderen stellt Klärschlamm die Schadstoffsenke in der Abwasserreinigung dar, was den direkten landwirtschaftlichen Einsatz dieses Abfalls bedenklich macht. Zwar nahm die Konzentration der umweltrelevanten Schwermetalle in Klärschlämmen in den vergangenen Jahren deutlich ab (Wiechmann, Dienemann et al. 2012), die Belastung mit organischen Schadstoffen (z.B. Pharmazeutika, endokrin wirksame Substanzen, persistente organische Stoffe etc.) ist jedoch unüberschaubar und dürfte sich einem sinnvollen „Monitoring“ weitgehend entziehen. Das daraus resultierende Risiko für Mensch und Umwelt ist schwer einschätzbar und wird aufgrund der Beschaffenheit der diffusen Schadstoffquellen (z.B. Eintrag von Pharmaka und deren Metabolite über menschliche Ausscheidungen) unkalkulierbar bleiben.
In Deutschland werden gegenwärtig etwa 53 % der kommunalen Klärschlämme in Mono- und Müllverbrennungsanlagen sowie Kraft- und Zementwerken thermisch und 47 % stofflich, d.h. landwirtschaftlich (30 %) oder in landschaftsbaulichen Maßnahmen u.ä. verwertet (17 %). Bezogen auf die etwa 9,4 Mrd Kubikmeter Abwasser, die pro Jahr in Deutschland anfallen, kann von einem Potenzial von 75000 t Phosphor/a (davon ca. 67500 t P/a im Klärschlamm) ausgegangen werden. Bislang werden in der Literatur ca. 37 Verfahren zur abwasserseitigen Rückgewinnung von Phosphor beschrieben, die im Labor, im Technikumsmaßstab oder in Pilotanlagen erprobt worden sind. Weltweit wurden bisher ca. 12 davon großtechnisch umgesetzt. Etwa vier verschiedene Verfahrenstechniken lassen sich derzeit in Deutschland finden. Einige Verfahren arbeiten mit Ionenaustauschern, zum überwiegenden Teil handelt es sich jedoch um Fällungs- oder Kristallisationsverfahren, bei denen Magnesium-Ammonium-Phosphat (MAP, Struvit) entweder aus den Haupt- oder Nebenströmen einer Kläranlage oder aus dem Schlamm bzw. Schlammwasser gewonnen wird. Für den Nährstoff Phosphor konnte in den Klärschlammaschen ein Gehalt von durchschnittlich 7 % ermittelt werden. Daraus ergibt sich ein Potenzial von etwa 17500 t P/a (Tendenz steigend). Je nach Verfahren kann mit einem Rückgewinnungspotenzial von etwa 60 bis 90 % Phosphor aus der Asche (ca. 55 bis 80 % Pges bezogen auf den Kläranlagenzulauf) gerechnet werden
Sewage sludge ashes (SSA) contain considerable mass fractions of phosphorus (5-10 w-% P) suitable for fertiliser production. Unfortunately, also most of the heavy metals remain in the ashes. A thermochemical process was developed for the treatment of SSA to i) remove heavy metals and ii) transform phosphates into bio-available mineral phases. The technology was already demonstrated in technical scale (capacity of 300 kg/h) and the company OUTOTEC is currently planning the first industrial plant. In order to manufacture a marketable multi-nutrient fertiliser from the thermochemically treated SSA further wastes and industrial by-products were taken into account. Ammonium sulphate occurs as a by-product of the caprolactam production and was chosen as N-carrier (21 w-% N). Straw ash was tested as potassium carrier (11-15 w-% K). Granulation campaigns were carried out with intensive mixers in lab-, medium-and technical scale. NPS-and NPKS-fertilisers were produced that were characterised by suitable particle size distributions and strength.
Sewage sludge ashes (SSA) represent a possible source of phosphorus (P) and technology metals. So far, the sole external source for P, that is indispensable for all animate beings, is rock phosphate, a finite resource. The EU depends completely on the import from possible politically instable regions. Furthermore, rock phosphate is often contaminated with heavy metals like Cd and U, leading to health and environmental hazards. P recovered from SSA might diminish these problems. SSA of German mono incineration facilities are analyzed for their elemental composition. More than 95% of the emerging SSA is monitored, enabling the determination of complete mass flows of the ashes and their components. Initial results indicate a P recovery potential of 16,000 t/a (11% of the annual P demands for fertilizer production). The concentrations of Cd and U in SSA are one to two Orders of magnitude lower than in rock phosphate and could help to reduce possible hazards.
The annual demand of phosphorus (P) for fertilizer use is more than 550,000 t in Germany. Several of the input streams like mineral fertilizer produced from phosphate rock or sewage sludge are problematic due to contamination with heavy metals and organic pollutants and/or supply risks. Phosphorus recovery from the wastewater stream might help to diminish these problems. Several procedures have been developed to recover P from either sewage sludge ash (SSA) or wastewater via precipitation or crystallization as struvite. This study determined the elemental composition and trace metal mass fractions of 252 SSA samples and 17 struvite samples from wastewater treatment plants (WWTP) to see whether there are differences related to the source of the sludge (municipal/industrial) or the stage of the WWTP where the struvite is produced. Results indicated that there is no clear trend of the elemental mass fractions related to the source of the sludge. All struvite samples show very low heavy metal contents, even though the struvite precipitated from the sludge shows slightly elevated heavy metal mass fractions compared to struvite crystallized from sludge liquor. Struvite is a clean and plant available material for fertilizer production from wastewater streams and has some advantages for the wastewater treatment plant. However, since the P recovery rate of struvite processes is low (7–11%) compared to that from SSA processes (90%), recovery from SSA is necessary for a substantial P recovery from the wastewater stream.
There is a growing concern of the direct use of contaminated sewage sludge in agriculture due possible environmental and health hazards. Thus, incineration of the sludge and thus increasing amounts of sewage sludge ashes (SSA) are to be expected in the future. SSA contains considerable amounts of phosphorus (P) and technology metals that might be recovered as secondary raw materials. Since the EU for instance depends completely on the import of rock phosphate needed for fertilizer production, alternative sources for P are in order. Furthermore, rock phosphate is often contaminated with heavy metals like Cd and U, leading to health and environmental hazards. P recovered from SSA might diminish these problems. To determine the possible recovery potential of P and technology metals from SSA, we conducted a survey of German mono incineration facilities and analyzed the respective SSA for their elemental composition. More than 95% of the emerging SSA was monitored. Results indicate a P recovery potential of 18,000 t/a (up to 13% of the annual P demands for fertilizer production). The concentrations of Cd and U in SSA are one to two Orders of magnitude lower than in rock phosphate and could help to reduce possible hazards.
Recovery of rare earth elements - optimized elemental analysis of fluorescent lamp shredder waste
(2016)
Rare earth elements (REE) are a crucial component of fluorescence lamps. Several procedures have been developed to recovery these technological important elements. Nevertheless, actual REE recycling from fluorescence lamps is scarce so far (recovery rate of less than 1 %), with current recycling approaches concentrating on glass recovery. Since most recycling processes include several, also wet-chemical steps, a complete knowledge of the actual elemental composition of the respective mass flows is necessary for an efficient REE recovery. We tested seven different reagent mixtures for microwave-assisted digestion of fluorescent lamp shredder, including HF, HClO4, and H2O2. We determined the concentrations of 25 of the most relevant rare earth and other trace elements in the respective dilutions. Two independent digestions, one a mixture of perchlorid/nitric/hydrofluoric acid and the other aqua regia, showed the highest concentrations of 23 of these elements, excluding only Sn and Tb. The REE concentrations in the tested lamp shredder sample (stated in g/kg) were 10.2 (Y), 12.1 (La), 7.77 (Ce), 6.91 (Eu), 1.90 (Gd), and 4.11 (Tb).