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- Klärschlammasche (3)
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Eingeladener Vortrag
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The amount of sewage sludge produced worldwide is expected to further increase due to rising efforts in wastewater treatment. There is a growing concern against its direct use as fertilizer due to contamination of the sludge with heavy metals and organic pollutants. Incinerating the sludge degrades organic compounds almost completely and concentrates heavy metals and phosphorus. However, the sewage sludge ash (SSA) is almost completely disposed of and with it all resources are removed from the economic cycle. Comprehensive knowledge of the composition of SSA is crucial to assess the resource recovery potentials. We conducted a survey of all SSA emerging in Germany and determined the respective mass fractions of 57 elements over a period of one year. The median content of phosphorus was 7.9%, indicating an important recovery potential. Important trace elements were Zn (2.5 g/kg), Mn (1.3 g/kg), and Cu (0.9 g/kg). Mass fractions of technology metals such as V, Cr, Ga, Nb, and rare earths were comparatively low. Considering the possible use of SSA as secondary raw material for fertilizer production it should be noted that its Cd and U content (2.7 mg/kg and 4.9 mg/kg respectively) is significantly lower than that of rock phosphate based mineral fertilizers.
Complete survey of German sewage sludge ashes - phosphorous and technology metal recovery potential
(2014)
Sewage sludge ashes (SSA) are a possible source for recovery of phosphorous (P) and technology metals. Phosphorous is essential for all animate beings. So far, its sole external source is rock phosphate, which is a finite resource. The EU import dependency on P from regions with high domestic demands or political instabilities is almost complete. Furthermore, rock phosphate often contains high amounts of heavy metals like Cd and U that might lead to health and environmetal hazards. P recovered from SSA might diminish these problems. We analyzed SSA from German mono incineration facilities in terms of their elemental composition and respective recovery potential. More than 97% of the emerging SSA is monitored, enabling the determination of complete mass flows of the ashes and their components. Results indicate a P recovery potential of 19,000 t/a (more than 12% of the annual P demands for fertilizer production). The concentrations of Cd and U in SSA are significantly lower than in rock phosphate and thus might help to reduce possible envrionmental and health hazards.
Due to the great quantities of construction and demolition waste in Europe, the reuse of building material waste as secondary raw materials is of particular importance. A recycling of building materials can meet the requirements of sustainability in several aspects: the extended time availability of primary raw materials and the related protection of natural resources as well as the saving of landfill sites. In recent years sulphates originating from gypsum in secondary building materials, made of construction and demolition waste, received growing attention. Sulphates are unwanted in secondary building materials for concrete production as well as for other use e.g. in unbounded layers. The content of gypsum resp. sulphates in secondary aggregates can be reduced by different methods. Depending on the type of material different steps for selective dismantling can be used to separate gypsum containing residues from concrete rubble. Furthermore some steps for the processing of building rubble are suitable for the reduction of sulphates in the produced aggregates. An environmental evaluation of different ways for the production of recycled concrete aggregates was performed with regard to a reduction of sulphates in the secondary building material. The use of techniques for selective dismantling was environmentally advantegous for the deconstruction of three investigated model houses.
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.
Phosphorous is essential for all living creatures and indispensable in its functions regarding bone structure, energy metabolism, and storage and expression of genetic information. It is removed from the soils through farming and husbandry and has to be replaced. Annually, more than 17 million tons of phosphorous are applied to soils in the form of mineral fertilizer worldwide (Killiches, 2013). In Germany, this accounts for 150,000 t/a. This fertilizer is produced from rock phosphate, which is often contaminated with heavy metals (Dissanayake et al., 2009), especially cadmium and uranium. Cd concentrations are around 30 mg/kg, U around 100 mg/kg, with even higher peak values. Since rock phosphate is treated with acids to make the phosphorous bioavailable, those heavy metals were also mobilized. When the fertilizer is applied to the farmland, the contaminants can pollute the soil and pose an environmental risk. Sewage sludge is another phosphorous source for farming. In Germany, 15,000 t/a phosphorous are applied to farmlands this way. However, there is a growing concern of possible environmental hazards since sewage sludge contains not only inorganic contaminants but also organic pollutants, especially pharmaceuticals and personal care products (Guardia et al., 2004).
Sewage sludge ash (SSA) based P-fertilisers were produced by thermochemical treatment of SSA with Cl-donors at approximately 1000 °C. During this thermochemical process heavy metals are separated as heavy metal chlorides via the gas phase. Chromium cannot be separated under normal conditions. The risk of the development of toxic Cr(VI) during the thermochemical process was investigated. X-ray Absorption Spectroscopy measurements showed that SSA and thermochemically treated SSA with CaCl2, MgCl2 and NaCl contain Cr(III) compounds only. In contrast, treating SSA with elevated quantities of Na2CO3, to enhance the plant-availability of the phosphate phases of the fertiliser, developed approximately 10–15% Cr(VI). Furthermore, Raman microspectroscopy showed that using Mg-carbonate reduces the risk of a Cr(VI) development during thermochemical treatment. Additionally, leaching tests showed that only a Crwater solubility >10% is an indicator for Cr(VI) in SSA based P-fertilisers.
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.
This study focuses on the question whether heat treated sewage sludge ashes are more favourable as fertilizers than untreated sewage sludge ashes (USSA) and whether their fertilization effects are comparable with commercial triple superphosphate (TSP). In a pot experiment, maize was fertilized either with one of three heat treated and Na-, Ca- and Si-compounds amended sewage sludge ashes (two glown phosphates, steel mill slag + sewage sludge ash) or USSA or TSP as control. Fertilization with USSA did not increase the biomass yield and the P uptake of maize in comparison to the P0 treatment (7.25 resp. 8.35 g dry matter/pot). Fertilization with heat treated sewage sludge ashes and TSP resulted in significantly higher yields and plant P uptakes which are on average eight times higher than treatment with USSA and P0 control. Biomass yields and P uptake of maize after fertilization with heat treated sewage sludge ashes are not significantly different from those after TSP fertilization. The main P compound in USSA is Ca3(PO4)2. By heat treatment and amendment with different sodium, calcium, sulphur and silicon containing additives or steel mill converter slag, Ca3(PO4)2 is converted to Ca- and Na-silico-phosphates, which have a higher water solubility than Ca3(PO4)2. This increased solubility is responsible for the high plant availability of this phosphates.