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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.
Phosphorus (P) for fertilizer use can be recovered from sewage sludge ash (SSA). To enhance the bioavailability of P and reduce the heavy metal content of SSA, it can be treated thermochemically with Na2CO3 or Na2SO4 at 950 °C in a rotary kiln using dry sewage sludge or lignite as reducing agent. These processes were investigated by thermogravimetry/differential thermal analysis coupled with gas analysis. Reducing conditions in this experimental setup were provided by 2 % hydrogen in the Ar carrier gas. During SSA + Na2CO3 treatment CO2, CO and water were detected in the off-gas. During SSA + Na2SO4 treatment SO2, some CO2 and water were detected. Heavy metal removal was more efficient for SSA + Na2CO3 compared to the sulfate variant. A SSA + Na2SO4 + lignite variant which also formed CO shifted the heavy metal removal to the results obtained with Na2CO3 which was obviously due to the additional reduction potential. However, Zn evaporation was not achieved with the Na2SO4 variants which were most probably due to immobilization as ZnS.
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
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.
Determination of chromium (VI) in primary and secondary fertilizer and their respective precursors
(2017)
Hexavalent chromium species (Cr(VI)) are often carcinogenic, of high acute toxicity, highly mobile, and thus pose a severe risk to health and environment. Fertilizers usually contain significant amounts of chromium. Therefore, a reliable analysis of chromium and the fraction of Cr(VI) are crucial for safe use of fertilizers. This problem is expected to increase in the future, since more and more recycled fertilizers emerge due to increasing fertilizer demand and respective supply risks. However, existing analytical methods have been developed for conventional fertilizers and have to be tested whether they are suitable for the new materials. Thus, we performed a wet-chemical extraction for Cr(VI) on several matrices as well as respective quality control experiments including spiking with Cr(III) and Cr(VI)compounds. We found the Cr(VI) amounts to be below 2 mg/kg except for a thermally post-treated sewage sludge ash (SSA) that showed 12.3 mg/kg. The presence of organic matter e.g. in sludge or precipitated struvite caused a reduction of spiked Cr(VI) and thus no satisfying recovery for Quality control. Cr(VI) reduction was also observed for SSA, presumably due to the presence of Fe(II) compounds.
Even though the tested procedure can be hampered in some recycled fertilizer matrices, it might be adapted to be applicable also for these complex samples.
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.
This study compares the mercury distribution in the vapor phase, the phosphor powder and the glass matrix of new and spent fluorescent lamps. The spent fluorescent lamps were obtained at the collection yards of a public waste management company in Hamburg, Germany. An innovative systematic sampling method is utilized to collect six spent and eight corresponding new, off-the-shelf fluorescent lamp samples. The efficiency of several acid digestion methods for the determination of the elemental composition was studied and elemental mass fractions of K, Na, Y, Ca, Ba, Eu, Al, Pb, Mg, Hg, and P were measured. The study also finds aqua regia to be the best reagent for acid digestion. However, no significant difference in mercury distribution was found in the different phases of the new and spent fluorescent lamps.
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.