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Sewage sludge ashes (SSA) contain up to 13% P and are thus promising raw materials for fertilizer production. However, SSAs also contain heavy metals and the main P-bearing mineral phases whitlockite and aluminium phosphate are poorly bioavailable. We developed and patented a process that produces fertilizers from SSA addressing the above mentioned challenges. SSA is thermo-chemically treated at ~950°C under reducing conditions (reductive: dry sewage sludge) in a rotary kiln together with an alkali compound such as sodium or potassium sulfate, hydroxide or carbonate. Some undesired trace elements such as As, Hg, Pb and Cd are evaporated and separated via off gas treatment system. The poorly soluble phosphates are transformed into calcium alkali phosphates (CaNaPO4 or CaKPO4) that are not water soluble but completely soluble in neutral ammonium citrate solution (NAC). Pot experiments showed that the fertilizer performance of the treated SSA containing calcium alkali phosphates as the single P-bearing mineral phase was comparable to the performance of triple superphosphate. The PNAC-solubility of fertilizer products correlated very well with the resulting contents of calcium alkali phosphates and the P-uptake of plants in pot experiments. The effect of the ratio alkali/P on the PNAC-solubility was investigated by crucible experiments and trial series with a medium scale rotary kiln using different SSAs and alkali phosphates. The effects of operational parameters such as the temperature and the retention time were investigated as well as concurring side reactions of the alkali compounds e.g. with SiO2 present in SSA. The alkali/P ratio must be roughly adjusted at 2 to achieve 100% PNAC-solubility for a common type of SSA. A demonstration trial with an output of 2 t recycling fertilizer was carried out in an industrial rotary kiln (product output 30 kg/h). The PNAC-solubility of the product varied between 60% and 80% during the 4 days campaign showing that the transformation of the mineral P-phases to calcium alkali phosphates was not complete. This was observed although the amount of Na2SO4 additive was dosed according to the results of the pre-investigations. Structure analysis by XRD showed that besides the target compound CaNaSO4 also some Ca3(PO4)2 remained in the SSA as well as some unreacted Na2SO4 additive. Obviously, the conditions in the industrial rotary kiln were not optimal for the process showing that some process aspects have to be reconsidered for the scale-up.
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.
Chemical state of chromium, sulfur, and iron in sewage sludge ash based phosphorus fertilizers
(2015)
As an essential element of all life forms, phosphorus (P) is vital to the fertilizer industry. With decreasing quantity and quality of phosphate rock resources, recycling P-fertilizers from wastewater is of increasing interest. The P-fertilizer products of a recently developed thermochemical process for P recovery from sewage sludge ash (SSA) were investigated by chromium, sulfur, and iron K-edge X-ray near-edge structure (XANES) spectroscopy. This paper focuses the formation and prevention of toxic chromium(VI) and toxic sulfides during the thermochemical processes. Reducing conditions prevent the oxidation of chromium(III) in the SSA to toxic chromium(VI). Sulfides formed under the reducing conditions are nontoxic iron sulfides. Hematite (Fe2O3) present in the SSA is reduced to magnetite (Fe3O4). A gentle post-treatment at 400 °C under oxidizing conditions converts the iron sulfides into plant-available iron sulfates. This oxidative post-treatment does not form undesired chromium(VI) compounds.
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) fertilizers from secondary resources became increasingly important in the last years. However, these novel P-fertilizers can also contain toxic pollutants such as chromium in its hexavalent state (Cr(VI)). This hazardous form of chromium is therefore regulated with low limit values for agricultural products even though the correct determination of Cr(VI) in these fertilizers may be hampered by redox processes, leading to false results. Thus, we applied the novel diffusive gradients in thin-films (DGT) technique for Cr(VI) in fertilizers and compared the results with the standard wet chemical extraction method (German norm DIN EN 15192) and Cr K-edge X-ray Absorption near-edge structure (XANES) spectroscopy. We determined an overall good correlation between the wet chemical extraction and the DGT method. DGT was very sensitive and for most tested materials selective for the analysis of Cr(VI) in P-fertilizers. However, hardly soluble Cr(VI) compounds cannot be detected with the DGT method since only mobile Cr(VI) is analyzed. Furthermore, Cr K-edge XANES spectroscopy showed that the DGT binding layer also adsorbs small amounts of mobile Cr(III) so that Cr(VI) values are overestimated. Since certain types of the P fertilizers contain mobile Cr(III) or partly immobile Cr(VI), it is necessary to optimize the DGT binding layers to avoid aforementioned over- or underestimation. Furthermore, our investigations showed that the Cr K-edge XANES spectroscopy technique is unsuitable to determine small amounts of Cr(VI) in fertilizers (below approx. 1% of Cr(VI) in relation to total Cr).
Determination of phosphate phases in sewage sludge ash-based fertilizers by Raman microspectroscopy
(2013)
The chemical form of phosphate phases in sewage sludge ash (SSA)-based fertilizers was determined by Raman microspectroscopy. Raman mapping with a lateral resolution of 5 × 5 µm² easily detected different compounds present in the fertilizers with the help of recorded reference spectra of pure substances. Quartz and aluminosilicates showed Raman bands in the range of 450-520 cm-1. Phosphates with apatite structure and magnesium triphosphate were determined at around 960 and 980 cm-1, respectively. Furthermore, calcium/magnesium pyrophosphates were detected in some samples.
The reaction mechanisms of phosphate-bearing mineral phases from sewage sludge ash-based fertilizers in soil were determined by Raman and synchrotron infrared microspectroscopy. Different reaction mechanisms in wet soil were found for calcium and magnesium (pyro-) phosphates. Calcium orthophosphates were converted over time to hydroxyapatite. Conversely, different magnesium phosphates were transformed to trimagnesium phosphate. Since the magnesium phosphates are unable to form an apatite structure, the plant-available phosphorus remains in the soil, leading to better growth results observed in agricultural pot experiments. The pyrophosphates also reacted very differently. Calcium pyrophosphate is unreactive in soil. In contrast, magnesium pyrophosphate quickly formed plant-available dimagnesium phosphate.
Evidence is provided that the tridymite component observed in the X-ray diffraction patterns of some sewage sludge ashes (SSAs) should not be interpreted as the tridymite modification of SiO2 but as the tridymite form of AlPO4. This proof is based on a combined X-ray Powder Diffraction (XRD), X-ray fluorescence (XRF) and Mossbauer spectroscopy investigation of two SSAs produced at two fluidized bed incineration facilities, located in different municipalities and operated differently. The structural and chemical characterization was carried out on the 'as received' SSA samples as well as on the residues of these two SSAs pretreated by leaching in citric acid. In addition, direct proof is presented that the tridymite form of AlPO4 does crystallize from X-ray amorphous precursors under conditions that mimic the huge heating rate and short retention time (just seconds at T ≈ 850 °C) typical for fluidized bed incinerators.
Sewage sludge and sewage sludge ashes (SSA) are produced in huge amounts at municipal waste water treatment plants (WWTP) all around the world and have become an issue for many urbanized areas. To deal with this unceasing mass flow in an ecologically and economically responsible way a comprehensive chemical and structural characterization of all types of SSA is needed. X-ray powder diffraction (XRD) is one of the most promising analytical methods for this task. Although, there has been ample chemical evidence showing that many SSA contain aluminium phosphate as a major component up to now no aluminium phosphate or aluminium-rich mixed phosphate phase has been reported to be identified by XRD in a SSA produced at a mono-incineration facility. The outcome of the present com-bined XRD and Mossbauer spectroscopy investigation provides comprehensive evidence closing this gap for the first time.