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Municipal sewage sludge is a carrier of nutrients most important phosphorus (P) but also contains organic pollutants and heavy metals. A two steps thermal treatment based on 1. mono-incineration of sewage sludge under destruction of organic pollutants and 2. thermochemical treatment of the resulting ashes to remove heavy metals and increase P-bioavailability is suggested by the authors. The process aims at P-fertilisers production from sewage sludge. Seven sewage sludge ashes of different origin and composition and one residue from gasification of sewage sludge were thermochemically treated. The raw ashes were mixed with a chlorine donor (e.g. MgCl2) and thermally treated in a gas tight lab-scale rotary furnace. The distributions of the heavy metals between solid and gas phase were determined. The heavy metals concentrations of the treated ashes matched the legislative limits in most cases. The products from thermochemical treatment are suited raw materials for P-fertiliser production. Relevant process parameters such as temperature, type and concentration of Cl-donor and retention time were optimised for the thermochemical treatment step.
Phosphorus (P) is an essential element for all living organisms and cannot be replaced. Municipal sewage sludge is a carrier of phosphorus, but also contains organic pollutants and heavy metals. A two-step thermal treatment is suggested, including mono-incineration of sewage sludge and subsequent thermochemical treatment of the ashes. Organic pollutants are completely destroyed by mono-incineration. The resulting sewage sludge ashes contain P, but also heavy metals. P in the ashes exhibits low bioavailability, a disadvantage in farming. Therefore, in a second thermochemical step, P is transferred into mineral phases available for plants, and heavy metals are removed as well. The thermochemical treatment was investigated in a laboratory-scale rotary furnace by treating seven different sewage sludge ashes under systematic variation of operational parameters. Heavy metal removal and the increase of the P-bioavailability were the focus of the investigation. The present experimental study shows that these objectives have been achieved with the proposed process. The P-bioavailability was significantly increased due to the formation of new mineral phases such as chlorapatite, farringtonite and stanfieldite during thermochemical treatment.
The sequence of reactions accompanying the thermochemical treatment of an iron- and aluminium-bearing sewage sludge ash was ascertained by investigating two systematic series of samples. The ash was thermochemically treated in a lab-scale rotary furnace after mixing it with a chlorine-donor, either CaCl2 or MgCl2. Within each of these two sample series only a single process parameter, the reaction temperature, was varied, namely between 350 and 1050°C. It was found, that among the numerous crystalline phases present in the raw ash only quartz and hematite continue to exist after thermochemical treatments carried out at 1050°C, whereas all other components undergo at least one decomposition-recrystallization cycle. Some of the components re-crystallize even several times. It was proved that the restructuring of the calcium- and phosphorus-bearing mineral phases proceeds via the formation of chlorspodioside, Ca2PO4Cl. The influence of the type of chlorine-donor on the final product was elucidated in detail and - to the best of our knowledge - for the first time crystalline AlPO4 was found in a sewage sludge ash and its decomposition was investigated, too.
A high temperature (1000 °C) thermochemical process for heavy metal removal from sewage sludge ash via the chloride pathway was investigated by thermogravimetry/differential thermal analysis (TG/DTA). TG and DTA measurements gave information about secession and evaporation of water, HCl, and heavy metal chlorides at different temperatures. Additionally, gaseous water and hydrochloric acid which occurred in the process were detected by an FT-IR detector that was coupled to the TG/DTA-system. Heavy metal chlorides which were also formed in the process cannot be detected by this technique. For that reason the outlet gas of the TG/DTA-system was discharged into washing flasks filled with water for absorption. The washing flasks were replaced in temperature steps of 50 °C and the heavy metal concentrations of the solutions were determined by ICP-OES. The temperature-dependent formation/evaporation of different heavy metal chlorides was analyzed and compared for two different thermochemical processes using magnesium chloride hydrate or calcium chloride hydrate as Cl-donors. In both cases evaporation of Cd, Cu, Pb, and Zn was observed from 600 °C, whereas As, Cr, and Ni remained in the solid state. The results were discussed against the background of thermodynamic calculations.
Inorganic phosphorus and nitrogen-phosphorus-potassium (NPK) fertilizers based on phosphates from thermochemically treated sewage sludge ash were analyzed using mid-infrared (mid-IR) and far-infrared (FIR) spectroscopy. The different compounds present in the fertilizers were qualitatively determined with the help of recorded reference spectra of pure substances. Differentiation between various phosphates and other compounds such as sulfates, nitrates, and oxides was possible using combined interpretation of the mid-IR and FIR spectra. The results are in agreement with previous X-ray diffraction (XRD) measurements of the same samples. The main phosphate phases detected were NH4H2PO4, MgHPO4·3H2O, Mg3(PO4)2, Ca5(PO4)5Cl, CaHPO4·2H2O, Ca(H2-PO4)2·H2O, and AlPO4. Furthermore, K2SO4, NH4NO3, Fe2O3, and SiO2 were identified in the IR spectra. However, ammonium and sulfate compounds were only identified in the mid-IR region but were not detectable in the FIR region.
Sewage sludge ash (SSA) is a suitable raw material for fertilizers due to its high phosphorus (P) content. However, heavy metals must be removed before agricultural application and P should be transferred into a bioavailable form. The utilization of gaseous hydrochloric acid for thermochemical heavy metal removal from SSA at approximately 1000 °C was investigated and compared to the utilization of alkaline earth metal chlorides. The heavy metal removal efficiency increased as expected with higher gas concentration, longer retention time and higher temperature. Equivalent heavy metal removal efficiency were achieved with these different Cl-donors under comparable conditions (150 g Cl/kg SSA, 1000 °C). In contrast, the bioavailability of the P-bearing compounds present in the SSA after thermal treatment with gaseous HCl was not as good as the bioavailability of the P-bearing compounds formed by the utilization of magnesium chloride. This disadvantage was overcome by mixing MgCO3 as an Mg-donor to the SSA before thermochemical treatment with the gaseous Cl-donor. A test series under systematic variation of the operational parameters showed that copper removal is more depending on the retention time than the removal of zinc. Zn-removal was declined by a decreasing ratio of the partial pressures of ZnCl2 and water.
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.