Article pubs.acs.org/est
Heavy Metal Removal from Sewage Sludge Ash by Thermochemical Treatment with Polyvinylchloride Christian Vogel,†,‡,* Robert M. Exner,†,§ and Christian Adam† †
BAM Federal Institute for Materials Research and Testing, Division 4.4 Thermochemical Residues Treatment and Resource Recovery, Unter den Eichen 87, D-12205 Berlin, Germany ‡ School of Chemistry, Monash University, Wellington Road, 3800 VIC Clayton, Australia § Institute for Technical Ecology, Technical University of Berlin, Straße des 17. Juni 135, D-10623 Berlin, Germany ABSTRACT: Sewage sludge ash (SSA) is a prospective phosphorus source for the future production of recycling Pfertilizers. Due to its high heavy metals contents and the relatively low P plant-availability, SSA must be treated before agricultural utilisation. In this paper SSA was thermochemically treated with PVC in a bench-scale rotary furnace in order to remove heavy metals via the chloride pathway. PVC has a high Cl-content of 52−53% and a high heating value that can be beneficially used for the thermochemical process. Large amounts of waste PVC are already recovered in recycling processes, but there are still some fractions that would be available for the proposed thermochemical process, for example, the low quality near-infrared(NIR)-fraction from waste separation facilities. Heavy metals were effectively removed at temperatures in the range of 800−950°C via the gas phase by utilisation of PVC as Cl-donor. The resulting P plant-availability was comparable to SSA thermochemically treated with MgCl2 as Cl-donor if MgO was used as an additive (Mg-donor). A further increase of the plant availability of phosphorus was achieved by acid posttreatment of the thermochemically treated SSA.
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INTRODUCTION Phosphate rock, a non-renewable resource, is mainly used for Pfertilizer production. Due to the sudden increases of the price for phosphate rock in the last years and in order to save the limited resources1,2 new options for the production of phosphate fertilizers from secondary resources have to be developed.3 An important secondary phosphate source is sewage sludge. Phosphorus is removed from wastewater by enhanced biological phosphorus removal (EBPR) and/or by precipitation with Ca2+-, Fe2+-, Fe3+-, or Al3+-salts in wastewater treatment plants (WWTP) and is accumulated in the sewage sludge. In the European Union more than 11 Mio Mg (dry matter) of sewage sludge are generated annually. However, direct agricultural application of sewage sludge as a P-fertilizer became a controversial issue mainly due to the presence of organic pollutants (e.g. hormones, antibiotics, endocrine disrupters, and persistent organic pollutants) and heavy metals. In the 6th EU Framework Program (FP6) research project SUSAN (Sustainable and Safe Re-use of Municipal Sewage Sludge for Nutrient Recovery)4 a two steps thermal process was developed, which transfers the important nutrient phosphorus contained in sewage sludge into a mineral phosphate fertiliser raw material with low heavy metals content.5 In the first step sewage sludge is incinerated (mono-incineration) at 800−900 °C, destructing all organic matter including the organic pollutants. The resulting sewage sludge ashes (SSA) are rich in phosphorus (5−10% P) and other nutrients (calcium, magnesium, potassium etc.) but still contain most of the heavy © 2012 American Chemical Society
metals from the original sludge. In the second step, the sewage sludge ashes are thermochemically treated at approx. 1000 °C for about 30 min after mixing with a solid Cl-donor (e.g. CaCl2, MgCl2, NaCl)6 or with gaseous Cl2 and HCl,7 respectively. Heavy metals can be effectively removed by the chloride pathway and new bio-available mineral phosphate phases are formed if sufficient Mg is present during thermochemical treatment or if a post treatment with acid was carried out. In another research projectSUSYPHOS (Sustainable Symbiotic Fertilizer Production from Two Renewable Materials)meat and bone meal was added to SSA in the thermal process because of its high content of phosphorus and energy. During this project also experiments with polyvinylchloride (PVC) as Cl-donor and energy carrier were carried out. PVC is used as plastic for many applications because of its good mechanical and thermal properties and high resistance against corrosive chemicals. In Germany more than 1.7 Mio Mg of PVC are used annually. Most of the PVC can be recycled after its application,8 but there are still some flow patterns of PVC which were not picked up from the recycling system e.g. the near-infrared(NIR)-fraction of waste separation facilities. PVC has a high Cl-content of 52−53 % and high heat energy of approx. 18 MJ/kg. PVC often contains heavy metals such as Received: Revised: Accepted: Published: 563
February 13, 2012 September 17, 2012 November 28, 2012 November 28, 2012 dx.doi.org/10.1021/es300610e | Environ. Sci. Technol. 2013, 47, 563−567
Environmental Science & Technology
Article
investigations with other chlorine donors5−7 in order to be able to compare the results. Thus, the PVC addition was not adjusted to a certain energetic input but on a certain Clcontent. MgO was dosed with 95 g/kg SSA. It was required as an adjuvant to avoid caking in the furnace. Furthermore, it was used as an Mg-donor aiming at the enhancement of the plantavailability of phosphorus. Thermogravimetry (TG) Coupled with FT-IR Spectrometer. Thermogravimetry/FT-IR experiments were carried out with a Netzsch STA 449 F3 Jupiter (Selb, Germany) coupled by a heated transfer line (heated at 200°C) to a Bruker Alpha FT-IR spectrometer (Ettlingen, Germany) with a DTGS detector. Samples of 20−30 mg (experiments with pure SSA: 1250 mg) were heated with a heating rate of 5°C/min (pure SSA: 10°C/min) from 30 to 1000 °C. Afterwards the temperature of 1000 °C was kept constant for 30 min. The flow rate of the carrier gas (air) was 50 mL/min. FT-IR gasphase spectra were collected with a spectral resolution of 2 cm‑1 and 18 scans were coadded per spectrum. Thermal Treatment in a Rotary Furnace. For the thermochemical experiments an indirectly heated small-scale rotary furnace (Tmax: 1700°C) from Thermal Technology GmbH (Bayreuth, Germany) with a corundum torque tube (length: 2000 mm, diameter: 100 mm) was used. For all experiments an inclination angle of 5° was adjusted. The torque tube was infinitely speed variable until 9 U/min. The retention time of the ashes in the hot zone of the torque tube (700 mm) was aligned to 20 min by adjustment of the speed of the torque tube. An air flow of 5 L/min was adjusted in co-current flow. The temperature in the tube was measured by a thermocouple (NiCr-Ni) in the hot zone near to the inner wall. At the end of the tube a collecting container was set up. The evaporated heavy metal chlorides were withdrawn by suction over a gas cooler and an alkaline wet scrubber system. The elemental composition of the thermochemically treated SSAs were measured by ICP-OES (Thermo IRIS Intrepid II XSP in combination with Thermo Timberland IIS Autosampler) after total digestion (HNO3/HCl/HF) in a microwave (mikroPrepA, MLS GmbH, Leutkirch, Germany; heating with 1000 W; 20 min isotherm segment at 210°C). For all samples digestion and ICP-OES measurement were carried out three times. P-Solubility Neutral Ammonium Citrate (Pnac). For the determination of the P-solubility in neutral ammonium citrate (Pnac) 3 g sample was extracted in 100 ml extractant for exactly one hour at constant temperature (65 °C) and stirring. After immediate cooling to ambient temperature the suspension was quantitatively transferred into a graduated 500-ml flask with a jet of water. The flask was made up to the volume with water and mixed thoroughly. At the end the solution was filtered through a phosphate-free filter (EU Directive 2003/2003 method 3.1.4) and analyzed by ICP-OES. Three replication of the whole extraction test were carried out for each sample.
cadmium and lead but these heavy metals are effectively separated via the gas phase in the thermochemical process. Thus, PVC fulfills the requirements of a useful and cheap Cldonor and energy carrier for the thermochemical treatment of sewage sludge ashes. In the past Sasabe et al.9 and Rio et al.10 already used PVC for the evaporation of heavy metals from solid wastes. Consequently, thermochemical treatment of SSA was investigated using PVC as Cl- and energy carrier focusing on heavy metal removal and the plant-availability of phosphorus contained in the treated ashes. The plant-availability was estimated by the extraction test neutral ammonium citrate (Pnac), which is established in the German Fertilizer Ordinance.11 Kuderna12 found that the results of greenhouse pot experiments carried out with thermochemically treated SSA showed very good correlation with the results of the Pnac test. Furthermore, the thermochemical treatment process was analyzed by thermogravimetry. The off-gas of the thermogravimetry was analysed by an FT-IR spectrometer.
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EXPERIMENTAL SECTION Material and Instrumentation. The thermochemical experiments were carried out with a SSA from a Dutch incineration plant that was already used in former investigations.5−7 The composition of the SSA is given in table 1. PVC from Solvin AG (Rheinberg, Germany) in powder-form and magnesium oxide (MgO) (Kauster, Leoben, Austria) were used for the thermochemical treatment. SSA was mixed with PVC and MgO in a gyrowheel-mixer. Chlorine contents between 100 and 200 g/kg SSA were adjusted by addition of PVC to the SSA. These amounts of chlorine added to SSA were in the same range of former Table 1. Mass Fractions and Standard Deviations (SD; n = 3) of Main Elements and Trace Elements Determined for the Used SSA by ICP-OES after Total Digestion with HNO3/HCl/HF in a Microwave (210°C) SSA mass fractions Al Ca Fe K Mg Mn Na P S Si As Cd Cr Cu Hg Mo Ni Pb Sn Tl Zn
SD
mg/kg
mg/kg
50 736 90 776 95 626 13 893 14 783 851 602 90 650 10 333 103 533 22.6 3.54 151 1073
