ARTICLE pubs.acs.org/est
Heavy Metal Removal from Sewage Sludge Ash by Thermochemical Treatment with Gaseous Hydrochloric acid Christian Vogel and Christian Adam* Division 4.3 Waste Treatment and Remedial Engineering, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin ABSTRACT: 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.
’ INTRODUCTION Phosphorus is an essential element of all life forms. It is necessary for the metabolism process (ADP/ATP) and part of the DNA. Accordingly, phosphorus is inserted in form of phosphate fertilizers in the agricultural food production. Usually, the limited natural resource phosphate rock is used for fertilizer production. Due to the increase of the phosphate rock price and decreasing amount and quality of mineral phosphate resources1 new alternatives for the production of phosphate fertilizers for agriculture must be found. An important source of secondary phosphates is sewage sludge. To protect local receiving waters from eutrophication P in wastewater is removed by enhanced biological phosphorus removal (EBPR) or by precipitation with Ca2+, Fe2+, Fe3+, or Al3+ salts in the wastewater treatment plant (WWTP) and ends up in the sewage sludge. Monoincineration of sewage sludge transfers P into the remaining sewage sludge ashes resulting in relatively high P mass fractions of 5 10%. In the European FP6-project2 SUSAN (Sustainable and Safe Reuse of Municipal Sewage Sludge for Nutrient Recovery) a thermochemical process was developed that separates heavy metals from sewage sludge ash and transfers the ash into a fertilizer containing mineral phosphate phases that are bioavailable. Adam et al.3 investigated the heavy metal elimination from sewage sludge ash using alkaline earth metal chlorides as Cl-donor at an operating temperature range of 750 1050 °C. The thermochemical transformation of phosphorus compounds into new bioavailable mineral phosphate phases was determined by XRD measurements.4 The improved P-fertilization performance of the thermochemically r 2011 American Chemical Society
treated sewage sludge ashes was ascertained by pot and field experiments.5 Pot trials with SSA thermochemically treated with MgCl2 showed dry matter yields comparable to those variants fertilized with super phosphate. The patented process6 was already operated in technical scale by the Austrian company ASH DEC Umwelt AG (a partner in the European projects SUSAN and SUSYPHOS). This article focuses on investigations with gaseous hydrochloric acid used as a Cl-donor for the thermochemical process. The utilization of gaseous Cl-donors for the thermochemical process3 could also become an option for large-scale industrial applications. The evaporation of heavy metals with gaseous HCl at high temperature is already mentioned in literature for other applications.7,8 Gaseous Cl-donors have the advantage that they do not have to be mixed with the SSA before thermochemical treatment. Utilization of magnesium chloride or calcium chloride that contain crystal water (e.g., MgCl2 3 6H2O, CaCl2 3 2H2O) requires cooling down the SSA before mixing it with the Cl-donor and evaporating the water in the following thermochemical process step. The energy demand for evaporating the water bound to the solid Cl-donor and the impossibility to treat hot ashes (800 °C) coming directly from the incineration facility (separated by high temperature cyclones) are the main disadvantages of the Received: March 4, 2011 Accepted: August 5, 2011 Revised: July 8, 2011 Published: August 05, 2011 7445
dx.doi.org/10.1021/es2007319 | Environ. Sci. Technol. 2011, 45, 7445–7450
Environmental Science & Technology
ARTICLE
Table 1. P-Solubilities Pcit, Pnac (Measured) and PlantAvailabilities10 for Some Pure Phosphates Present in SSA and Thermochemically Treated SSA
AlPO4
Pcit
Pnac
0%
3%
plant availability
Table 2. Mass Fractions and Standard Deviations (SD; n = 5) of Main Elements and Trace Elements Determined for SSA 1 to SSA 3 by ICP-OES after Total Digestion with HNO3/HCl/ HF in a Microwave (210°C) SSA 1
Ca5(PO4)3Cl
55%
11%
Ca3(PO4)2
83%
21%
0
Mg3(PO4)2
96%
24%
+
solid Cl-donors (containing crystal water) that could be overcome by utilization of gaseous Cl-donors. Therefore, thermochemical investigations were carried out with focus on the performance of HCl-gas as Cl-donor in comparison to the well investigated solid Cl-donors magnesium chloride and calcium chloride. The thermochemical experiments focused on heavy metal removal and the plant-availability of phosphorus contained in the treated ashes. The plant-availability was estimated by extraction tests which are established in the German Fertilizer Ordinance:9 the P-solubility in 2% citric acid (Pcit) and neutral ammonium citrate (Pnac). The P-solubilities (measured) of pure mineral phosphate phases that were detected in SSA and in thermochemically treated SSA are displayed in Table 1 together with specifications on plant-availability for these substances taken from literature.10 For both extraction tests magnesium phosphate (Mg3(PO4)2) shows the best P-solubility followed by whitlockite (Ca3(PO4)2), chlorapatite (Ca5(PO4)3Cl) and aluminum phosphate (AlPO4). The results are in agreement with the plant-availability data for these phosphates. However, Kuderna11 found that the Pcit test showed worse correlation with the results of pot experiments carried out with thermochemically treated SSA whereas the Pnac test showed very good correlation in the same investigation. The results of both extraction tests are presented and discussed in this article. However, due to the findings of Kuderna, the Pnac solubility is considered to be more eligible to estimate the plant availability of different SSAs before and after thermochemical treatment.
’ MATERIAL AND METHODS The thermochemical experiments were carried out with three different sewage sludge ashes (SSA 1 3) from large-scale industrial incineration facilities. SSA 1 stemmed from an incineration plant in The Netherlands and originated from WWTPs primarily using Fe-salts for phosphate precipitation (Fe-rich SSA). SSA 2 originated from a German incineration plant where P-precipitation in the WWTP was done with aluminum salts (Al-rich SSA). SSA 3 also stemmed from a German incineration plant. In contrast to SSA 1 and SSA 2 that were produced in a common fluidized bed incinerator SSA 3 stemmed from a grate firing system normally used for incineration of wastes from the paper mill. SSA 1 and SSA 2 are fine materials with a median particle size of approximately 50 μm whereas SSA 3 from the grate firing system is coarser with a medium particle size of approximately 400 μm. The mass fractions of main and trace elements present in the SSAs are listed in Table 2. Hydrochloric acid gas (HCl 2.8 gas, Linde, Leuna, Germany), calcium chloride (CaCl2 3 2 H2O; extra pure, Merck, Darmstadt, Germany) and magnesium chloride (MgCl2 3 6 H2O; cryst., Merck, Darmstadt, Germany) were used as chlorine donors for the thermochemical experiments. Furthermore, magnesium carbonate (MgCO 3 ; basic heavy, Fluka, Germany) was used in combination with HCl
mass fraction
SSA 2 SD
mass fraction
SSA 3 SD
mg/kg
mg/kg
mg/kg
mg/kg
Al
55 920
359
105 300
Ca
134 200
190
85 950
Fe
100 100
1177
21 660
mass fraction
SD
mg/kg
mg/kg
1702
72 198
573
487
103 315
717
134
60 266
150
K
10 580
72
7193
53
1189
471
Mg
15 020
146
8355
159
20 160
196
Mn
1042
Na
5586
30
6238
18
6364
45
90 040 12 970
68 92
78 440 3949
171 183
74 845 6967
423 309
P S As Cd
4.3
34.1
0.2
6.53
0.5
Cr
145
1.0
Cu
1113
3.0
Hg
