Size Distribution of Metals in Bottom Ash of Municipal Solid Waste Incinerators

Previous researches have examined the emission and management of heavy metals from municipal solid waste incinerations for compliances with regulations, and also, to address the concerns on various environmental issues. However, the size distribution of heavy metals in bottom ash has not been addressed to such extent yet. This study investigated the size distribution of bottom ash and also the heavy-metal weight fractions in different particle sizes. The samples were digested in a microwave digester and then analyzed using an inductive coupled argon plasma/mass selective detector. This characterization may be used to assess the reutilization of bottom ash and the associated environmental hazards in order to achieve the goal of zero waste. Experimental results showed that about 20-40% (w/w) of heavy metals were found in fine particles (< 0.21 mm), about 30-40% of heavy metals showed up in medium particles (0.21 to 2.36 mm) and the remaining 3050% of heavy metals were located in coarse particles (larger than 2.36 mm). Specifically, a greater fraction of mercury and cadmium existed in smaller particles, while a greater fraction of lead, calcium, manganese and iron existed in larger particles. The results showed that sieving the bottom ash before considering reutilization is necessary because the government policy for incineration residues advocates their reuse as road sub-bases or secondary building materials in Taiwan. From the view point of heavy metals, the suggested cutoff size for reutilization of the bottom ash is 0.6-2.36 mm. Ash with metal content exceeding the legal limit should be controlled properly before being disposal in a landfill. *Corresponding author Email: [email protected] INTRODUCTION Waste-to-energy incineration was considered as a mainstream strategy for municipal solid waste (MSW) management due to the lack of landfill spaces and the associated risks to air, water and soil. Approximately 85% of the waste volume and 60-75% of the mass of MSW could be reduced by MSW Incinerators (MSWI) [1,2]. In addition, MSWI could be used to generate electricity and/or heat from the energy content of MSW. Nevertheless, there are still negative impacts on the environment from MSWIs. The emission and management of heavy metals from MSWI have been studied extensively because of the regulations and also issues with the environment [216]. For example, the particle size distributions of Zn, Pb, and Cu were in bimodal forms and mass concentration in each fraction of the particle size was in descending order of Zn, Pb, Cu in the stack of two largescale MSWI [8]. Yoo et al. [12] concluded that more volatile metals such as Cd, Pb and Zn showed higher enrichment in the particular matter (PM) emitted through stack instead of bottom ashes. Cu, Pb and Zn associated with PM smaller than 2.5 mm accounted 106 J. Environ. Eng. Manage., 18(2), 105-113 (2008) for approximately 90% of the total mass of each metal in PM10. Lithophilic metals such as Fe, Cu, Cr, and Al remained mainly in the bottom ash while Cd volatilized from the furnace and condensed and present in fly ash. Despite the high volatility of Pb and Zn, Jung et al. [14] observed that about two thirds of the species were in the bottom ash. The bottom and fly ashes, both of which can be significant, must be carefully managed. There are 19 MSWIs in Taiwan producing approximately 0.86 Mt of bottom ash annually, most of which is land-filled. Bottom ash is a highly heterogeneous burnt-out mixture of slag, ferrous and non-ferrous metal, ceramics, glass, other non-combustibles and residual organic matter. The major elements in bottom ash are O, Si, Fe, Ca, Al, Na, K and C [1]. Moreover, many metals in the bottom ash are in the form of metallic oxides. Bottom ash thus presents a similar composition of natural geological materials [1,5,13,17]. Bottom ash is used as a secondary building material or other similar purposes, such as road sub-bases and the construction of embankments, wind and noise barriers or other civil engineering applications [1,2]. However, the high level of heavy metal leaching is an obstacle for reusing the bottom ash. Despite the significant volume reduction from incineration, waste recycling is important to ensuring the future welfare of mankind. Possibilities other than landfill have been used in some European countries such as land-filling material in the quarries regeneration or as road sub-base [13,18-20]. In the United States, demonstrative experimental projects, including asphalt pavement, construction blocks, artificial reefs, shoreline erosion control and similar applications, have been carried out [13]. However, bottom ash contains varying amount of heavy metals and soluble salts that, potentially, could have adverse effects on the environment if it was improperly managed. Although the heavy metals in bottom ash were studied extensively for ensuring the compliances with regulations, heavy metals present in different particular sizes in bottom ash are seldom addressed in the literature. Heavy metals that appear in fine particles can get into our lungs much easier than coarse particles. Therefore, the examination of size distribution for the heavy metals in the bottom ash is essential. In this study, bottom ash was sampled from four MSWIs in Taiwan and sieved to size classes of < 0.074 mm, 0.075-0.125 mm, 0.125-0.21 mm, 0.21-0.3 mm, 0.3-0.6 mm, 0.6-1.0 mm, 1.0-2.36 mm, 2.36-4.75 mm and 4.75-9.5 mm. The size distribution of the bottom ash and the heavymetal weight fractions of different particle sizes were then determined. This characterization may be used to assess the environmental hazards associated with the reutilization of bottom ash while achieving the goal of zero waste. EXPERIMENTAL METHODS 1. Four Selected Municipal Solid Waste Incineration The bottom ash was collected from four MSWIs (A-D) in Taiwan. The operation of MSWI A began in 2000 and its capacity is 1200 t d. Electric power produced by the plant is 14,700 MWh. The operation of MSWI B was commenced in 1992 with capacity of 900 t d. It is generating 3460 MWh of electric power. MSWI C was inaugurated in 1995 and its capacity is 1350 t d with 14,300 MWh of electric power output. MSWI D, opened in 2000, has 900 t d capacity with 13800 MWh of electric power produced. There are three incinerators in MSWI A, B and C, while MSWI D operates two incinerators. Each incinerator has its own heat recovery systems (350 °C), selective noncatalytic reduction, dry scrubber (250-230 °C), activated carbon injection, fabric filter (180-160 °C) and stack. 2. Sampling and Sieving To obtain good representative samples, bottom ash was sampled four times each day at intervals of 2 h for 1 wk to collect 15 kg of the bottom ash from each of the four MSWIs. The ferrous and non-ferrous metals, glass and stone were removed and after wellmixing and diagonal sectioning, 5 kg of bottom ash was retained. The samples were then spread out on a clean aluminum foil and naturally dried before they were sieved into different sizes. Sieving was performed by mechanical shaking with stainless steel mesh screens to particle size classes of ASTM No. 2, 4, 8, 18, 30, 50, 70, 120 and 200, i.e. 9.5, 4.75, 2.36, 1.0, 0.6, 0.3, 0.21, 0.125 and 0.075 mm, respectively. For the determination of the heavy metals in the samples, 20 g was taken from each of the different size ranges of < 0.21, 0.21-0.6, 0.6-2.36, 2.36-4.75 and 4.75-9.5 mm after homogenization. There are only 5 size ranges for metal size distribution due to low heavy-metal contents in bottom ash. These five ranges can be classified as fine (< 0.21 mm), medium (0.212.36 mm) and coarse particles (> 2.36 mm). 3. Analyses of Heavy Metals The samples were digested in a microwave digester and then analyzed using an inductive coupled argon plasma/mass selective detector (ICP/MS) (Agilent 7500, Japan). The digestion procedure included two steps: each sample was digested with 10 mL of HNO3 in the primary stage and then with 2 mL of HF in the secondary stage. After cooling the samples to room temperature, the solution was transferred to a 25 mL calibrated flask and then diluted with distilled water. The ICP/MS were calibrated using a diluted standard solution purchased from Merk Co. The standard reference material was digested with the Chen et al.: Metal Distribution in Bottom Ash 107 standard experimental procedure to ensure the accuracy and reliability of the analytical results. Many metals were analyzed; however, some of them are not significant. Therefore, only Hg, Cd, Pb, Ca, Mn and Fe are shown in this study. Hg would cause damage to kidney, liver and brain. Cd would cause hypertension. Pb would cause damage to kidney, brain and the central nervous system. Mn would cause damage to skin and the respiratory system. Fe would cause pneumoconiosis. Recovery efficiencies of the above heavy metals were determined by processing a solution with a known heavy metal concentration through the aforementioned experimental procedure. This study showed that the recovery efficiency varied from 93103%, while control levels ranged 80-120%. The blank tests for heavy metals were carried out by the same procedure. Analysis of duplicate experiments could be calculated in terms of relative percentage difference which ranged from 5-13% to assure the precision of analysis. Low loss was observed for heavy metals as well as from external contaminations. RESULTS AND DISCUSSION 1. Particle Size Distributions of Bottom Ash In this study, the bottom ash samples were obtained from four MSWIs in Taiwan and sieved to size classes of < 0.075, 0.075-0.125, 0.125-0.21, 0.21-0.3, 0.3-0.6, 0.6-1.0, 1.0-2.36, 2.36-4.75 and 4.75-9.5 mm. Figure 1 shows the size distributions of particle mass in the bottom ash of four MSWIs (A to D). For MSWI A, the highest peak was located in the range of 1.02.36 mm, for which the weight percentage for the range was 33%. The highest peaks of MSWI B, C and D were all located in the range of 2.36-4.75 mm, where the weight percentages were 32, 39 and 28%, respectively. The particle size distributions in terms of cumulative mass fractions (F%) were used to compare the contributions of fine and coarse particles. The F% for the bottom ash was calculated from the data. The F% for particle sizes below 0.21 and 2.36 mm were about 7 and 66% (w/w) for MSWI A, 9 and 51% for MSWI B, 8 and 47% for MSWI C, and, 13 and 61% for MSWI D. The averaged values were 9% for particles smaller than 0.21 mm and 56% for particles smaller than 2.63 mm. These results showed that the sampleobtained from MSWI A contained the least fine particles (7% < 0.21 mm) among all the samples; while MSWI D had the highest percentage (13%) of fine particles. For medium particles (0.21-2.36 mm), MSWI A had the highest fraction (59%) with MSWI C the least fraction (39%). In addition, the maximumpercentage of coarse particles (> 2.36 mm) was observed in MSWI C (53%) with the minimum percentage of coarse particles (34%) from the sample ob tained from MSWI A. The observations were consisFig. 1. Size distributions of particle mass in bottom ash. tent with those obtained by Forteza et al. [13], although their particle size of bottom ash is between 0.075-25 mm. The mass median diameter (MMD) can be determined from the particle diameter at 50% of the cumulative fractions, d50, of the particles. The geometric standard deviation (σg) measures the dispersiveness of particle sizes within a given size range. The σg was calculated from (d84/d16) , where d84 and d16 represent the diameters at 84 and 16%, respectively, of the cumulative fraction. Table 1 shows the MMD and σg for the bottom ash from the four MSWIs. The averaged MMD for the bottom ash from the four MSWIs, A to D, was 2.1 mm, and, the related standard deviation (RSD) was 20%. The averaged σg was 3.2 with the RSD 15%. As shown in Table 1, a large fraction of the bottom ash obtained from MSWI C existed as coarse particles and as medium particles for the samples from MSWIs A, B and D. In addition, the distribution of particle size for MSWI D was more dispersive than the samples from the other three MSWIs. On the other hand, particle sizes are more concentrative for the samples from MSWI A and C. Each incinerator has its own heat recovery systems, selective noncatalytic reduction, dry scrubber, activated carbon injection and fabric filter for those four MSWIs. In the combustion process, no metal was formed or destroyed. In other words, total mass input of metal from waste equals to total mass output of metal from bottom/fly ash and stack. Thus, main factors affecting the size distributions for the 4 MSWIs are waste properties and the operating conditions. Therefore, metal recycle policy should be executed in a systematic approach. A B