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Development of lightweight aggregate from dry sewage sludge and coal ash
Waste Management 29 (2009) 1330–1335
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Development of lightweight aggregate from dry sewage sludge and coal ash Xingrun Wang a,b, Yiying Jin b,*, Zhiyu Wang b, Yongfeng Nie b, Qifei Huang a, Qi Wang a a b
Chinese Research Academy of Environmental Science, Beijing 100012, China Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Accepted 22 September 2008 Available online 12 November 2008
a b s t r a c t In this study, dry sewage sludge (DSS) as the principal material was blended with coal ash (CA) to produce lightweight aggregate. The effects of different raw material compositions and sintering temperatures on the aggregate properties were then evaluated. In addition, an environmental assessment of the lightweight aggregate generated was conducted by analyzing the fixed rate of heavy metals in the aggregate, as well as their leaching behavior. The results indicated that using DSS enhanced the pyrolysis–volatilization reaction due to its high organic matter content, and decreased the bulk density and sintering temperature. However, the sintered products of un-amended DSS were porous and loose due to the formation of large pores during sintering. Adding CA improved the sintering temperature while effectively decreasing the pore size and increasing the compressive strength of the product. Furthermore, the sintering temperature and the proportion of CA were found to be the primary factors affecting the properties of the sintered products, and the addition of 18–25% of CA coupled with sintering at 1100 °C for 30 min produced the highest quality lightweight aggregates. In addition, heavy metals were fixed inside products generated under these conditions and the As, Pb, Cd, Cr, Ni, Cu, and Zn concentrations of the leachate were found to be within the limits of China’s regulatory requirements. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Due to the rapid growth of wastewater output and the reinforcement of the regulations on its treatment in China, sewage sludge, as an inevitable by-product of the treatment process of wastewater, is increasing very fast. In 2005, the daily output of dry sewage sludge in China was 8000 tonnes, and this value has been increasing at an average rate of 10% per year. To date, sewage sludge has primarily been disposed of by two methods, landfilling and soil application. However, landfilling may no longer be appropriate due to the scarcity of land and increasingly stringent environmental controls (Merino et al., 2007). In addition, the presence of heavy metals in the sewage sludge makes soil application difficult due to the potential for contamination of the aquatic and terrestrial environments (Kim et al., 2005). Therefore, it is imperative to investigate new environmentally sustainable applications for this type of waste. Ceramic is a kind of lightweight aggregate that can be used to produce concrete mixtures. It is currently common to use clay to produce ceramic (Jordán et al., 1999). To get fine lightweight aggregate, the chemical composition of the raw material used * Corresponding author. Tel.: +86 10 62794697; fax: +86 10 62789748. E-mail address: [email protected] (Y. Jin). 0956-053X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2008.09.006
should be similar to clay and satisfy the following requirements: SiO2 48–70%; Al2O3 8–25%; Fe2O3 + FeO 3–12%; CaO + MgO 1– 12%; K2O + Na2O 0.5–7% (Riley, 1950). The characteristics of sewage sludge are similar to that of clay, and the total lightweight aggregate output in China in 2004 is about 420 million m3. Therefore, the use of sewage sludge as a possible substitute for clay has the potential to provide plenty of raw materials while reducing the consumption of clay. At the same time, the environmental impact caused by sewage sludge can also be reduced. Previous studies conducted to evaluate the production of lightweight aggregate from sewage sludge have focused on the use of dewatered sewage sludge (Elkins et al., 1985; Tay et al., 1991; Tay and Show, 1997) or sewage sludge incineration ash (Wiebusch and Seyfried, 1997; Wainwright and Cresswell, 2001; Endo et al., 1997). However, the results of these studies showed that, when un-amended dewatered sewage sludge is used, porous and loose aggregates are produced due to the high organic matter and water content (e.g., approximately 10% and 80%, respectively). Indeed, to obtain a fine lightweight aggregate from sewage sludge, the proportion of dewatered sewage sludge in the raw materials can be no greater than 30% (Yan, 2005); therefore, the use of dewatered sewage sludge is not a feasible method for the disposal of the total output of sewage sludge in China. Conversely, when only sewage sludge ash is used, two heat treatment technologies, combustion
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and sintering, are required. The need for both of these technologies increases the cost of construction and operation, and results in more energy being consumed during production due to the lower level of organic matter in the starting materials (Chiou et al., 2006). Therefore, this study was conducted to evaluate the feasibility of using DSS as the primary material for the production of lightweight aggregates. Because sewage sludge contains a lower content of SiO2 and Al2O3 than is required to produce a lightweight aggregate, CA was introduced as an additive. Specifically, we evaluated the effects of adding 10–32% CA to DSS and then sintering the mixtures at 1050–1100 °C to determine if ceramic could be produced. In addition, we evaluated the fixed rate and leaching behavior of the heavy metals in the generated products to determine if they were environmentally safe.
The mechanical quality of the sintered products was estimated by testing the compressive strength, the bulk density and the 1-h water absorption rate. For the compressive strength test, a single sintered product was pressed down by a steel puncheon until it was crushed. The compressive strength was then calculated by measuring the ratio between the load and the surface area of the sintered product, in stress units. The values of 1-h water absorption (h) and bulk density (q) were obtained using the following formulas:
h ¼ 100
m1 m0 m0
ð1Þ
where m1 is the 1-h saturated surface-dry weight of the ceramic bodies (g) and m0 is the dry weight of the ceramic bodies (g)
q¼ 2. Materials and methods
m V
ð2Þ
where m is the weight of the ceramic bodies (g) and V is the volume of the ceramic bodies (cm3). After being crushed, the sintered products were evaluated by scanning electron microscopy using a JSM-6460LV scanning electron microscope. In addition, the concentration of a variety of heavy metals in the products before and after sintering, including As, Cd, Cr, Cu, Ni, Pb and Zn, was determined according to an ASTM method after the products were digested (ASTM, 2000; Wan et al., 2006). The heavy metals were measured by inductively coupled plasma mass spectrometry (ICP-MS, Perkin–Elmer Elan 6000, USA). The fixed rate of the heavy metals was defined as the ratio of the weight of heavy metals in the products prior to sintering to that of the products after sintering. The standard method for determining the leaching toxicity of solid wastes by horizontal vibration extraction procedure (HVEP) (GB5086.2, 1997) was used to evaluate the leaching of heavy metals from the product.
2.1. Sintering ceramic product The sewage sludge used in this study was obtained from a municipal wastewater treatment plant in Beijing. The sludge was dewatered mechanically via a belt press, dried in a stove at a temperature of 105 °C for 24 h, after which it was pulverized with a ball mill until it could pass through a 45-mm sieve. CA was collected from the Beijing Power Plant and dried in a stove at 105 °C for 24 h and then pulverized with a ball mill until it could pass through a 45-mm sieve. Five different weight percentages of CA to DSS were evaluated: 0%, 10%, 18%, 25% and 32% (SA, SB, SC, SD and SE, respectively). The materials were finely mixed using a roll-shaker for 24 h and then shaped into cylindrical specimens with a diameter of 20.4 mm and a height of 14 mm by uniaxial pressing at a pressure of 8 MPa. The cylinders were then sintered in an electric kiln at temperatures ranging from 1050 to 1100 °C for 30 min. A ramp rate of 9 °C/min was used during the sintering process. In addition, the temperature was held at 420 °C for 20 min during sintering to allow pyrolysis and volatilization of the organic matter to ensure that the bulk density of the sintered products was low.
3. Results and discussion 3.1. Properties of the materials The results of the fuel analysis of DSS and CA are shown in Table 1. For DSS, the heating value was nearly equal to that of peat, which is widely used as a support fuel in power plants. Therefore, the use of DSS to produce ceramic products can reduce the consumption of fuel. In addition, the total organic matter content was found to be very high, which indicates that the sintered products should be lightweight due to the release of organic matters during sintering. However, the sulfur content was also found to be relatively high, indicating that a sulfur removal procedure would be required to remediate the flue gas.
2.2. Characterization DSS and CA were analyzed to determine the fractions of ash and total organic matters, as well as the contents of carbon, hydrogen, nitrogen and sulfur, and the calorific heating values. All analyses were conducted using standardized methods for fuel analyses. Following combustion at 600 °C for 3 h, the chemical compositions of both the DSS and CA, including the P, Si, Ca, Mg, Fe, Al, K, Na and S contents, were determined using an X-ray fluorescence spectrometer (XRF, Shimadzu Lab Center XRF-1700, Japan).
Table 1 Fuel analyses of DSS and CA.
DSS CA
Ash (w-%(dry))
Total organic matter (w-%(dry))
Carbon (w-%(dry))
Hydrogen (w-%(dry))
Nitrogen (w-%(dry))
Sulfur (w-%(dry))
Calorific heating value (kcal/kg(dry))
27.15 92.72
65.36 8.28
28.06 6.65
4.20 0.53
3.64 0.19
1.72 0.81
3609.21 12.52
Table 2 Chemical composition of DSS and CA. Composition
P2O5
SiO2
Al2O3
CaO
MgO
Fe2O3
K2O
Na2O
TiO2
SO3
MnO
DSS ash (w-%) CA (w-%)
32.98 0.5
24.37 49.55
7.95 37.4
10.69 3.48
10.68 0.98
6.09 4.87
5.33 1.02
0.34 0.12
0.73 1.67
0.22 0.24
0.11 0.04
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Table 2 shows the chemical composition of DSS and CA determined by XRF after combustion at 600 °C for 3 h. The DSS ash was found to contain large amounts of P2O5 and SiO2 (32.98% and 24.37%, respectively). In addition, the DSS ash contained CaO (10.69%), MgO (10.68%), Al2O3 (7.95%), Fe2O3 (6.09%) and K2O (5.33%). The contents of all other oxides were less than 1%. The typical composition of CA was found to be SiO2 and Al2O3, which were present in concentrations of 49.55% and 37.4%, respectively. The melting points of CaO, MgO, Fe2O3 and K2O are all very low when compared with those of SiO2 and Al2O3; therefore, they are usually used as fusing agents in the production of lightweight aggregates. Conversely, SiO2 and Al2O3 are usually used to improve the strength of sintered products. Taken together, these results indicate that DSS has a lower melting temperature than CA. 3.2. Properties of the sintered products 3.2.1. SEM observation of the sintered products Fig. 1 shows the microstructure of the SA sintered products following sintering at 1050 and 1080 °C, respectively, as determined by scanning electron microscope (SEM). Fig. 1a and b clearly shows that the surface of the sintered SA appears to become sintered bonding with many large pores caused by the volatilization of organic matter in the DSS during the preheating stage. Although the generation of large pores will cause the sintered product to have a lightweight property, large pores will also decrease the compressive strength of the sintered products. When the sintering temperature was increased to 1080 °C, more small pores gathered with the appearance of prominent apertures (Fig. 1c and d). Fig. 1 also revealed that the melting temperature of DSS is approximately
1050 °C, which is very low compared with that of clay. This low melting temperature is due to the high content of CaO, MgO, Fe2O3 and K2O in DSS. Therefore, the use of DSS to produce lightweight aggregate requires a lower sintering temperature than clay, which would reduce fuel consumption during production. However, due to the high content of organic matters in SA, many large pores form in the sintered products. Therefore, SA must be amended with other materials. As seen in Fig. 2a and b, the sintered product of SB also contained many large pores. However, as CA was added, the pores became smaller and more homogeneous, as shown in Fig. 2c–f. These findings indicate that increasing the amount of CA can effectively decrease the pore size of the sintered products due to the lower organic matter content, as was observed in SC and SD. This reduction in pore size consequently enhances the compressive strength. However, CA contains a high concentration of SiO2 and Al2O3 (Table 2); therefore, the melting temperature is improved with the addition of CA. When starting materials that contain greater than 25% CA, such as SE, are used, they cannot be properly sintered at this temperature. This was demonstrated by the presence of a powdery film on the surface of the sintered SE. 3.2.2. Water absorption of the sintered products The quality of the sintered products has a fundamental relationship with the absorption of water, which is also a function of the compressive strength and bulk density of the sintered products (Wang et al., 1998). As shown in Fig. 3a, when the sintering temperature was 1050 °C, the water absorption of the sintered SB decreased to 3.65%, whereas the water absorptions of the sintered SC, SD and SE ranged from 23.26% to 63.25%. Furthermore, when the
Fig. 1. SEM micrographs of SA sintered products at 1050 and 1080 °C.
X. Wang et al. / Waste Management 29 (2009) 1330–1335
Fig. 2. SEM micrographs of SB, SC, SD and SE sintered products at 1100 °C.
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Fig. 4. Appearance of SC, SD and SE sintered products.
sinter SC and SD. When the sintering temperature was 1080 °C, the bulk density of the SC and SD sintered products increased significantly to 1.48–1.54 g/cm3. In addition, sintered bonding appeared on the surface of the sintered products (Fig. 4a and b), and their volume decreased due to the presence of the liquid phase, facilitating densification. At higher temperatures, only a slight change in bulk density was observed (1.46–1.51 g/cm3). Finally, the sintered SE products had a particulate nature (Fig. 4c) and a bulk density that was less than 1.3 g/cm3 when the sintering temperature was between 1050 °C and 1100 °C, indicating that SE requires a higher temperature to be sintered.
Fig. 3. Changes in aggregate properties of SB, SC, SD and SE sintered products.
sintering temperature was 1080, the water absorption of the sintered SC was less than 8%. Finally, when the sintering temperature was 1100 °C, the water absorption of the sintered SD decreased to 6.21%, while the water absorption of the SE sintered product was only reduced to 27.36%. Taken together, these findings indicate that the sewage sludge has a low sintering temperature, and, therefore, the sintered product has a low water absorption rate. Increasing the proportion of CA improves the sintering temperature, and increases the water absorption rate. This was demonstrated by SC and SD being completely sintered after being heated to 1100 °C for 30 min. 3.2.3. Bulk density of the sintered products Fig. 3b shows the relationship between sintering temperature and bulk density. For SB, sintering was possible at 1050 °C. This resulted in the formation of a glass phase on the surface, which prevented the density from changing when the sintering temperature was increased to 1100 °C. However, when SC and SD were subjected to sintering at 1050 °C, the surface of the sintered products had a particulate nature and the bulk density was very low, ranging from 1.14 to 1.28 g/cm3. The bulk density and particulate nature of these products indicate that 1050 °C is not sufficient to
3.2.4. Compressive strength of the sintered products The compressive strengths of the SB, SC, SD and SE sintered products are shown in Fig. 3c. For the SB sintered products, the compressive strengths at 1050 °C, 1080 °C and 1100 °C were only 6.95, 8.82 and 9.96 MPa, respectively. This is due to the high volume of large pores in the sintered products, as shown in Fig. 2a and b. Additionally, when the SC was sintered at 1050 °C, the compressive strength of its sintered product was only 10.68 MPa. However, when SC was sintered at 1100 °C, its compressive strength was increased significantly to 19.3 MPa. This is likely because densification and sintering occurred at 1100 °C, and the pores in the sintered products of SC are smaller and more homogeneous, as shown in Fig. 2c and d. For the SD sintered products, the compressive strength of the products that were sintered at 1050 °C and 1080 °C were only 3.45 and 9.03 MPa, respectively. This may have been because these temperatures were lower than the sintering temperature of the SD, which resulted in its particulate nature. Conversely, when the SD was sintered at 1100 °C, its compressive strength was 23.69 MPa, indicating the occurrence of densification and the appearance of a glass phase. Finally, when the SE sintered products were sintered at 1050 °C, 1080 °C, and 1100 °C, the compressive strengths were all less than 15 MPa. This demonstrates that the sintering temperature of the SE is greater than 1100 °C, as indicated by the powdery film on the surface of the sintered products (Fig. 2g and h). 3.3. Properties of heavy metals 3.3.1. Fixed rate of heavy metals Fig. 5 shows the fixed rates of heavy metals after sintering products that contained 25% CA at 1100 °C for 30 min. Three par-
Fig. 5. Fixed rate and vapored rate of heavy metals for sintered products.
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X. Wang et al. / Waste Management 29 (2009) 1330–1335 Table 3 HVEP-results of heavy metals in sintered products before and after sintering.
Leaching concentration (mg/L) Leaching rate (%) Identification standard for hazardous wastes (mg/L) (GB5085.3, 1996) Environmental quality standards for surface water III (mg/L) (GB3838, 2002)
allel samples were researched. With the exception of Pb, only a small amount of the heavy metals were volatilized during the sintering process, and the fixed rates were all greater than 60%. A greater amount of Pb was volatilized due to its low boiling point, and its fixed rate was 37.6%. At the same time, Pb is the primary heavy metal present in sewage sludge in China; therefore, additional steps must be taken to control Pb volatilization during sintering. 3.3.2. Leaching characteristics of heavy metals The HVEP results of As, Cd, Cr, Cu, Ni, Pb and Zn analysis of the product produced by sintering the SD at 1100 °C for 30 min are shown in Table 3. The leaching rates of the heavy metals were all lower than 1% and the leaching concentrations were all in compliance with the China Identification Standard for Hazardous Wastes and the China Environmental Quality Standards for Surface Water. Therefore, these results indicate that sintering generally reduces the leaching concentration for all of the heavy metals evaluated here, and that the use of lightweight aggregate produced from DSS does not pose an environmental risk.
4. Conclusions Based on the results of this study, the following conclusions can be drawn: 1. The SiO2 and Al2O3 contents in DSS are lower than the requirements for lightweight aggregates. Besides, the sintered products produced using pure DSS are porous and loose, and have a low compressive strength due to high organic matter content. Therefore, it is necessary to amend DSS with a material that has a high SiO2 and Al2O3 content, such as CA, in order for it to be sintered. 2. DSS has a low sintering temperature of 1050 °C due to its high CaO, MgO, Fe2O3 and K2O content. However, the addition of CA improves the sintering temperature, while effectively decreasing the pore size of the sintered products and increasing the compressive strength. 3. When DSS and CA are used to sinter lightweight aggregates, the factors affecting the properties of the sintered products were the sintering temperature, the retention period and the raw material mixture rate, in that order. 4. A mixture containing 18–25% of CA that was preheated at 420 °C for 20 min and then sintered at 1100 °C for 30 min produced high quality lightweight aggregates. The leaching con-
As
Cd
Cr
Cu
Ni
Pb
Zn
0.001 0.13 61.5 60.05
0.001 0.41 60.3 60.005
0.022 0.18 610 60.05
0.058 0.20 650 61
0.009 0.13 610 60.02
0.056 0.56 63 60.05
0.279 0.33 650 61
centrations of the sintered products were all in compliance with the China Identification Standard for hazardous wastes and China Environmental Quality Standards for surface water.
Acknowledgments This work was supported by the national Key Technologies R and D Program of China (No. 2006BAC02A19), and we received considerable cooperation from Tsinghua University. All the support is greatly appreciated. References ASTM Standard Method D 6357-00a, 2000. Standard Test Methods for Determination of Trace Elements in Coal, Coke, and Combustion Residues from Coal Utilization Processes by Inductively Coupled Plasma Atomic Emission Spectrometry, Inductively Coupled Plasma Mass Spectrometry, and Graphite Furnace Atomic Absorption Spectrometry. Chiou, I.J., Wang, K.S., Chen, C.H., Lin, Y.T., 2006. Lightweight aggregate made from sewage sludge and incinerated ash. Waste Manage. 26, 1453–1461. Elkins, B.V., Wilson, G.E., Gersberg, R.M., 1985. Complete reclamation of wastewater and sludge. Water Sci. Technol. 17, 1453–1454. Endo, H., Nagayoshi, Y., Suzuki, K., 1997. Production of glass ceramics from sewage sludge. Water Sci. Technol. 36, 235–241. GB3838, 2002. Environmental Quality Standards for Surface Water. GB5085.3, 1996. Identification Standard for Hazardous Wastes – Identification for Extraction Procedure Toxicity. GB5086.2, 1997. Test Method Standard for Leaching Toxicity of Solid Wastes – Horizontal Vibration Extraction Procedure. Jordán, M.M., Boix, A., Sanfeliu, T., de la Fuente, C., 1999. Firing transformations of cretaceous clays used in the manufacturing of ceramic tiles. Appl. Clay Sci. 14, 225–234. Kim, E.H., Cho, C.K., Yim, S., 2005. Digested sewage sludge solidification by converter slag for landfill cover. Chemosphere 59, 387–395. Merino, I., Arévalo, L.F., Romero, F., 2007. Preparation and characterization of ceramic products by thermal treatment of sewage sludge ashes mixed with different additives. Waste Manage. 27, 1829–1844. Riley, C.M., 1950. Relation of chemical process to the bloating clay. J. Am. Ceram. Soc. 34 (4), 121–128. Tay, J.H., Show, K.Y., 1997. Resource recovery of sludge as a building and construction material – a future trend in sludge management. Water Sci. Technol. 36 (11), 256–266. Tay, J.H., Yip, W.K., Show, K.Y., 1991. Clay-blended sludge as lightweight aggregate concrete material. J. Environ. Eng. 117 (6), 834–844. Wainwright, P.J., Cresswell, D.J.F., 2001. Synthetic aggregate from combustion ashes using an innovative rotary kiln. Waste Manage. 21, 241–246. Wan, X., Wang, W., Ye, T.M., Guo, Y.W., Gao, X.B., 2006. A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure. J. Hazard. Mater. B134, 197–201. Wang, K.S., Chiang, K.Y., Perng, J.K., 1998. The characteristics study on sintering of municipal solid waste incinerator ashes. J. Hazard. Mater. 59, 201–210. Wiebusch, B., Seyfried, C.F., 1997. Utilization of sewage sludge ash in the brick and tile industry. Water Sci. Technol. 36 (11), 251–258. Yan, H.D., 2005. Study on the super-lighting ceramsite calcined by the modified life sludge. Environ. Pollut. Control 27, 63–67.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Development of lightweight aggregate from dry sewage sludge and coal ash Xingrun Wang a,b, Yiying Jin b,*, Zhiyu Wang b, Yongfeng Nie b, Qifei Huang a, Qi Wang a a b
Chinese Research Academy of Environmental Science, Beijing 100012, China Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Accepted 22 September 2008 Available online 12 November 2008
a b s t r a c t In this study, dry sewage sludge (DSS) as the principal material was blended with coal ash (CA) to produce lightweight aggregate. The effects of different raw material compositions and sintering temperatures on the aggregate properties were then evaluated. In addition, an environmental assessment of the lightweight aggregate generated was conducted by analyzing the fixed rate of heavy metals in the aggregate, as well as their leaching behavior. The results indicated that using DSS enhanced the pyrolysis–volatilization reaction due to its high organic matter content, and decreased the bulk density and sintering temperature. However, the sintered products of un-amended DSS were porous and loose due to the formation of large pores during sintering. Adding CA improved the sintering temperature while effectively decreasing the pore size and increasing the compressive strength of the product. Furthermore, the sintering temperature and the proportion of CA were found to be the primary factors affecting the properties of the sintered products, and the addition of 18–25% of CA coupled with sintering at 1100 °C for 30 min produced the highest quality lightweight aggregates. In addition, heavy metals were fixed inside products generated under these conditions and the As, Pb, Cd, Cr, Ni, Cu, and Zn concentrations of the leachate were found to be within the limits of China’s regulatory requirements. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Due to the rapid growth of wastewater output and the reinforcement of the regulations on its treatment in China, sewage sludge, as an inevitable by-product of the treatment process of wastewater, is increasing very fast. In 2005, the daily output of dry sewage sludge in China was 8000 tonnes, and this value has been increasing at an average rate of 10% per year. To date, sewage sludge has primarily been disposed of by two methods, landfilling and soil application. However, landfilling may no longer be appropriate due to the scarcity of land and increasingly stringent environmental controls (Merino et al., 2007). In addition, the presence of heavy metals in the sewage sludge makes soil application difficult due to the potential for contamination of the aquatic and terrestrial environments (Kim et al., 2005). Therefore, it is imperative to investigate new environmentally sustainable applications for this type of waste. Ceramic is a kind of lightweight aggregate that can be used to produce concrete mixtures. It is currently common to use clay to produce ceramic (Jordán et al., 1999). To get fine lightweight aggregate, the chemical composition of the raw material used * Corresponding author. Tel.: +86 10 62794697; fax: +86 10 62789748. E-mail address: [email protected] (Y. Jin). 0956-053X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2008.09.006
should be similar to clay and satisfy the following requirements: SiO2 48–70%; Al2O3 8–25%; Fe2O3 + FeO 3–12%; CaO + MgO 1– 12%; K2O + Na2O 0.5–7% (Riley, 1950). The characteristics of sewage sludge are similar to that of clay, and the total lightweight aggregate output in China in 2004 is about 420 million m3. Therefore, the use of sewage sludge as a possible substitute for clay has the potential to provide plenty of raw materials while reducing the consumption of clay. At the same time, the environmental impact caused by sewage sludge can also be reduced. Previous studies conducted to evaluate the production of lightweight aggregate from sewage sludge have focused on the use of dewatered sewage sludge (Elkins et al., 1985; Tay et al., 1991; Tay and Show, 1997) or sewage sludge incineration ash (Wiebusch and Seyfried, 1997; Wainwright and Cresswell, 2001; Endo et al., 1997). However, the results of these studies showed that, when un-amended dewatered sewage sludge is used, porous and loose aggregates are produced due to the high organic matter and water content (e.g., approximately 10% and 80%, respectively). Indeed, to obtain a fine lightweight aggregate from sewage sludge, the proportion of dewatered sewage sludge in the raw materials can be no greater than 30% (Yan, 2005); therefore, the use of dewatered sewage sludge is not a feasible method for the disposal of the total output of sewage sludge in China. Conversely, when only sewage sludge ash is used, two heat treatment technologies, combustion
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and sintering, are required. The need for both of these technologies increases the cost of construction and operation, and results in more energy being consumed during production due to the lower level of organic matter in the starting materials (Chiou et al., 2006). Therefore, this study was conducted to evaluate the feasibility of using DSS as the primary material for the production of lightweight aggregates. Because sewage sludge contains a lower content of SiO2 and Al2O3 than is required to produce a lightweight aggregate, CA was introduced as an additive. Specifically, we evaluated the effects of adding 10–32% CA to DSS and then sintering the mixtures at 1050–1100 °C to determine if ceramic could be produced. In addition, we evaluated the fixed rate and leaching behavior of the heavy metals in the generated products to determine if they were environmentally safe.
The mechanical quality of the sintered products was estimated by testing the compressive strength, the bulk density and the 1-h water absorption rate. For the compressive strength test, a single sintered product was pressed down by a steel puncheon until it was crushed. The compressive strength was then calculated by measuring the ratio between the load and the surface area of the sintered product, in stress units. The values of 1-h water absorption (h) and bulk density (q) were obtained using the following formulas:
h ¼ 100
m1 m0 m0
ð1Þ
where m1 is the 1-h saturated surface-dry weight of the ceramic bodies (g) and m0 is the dry weight of the ceramic bodies (g)
q¼ 2. Materials and methods
m V
ð2Þ
where m is the weight of the ceramic bodies (g) and V is the volume of the ceramic bodies (cm3). After being crushed, the sintered products were evaluated by scanning electron microscopy using a JSM-6460LV scanning electron microscope. In addition, the concentration of a variety of heavy metals in the products before and after sintering, including As, Cd, Cr, Cu, Ni, Pb and Zn, was determined according to an ASTM method after the products were digested (ASTM, 2000; Wan et al., 2006). The heavy metals were measured by inductively coupled plasma mass spectrometry (ICP-MS, Perkin–Elmer Elan 6000, USA). The fixed rate of the heavy metals was defined as the ratio of the weight of heavy metals in the products prior to sintering to that of the products after sintering. The standard method for determining the leaching toxicity of solid wastes by horizontal vibration extraction procedure (HVEP) (GB5086.2, 1997) was used to evaluate the leaching of heavy metals from the product.
2.1. Sintering ceramic product The sewage sludge used in this study was obtained from a municipal wastewater treatment plant in Beijing. The sludge was dewatered mechanically via a belt press, dried in a stove at a temperature of 105 °C for 24 h, after which it was pulverized with a ball mill until it could pass through a 45-mm sieve. CA was collected from the Beijing Power Plant and dried in a stove at 105 °C for 24 h and then pulverized with a ball mill until it could pass through a 45-mm sieve. Five different weight percentages of CA to DSS were evaluated: 0%, 10%, 18%, 25% and 32% (SA, SB, SC, SD and SE, respectively). The materials were finely mixed using a roll-shaker for 24 h and then shaped into cylindrical specimens with a diameter of 20.4 mm and a height of 14 mm by uniaxial pressing at a pressure of 8 MPa. The cylinders were then sintered in an electric kiln at temperatures ranging from 1050 to 1100 °C for 30 min. A ramp rate of 9 °C/min was used during the sintering process. In addition, the temperature was held at 420 °C for 20 min during sintering to allow pyrolysis and volatilization of the organic matter to ensure that the bulk density of the sintered products was low.
3. Results and discussion 3.1. Properties of the materials The results of the fuel analysis of DSS and CA are shown in Table 1. For DSS, the heating value was nearly equal to that of peat, which is widely used as a support fuel in power plants. Therefore, the use of DSS to produce ceramic products can reduce the consumption of fuel. In addition, the total organic matter content was found to be very high, which indicates that the sintered products should be lightweight due to the release of organic matters during sintering. However, the sulfur content was also found to be relatively high, indicating that a sulfur removal procedure would be required to remediate the flue gas.
2.2. Characterization DSS and CA were analyzed to determine the fractions of ash and total organic matters, as well as the contents of carbon, hydrogen, nitrogen and sulfur, and the calorific heating values. All analyses were conducted using standardized methods for fuel analyses. Following combustion at 600 °C for 3 h, the chemical compositions of both the DSS and CA, including the P, Si, Ca, Mg, Fe, Al, K, Na and S contents, were determined using an X-ray fluorescence spectrometer (XRF, Shimadzu Lab Center XRF-1700, Japan).
Table 1 Fuel analyses of DSS and CA.
DSS CA
Ash (w-%(dry))
Total organic matter (w-%(dry))
Carbon (w-%(dry))
Hydrogen (w-%(dry))
Nitrogen (w-%(dry))
Sulfur (w-%(dry))
Calorific heating value (kcal/kg(dry))
27.15 92.72
65.36 8.28
28.06 6.65
4.20 0.53
3.64 0.19
1.72 0.81
3609.21 12.52
Table 2 Chemical composition of DSS and CA. Composition
P2O5
SiO2
Al2O3
CaO
MgO
Fe2O3
K2O
Na2O
TiO2
SO3
MnO
DSS ash (w-%) CA (w-%)
32.98 0.5
24.37 49.55
7.95 37.4
10.69 3.48
10.68 0.98
6.09 4.87
5.33 1.02
0.34 0.12
0.73 1.67
0.22 0.24
0.11 0.04
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Table 2 shows the chemical composition of DSS and CA determined by XRF after combustion at 600 °C for 3 h. The DSS ash was found to contain large amounts of P2O5 and SiO2 (32.98% and 24.37%, respectively). In addition, the DSS ash contained CaO (10.69%), MgO (10.68%), Al2O3 (7.95%), Fe2O3 (6.09%) and K2O (5.33%). The contents of all other oxides were less than 1%. The typical composition of CA was found to be SiO2 and Al2O3, which were present in concentrations of 49.55% and 37.4%, respectively. The melting points of CaO, MgO, Fe2O3 and K2O are all very low when compared with those of SiO2 and Al2O3; therefore, they are usually used as fusing agents in the production of lightweight aggregates. Conversely, SiO2 and Al2O3 are usually used to improve the strength of sintered products. Taken together, these results indicate that DSS has a lower melting temperature than CA. 3.2. Properties of the sintered products 3.2.1. SEM observation of the sintered products Fig. 1 shows the microstructure of the SA sintered products following sintering at 1050 and 1080 °C, respectively, as determined by scanning electron microscope (SEM). Fig. 1a and b clearly shows that the surface of the sintered SA appears to become sintered bonding with many large pores caused by the volatilization of organic matter in the DSS during the preheating stage. Although the generation of large pores will cause the sintered product to have a lightweight property, large pores will also decrease the compressive strength of the sintered products. When the sintering temperature was increased to 1080 °C, more small pores gathered with the appearance of prominent apertures (Fig. 1c and d). Fig. 1 also revealed that the melting temperature of DSS is approximately
1050 °C, which is very low compared with that of clay. This low melting temperature is due to the high content of CaO, MgO, Fe2O3 and K2O in DSS. Therefore, the use of DSS to produce lightweight aggregate requires a lower sintering temperature than clay, which would reduce fuel consumption during production. However, due to the high content of organic matters in SA, many large pores form in the sintered products. Therefore, SA must be amended with other materials. As seen in Fig. 2a and b, the sintered product of SB also contained many large pores. However, as CA was added, the pores became smaller and more homogeneous, as shown in Fig. 2c–f. These findings indicate that increasing the amount of CA can effectively decrease the pore size of the sintered products due to the lower organic matter content, as was observed in SC and SD. This reduction in pore size consequently enhances the compressive strength. However, CA contains a high concentration of SiO2 and Al2O3 (Table 2); therefore, the melting temperature is improved with the addition of CA. When starting materials that contain greater than 25% CA, such as SE, are used, they cannot be properly sintered at this temperature. This was demonstrated by the presence of a powdery film on the surface of the sintered SE. 3.2.2. Water absorption of the sintered products The quality of the sintered products has a fundamental relationship with the absorption of water, which is also a function of the compressive strength and bulk density of the sintered products (Wang et al., 1998). As shown in Fig. 3a, when the sintering temperature was 1050 °C, the water absorption of the sintered SB decreased to 3.65%, whereas the water absorptions of the sintered SC, SD and SE ranged from 23.26% to 63.25%. Furthermore, when the
Fig. 1. SEM micrographs of SA sintered products at 1050 and 1080 °C.
X. Wang et al. / Waste Management 29 (2009) 1330–1335
Fig. 2. SEM micrographs of SB, SC, SD and SE sintered products at 1100 °C.
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Fig. 4. Appearance of SC, SD and SE sintered products.
sinter SC and SD. When the sintering temperature was 1080 °C, the bulk density of the SC and SD sintered products increased significantly to 1.48–1.54 g/cm3. In addition, sintered bonding appeared on the surface of the sintered products (Fig. 4a and b), and their volume decreased due to the presence of the liquid phase, facilitating densification. At higher temperatures, only a slight change in bulk density was observed (1.46–1.51 g/cm3). Finally, the sintered SE products had a particulate nature (Fig. 4c) and a bulk density that was less than 1.3 g/cm3 when the sintering temperature was between 1050 °C and 1100 °C, indicating that SE requires a higher temperature to be sintered.
Fig. 3. Changes in aggregate properties of SB, SC, SD and SE sintered products.
sintering temperature was 1080, the water absorption of the sintered SC was less than 8%. Finally, when the sintering temperature was 1100 °C, the water absorption of the sintered SD decreased to 6.21%, while the water absorption of the SE sintered product was only reduced to 27.36%. Taken together, these findings indicate that the sewage sludge has a low sintering temperature, and, therefore, the sintered product has a low water absorption rate. Increasing the proportion of CA improves the sintering temperature, and increases the water absorption rate. This was demonstrated by SC and SD being completely sintered after being heated to 1100 °C for 30 min. 3.2.3. Bulk density of the sintered products Fig. 3b shows the relationship between sintering temperature and bulk density. For SB, sintering was possible at 1050 °C. This resulted in the formation of a glass phase on the surface, which prevented the density from changing when the sintering temperature was increased to 1100 °C. However, when SC and SD were subjected to sintering at 1050 °C, the surface of the sintered products had a particulate nature and the bulk density was very low, ranging from 1.14 to 1.28 g/cm3. The bulk density and particulate nature of these products indicate that 1050 °C is not sufficient to
3.2.4. Compressive strength of the sintered products The compressive strengths of the SB, SC, SD and SE sintered products are shown in Fig. 3c. For the SB sintered products, the compressive strengths at 1050 °C, 1080 °C and 1100 °C were only 6.95, 8.82 and 9.96 MPa, respectively. This is due to the high volume of large pores in the sintered products, as shown in Fig. 2a and b. Additionally, when the SC was sintered at 1050 °C, the compressive strength of its sintered product was only 10.68 MPa. However, when SC was sintered at 1100 °C, its compressive strength was increased significantly to 19.3 MPa. This is likely because densification and sintering occurred at 1100 °C, and the pores in the sintered products of SC are smaller and more homogeneous, as shown in Fig. 2c and d. For the SD sintered products, the compressive strength of the products that were sintered at 1050 °C and 1080 °C were only 3.45 and 9.03 MPa, respectively. This may have been because these temperatures were lower than the sintering temperature of the SD, which resulted in its particulate nature. Conversely, when the SD was sintered at 1100 °C, its compressive strength was 23.69 MPa, indicating the occurrence of densification and the appearance of a glass phase. Finally, when the SE sintered products were sintered at 1050 °C, 1080 °C, and 1100 °C, the compressive strengths were all less than 15 MPa. This demonstrates that the sintering temperature of the SE is greater than 1100 °C, as indicated by the powdery film on the surface of the sintered products (Fig. 2g and h). 3.3. Properties of heavy metals 3.3.1. Fixed rate of heavy metals Fig. 5 shows the fixed rates of heavy metals after sintering products that contained 25% CA at 1100 °C for 30 min. Three par-
Fig. 5. Fixed rate and vapored rate of heavy metals for sintered products.
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X. Wang et al. / Waste Management 29 (2009) 1330–1335 Table 3 HVEP-results of heavy metals in sintered products before and after sintering.
Leaching concentration (mg/L) Leaching rate (%) Identification standard for hazardous wastes (mg/L) (GB5085.3, 1996) Environmental quality standards for surface water III (mg/L) (GB3838, 2002)
allel samples were researched. With the exception of Pb, only a small amount of the heavy metals were volatilized during the sintering process, and the fixed rates were all greater than 60%. A greater amount of Pb was volatilized due to its low boiling point, and its fixed rate was 37.6%. At the same time, Pb is the primary heavy metal present in sewage sludge in China; therefore, additional steps must be taken to control Pb volatilization during sintering. 3.3.2. Leaching characteristics of heavy metals The HVEP results of As, Cd, Cr, Cu, Ni, Pb and Zn analysis of the product produced by sintering the SD at 1100 °C for 30 min are shown in Table 3. The leaching rates of the heavy metals were all lower than 1% and the leaching concentrations were all in compliance with the China Identification Standard for Hazardous Wastes and the China Environmental Quality Standards for Surface Water. Therefore, these results indicate that sintering generally reduces the leaching concentration for all of the heavy metals evaluated here, and that the use of lightweight aggregate produced from DSS does not pose an environmental risk.
4. Conclusions Based on the results of this study, the following conclusions can be drawn: 1. The SiO2 and Al2O3 contents in DSS are lower than the requirements for lightweight aggregates. Besides, the sintered products produced using pure DSS are porous and loose, and have a low compressive strength due to high organic matter content. Therefore, it is necessary to amend DSS with a material that has a high SiO2 and Al2O3 content, such as CA, in order for it to be sintered. 2. DSS has a low sintering temperature of 1050 °C due to its high CaO, MgO, Fe2O3 and K2O content. However, the addition of CA improves the sintering temperature, while effectively decreasing the pore size of the sintered products and increasing the compressive strength. 3. When DSS and CA are used to sinter lightweight aggregates, the factors affecting the properties of the sintered products were the sintering temperature, the retention period and the raw material mixture rate, in that order. 4. A mixture containing 18–25% of CA that was preheated at 420 °C for 20 min and then sintered at 1100 °C for 30 min produced high quality lightweight aggregates. The leaching con-
As
Cd
Cr
Cu
Ni
Pb
Zn
0.001 0.13 61.5 60.05
0.001 0.41 60.3 60.005
0.022 0.18 610 60.05
0.058 0.20 650 61
0.009 0.13 610 60.02
0.056 0.56 63 60.05
0.279 0.33 650 61
centrations of the sintered products were all in compliance with the China Identification Standard for hazardous wastes and China Environmental Quality Standards for surface water.
Acknowledgments This work was supported by the national Key Technologies R and D Program of China (No. 2006BAC02A19), and we received considerable cooperation from Tsinghua University. All the support is greatly appreciated. References ASTM Standard Method D 6357-00a, 2000. Standard Test Methods for Determination of Trace Elements in Coal, Coke, and Combustion Residues from Coal Utilization Processes by Inductively Coupled Plasma Atomic Emission Spectrometry, Inductively Coupled Plasma Mass Spectrometry, and Graphite Furnace Atomic Absorption Spectrometry. Chiou, I.J., Wang, K.S., Chen, C.H., Lin, Y.T., 2006. Lightweight aggregate made from sewage sludge and incinerated ash. Waste Manage. 26, 1453–1461. Elkins, B.V., Wilson, G.E., Gersberg, R.M., 1985. Complete reclamation of wastewater and sludge. Water Sci. Technol. 17, 1453–1454. Endo, H., Nagayoshi, Y., Suzuki, K., 1997. Production of glass ceramics from sewage sludge. Water Sci. Technol. 36, 235–241. GB3838, 2002. Environmental Quality Standards for Surface Water. GB5085.3, 1996. Identification Standard for Hazardous Wastes – Identification for Extraction Procedure Toxicity. GB5086.2, 1997. Test Method Standard for Leaching Toxicity of Solid Wastes – Horizontal Vibration Extraction Procedure. Jordán, M.M., Boix, A., Sanfeliu, T., de la Fuente, C., 1999. Firing transformations of cretaceous clays used in the manufacturing of ceramic tiles. Appl. Clay Sci. 14, 225–234. Kim, E.H., Cho, C.K., Yim, S., 2005. Digested sewage sludge solidification by converter slag for landfill cover. Chemosphere 59, 387–395. Merino, I., Arévalo, L.F., Romero, F., 2007. Preparation and characterization of ceramic products by thermal treatment of sewage sludge ashes mixed with different additives. Waste Manage. 27, 1829–1844. Riley, C.M., 1950. Relation of chemical process to the bloating clay. J. Am. Ceram. Soc. 34 (4), 121–128. Tay, J.H., Show, K.Y., 1997. Resource recovery of sludge as a building and construction material – a future trend in sludge management. Water Sci. Technol. 36 (11), 256–266. Tay, J.H., Yip, W.K., Show, K.Y., 1991. Clay-blended sludge as lightweight aggregate concrete material. J. Environ. Eng. 117 (6), 834–844. Wainwright, P.J., Cresswell, D.J.F., 2001. Synthetic aggregate from combustion ashes using an innovative rotary kiln. Waste Manage. 21, 241–246. Wan, X., Wang, W., Ye, T.M., Guo, Y.W., Gao, X.B., 2006. A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure. J. Hazard. Mater. B134, 197–201. Wang, K.S., Chiang, K.Y., Perng, J.K., 1998. The characteristics study on sintering of municipal solid waste incinerator ashes. J. Hazard. Mater. 59, 201–210. Wiebusch, B., Seyfried, C.F., 1997. Utilization of sewage sludge ash in the brick and tile industry. Water Sci. Technol. 36 (11), 251–258. Yan, H.D., 2005. Study on the super-lighting ceramsite calcined by the modified life sludge. Environ. Pollut. Control 27, 63–67.