- Email: [email protected]
Evaluation of carbonization as a thermal pretreatment method for landfilling by column leaching tests
Available online at www.sciencedirect.com
Waste Management 28 (2008) 3–14 www.elsevier.com/locate/wasman
Evaluation of carbonization as a thermal pretreatment method for landfilling by column leaching tests I.H. Hwang, T. Matsuto
*
Laboratory of Solid Waste Disposal Engineering, Graduate School of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan Accepted 8 November 2006 Available online 30 January 2007
Abstract To evaluate carbonization as a thermal pretreatment method for landfilling, the releasing characteristics of organic and inorganic constituents from carbonization residue derived from shredded residue of bulky waste was investigated by means of batch and column leaching tests. Shredded residue of bulky waste itself and its incineration ash were tested together to compare pretreatment methods. In batch leaching tests at a liquid/solid ratio of 10, the release of organic carbon from carbonization residue was at a remarkably low level. Besides, carbonization contributed to immobilize heavy metals such as chromium, cadmium, and lead within its residue. In column tests, the discharges of organic constituents were lowest from carbonization residue under aerobic conditions due to microbial activity. The leaching of Cd, Cr, Pb, and Cu from carbonization residue was suppressed under anaerobic conditions; however, this suppression effect tended to be weaker under aerobic conditions. From the results showing that the total releasing amounts of organic and inorganic constituents from carbonization residue are so low as to be comparable to that of incineration ash, carbonization can be considered as one of the thermal pretreatment methods of organic wastes. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The early stabilization of landfill waste is one of the most important issues in solid waste management. In general, mechanical, biological, and thermal pretreatment methods are considered feasible ways to hasten landfill stabilization. The objectives of those pretreatment methods are summarized as follows (Fricke et al., 2005): (1) the reduction of waste volume and mass to secure landfill capacity; (2) the control of biological and chemical reactions to restrain gas and leachate generation; and (3) the immobilization of contaminants to prevent pollutants from migrating to the environment. Most countries in the European Union have recommended the mechanical–biological treatment of biodegradable waste prior to landfilling. On the other hand, several *
Corresponding author. Tel./fax: +81 11 706 6828. E-mail address: [email protected] (T. Matsuto).
0956-053X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2006.11.007
countries such as Switzerland, Japan, and Sweden have introduced municipal solid waste incinerators (MSWIs) for the thermal treatment of combustible waste. Incineration is considered to be remarkably effective at reducing waste volume and increasing sanitation. In particular, it can be one of the best options for countries having difficulties in securing proper sites for final disposal. Nevertheless, critical issues such as the emission of secondary pollutants and the high cost for their control still remain following incineration. Bertolini (2003) noted that the gross national product of countries treating more than 50% of waste at MSWIs corresponded to more than USD 10,000 per capita. Carbonization is a thermal process that produces carbonaceous materials commonly called ‘‘char’’. At actual carbonization plants for the treatment of solid wastes, rotary kilns or furnaces are operated in the range of 400–500 C. During the heating process under an inert atmosphere, moisture and volatiles are exhausted in their gas phases, whereas fixed carbon, ash, and some volatile matter form
4
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
residual materials with porous surfaces. In some facilities, evolved gas is recirculated in the carbonization system to be used as a subsidized fuel or atmospheric gas. Kistler et al. (1987) concluded that carbonization might have a positive effect on the immobility of heavy metals due to the high H+ buffering capacity and the alkaline nature of the slaked char. Honda et al. (1993) reported that an iodine adsorptive capacity of 680–720 mg/g was achieved by carbonization of waste ion exchange resin. Lu et al. (1995) noted that heavy metals, except mercury, are safely enclosed in the solid residues by pyrolysis. Caballero et al. (1997) and Inguanzo et al. (2002) suggested the pyrolysis of sludge be an alternative to incineration because it concentrated the heavy metals in the final residue and avoided the formation of toxic organic compounds. Sainz-Diaz and Griffiths (2000) reported that straw and furniture waste chars produced at 500 C had 50 m2/g and 40 m2/g of BET surface, respectively. In our previous research (Hwang et al., in press), chars derived from various municipal and industrial wastes were subjected to batch leaching tests. The release of heavy metals such as chromium, cadmium, and lead was significantly reduced by carbonization regardless of waste type. Furthermore, carbonization could degrade a considerable amount of organic matter in raw waste. Based on these results, we are going to investigate carbonization residue as a thermally pretreated material for landfilling using column tests. Column leaching tests have been used to simulate the leaching behavior of contaminants over the short, medium, and long term by relating the release, expressed as mg/kg leached, to the liquid/solid (L/S) ratio (Van der Sloot, 1996). Moreover, column tests provide information on the successive release characteristics of pollutants under the presence of microbial activity or given landfill conditions. Shredder residue of bulky waste discharged from households was chosen as the study material because of its high organic matter and metal content (Hwang et al., in press). ‘‘Char’’ will be referred to as ‘‘carbonization residue’’ in this study, because the objective of carbonization is not to recover fuel but to treat waste thermally before final disposal. In order to compare carbonization to other pretreatment methods, we also tested shredded residue of bulky waste itself as a mechanically treated waste and ash obtained from the incineration of shredded residue of bulky waste in parallel. The discharge of organic and inorganic constituents to leachate, as well as gas emissions, was measured to quantify environmental loads from shredded waste, incineration ash, and carbonization residue in an attempt to evaluate carbonization as a thermal pretreatment method. 2. Materials and methods 2.1. Materials The shredded residue of bulky waste collected from households was obtained from a shredding plant in Sap-
poro, Japan, in July 2004. Loaded bulky wastes were shredded by a hammer mill and then transported to a magnetic separator for ferrous metal recovery. The remaining residuals were subsequently divided into two fractions by a 50 mm screen. The fraction that passed through the screen was sampled as mechanically pretreated bulky waste (BWM). The composition of BWM was as follows: wood, 45%; plastics, 14.6%; paper, 1.6%; textiles, 0.6%; glass and porcelain, 0.2%; metals, 1.7%; and miscellaneous materials smaller than 2 mm, 36.3%. Approximately 50 kg of BWM were collected for the experiments and dried at 60 C until the moisture content reached a stable minimum. 2.2. Experiments 2.2.1. Carbonization and incineration About 10 kg of dried BWM were put into a rotary kiln type of reactor (B150 mm · 1200 mm) by a screw feeder to produce carbonization residues. The slope angle and rotation rate of the kiln were adjusted to 0.8 and 2 rpm, respectively, to maintain a retention time of 1 h. Temperature was 500 C and nitrogen was used as a carrier gas at a rate of 11 L/min to maintain a reducing atmosphere. On the other hand, about 25 kg of dried BWM was incinerated in the rotary kiln for 2 h to obtain incineration ash. The slope angle and rotation rate were set at 0.8 and 1 rpm, respectively. Incineration temperature was 600 C and air was introduced to the kiln at the rate of 50 L/min, taking a theoretical excess air ratio into consideration. After carbonization and incineration, the weight of carbonization residue (BWC) and incineration ash (BWI) was measured to estimate yields. 2.2.2. Composition of samples BWM was reduced in size to less than 1 mm using a laboratory-scale cutting mill. Carbonization residue (BWC) and incineration ash (BWI) were pulverized using a ball mill for 30 min to ensure homogeneity in the batch tests, and a sample of the fraction smaller than 1 mm was taken. The organic matter content was determined by proximate analysis; however, for the incineration ash, it was measured by ignition loss at 600 C for 3 h. Carbon, hydrogen, and nitrogen were measured by elementary analyzer (CHN recorder MT-5, Yanaco Co.). Sulfur was absorbed in hydrogen peroxide solution during the incineration of sample and its concentration was measured by ion chromatography (DX-500, Dionex Co.). The microwave-assisted acid digestion method was used to decompose the solid matrix for measuring the metal content of samples (USEPA, 1996). 2.2.3. Batch leaching test The release of organic matter and metals from the sample was investigated using the Japanese leaching test No. 13 (Environment Agency of Japan, 1973). We immersed 10 g of homogenized sample in a flask containing 100 mL of distilled water, which was capped and shaken horizontally for 6 h at 200 rpm. The mixture was filtered using 1 lm pore filter paper, and the filtrate was prepared for measuring
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
pH and the concentration of total organic carbon (TOC) and metals such as cadmium, chromium, lead, zinc, copper, magnesium, calcium, sodium, and potassium. 2.2.4. Column test As illustrated in Fig. 1, two types of columns were designed to simulate aerobic and anaerobic conditions. Transparent acrylic plastic columns were packed with shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI). To facilitate microbial activity under the given aerobic or anaerobic conditions, supernatant obtained from a mixture of soil and distilled water was seeded into the aerobic columns, whereas supernatant from anaerobically digested sludge was added to the anaerobic columns. The initial moisture content of the packed samples was adjusted to 20% and the temperature was maintained at 30 C. Table 1 outlines the conditions for each column. The gas composition, i.e., H2, CO2, O2, and CH4, was checked through gas sampling ports, but the gas generation amount was measured only at anaerobic columns. Every two weeks, 200 mL of distilled water were injected into each column to simulate rainfall. To prevent air permeation, micro-tubing pumps were used to sprinkle distilled water into the anaerobic columns. After 24 h, leachate was collected in a flask and weighed using an electric balance to confirm leachate quantity. The upper and lower parts of the anaerobic column were purged by nitrogen to drive out any air that may have permeated the column during the sampling procedure.
The pH and oxidation–reduction potential (ORP) of the leachate were checked as quickly as possible. Each leachate was filtered using a 1 lm pore filter paper prior to measuring TOC, inorganic carbon (IC), 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (CODMn), total nitrogen (TN), ammonia nitrogen (NH4–N), chloride (Cl), and sulfate ðSO2 4 Þ. To measure metal concentrations (Cd, Cr, Pb, Zn, Cu, Mg, Ca, Na, and K), the leachate was filtered through a 0.45 lm pore filter paper and acidified by nitric acid. The entire experimental scheme is presented in Fig. 2. 2.3. Analytical methods and instruments The concentrations of H2, CO2, O2, and CH4 were analyzed using gas chromatography with a thermal conductivity detector (MTI M200D for H2, column type: WG-100, flow rate of Ar: 33 mL/min; detector temperature: 50 C; Hitachi Type 164 for O2, CO2, and CH4, column type: WG-100, flow rate of He: 33 mL/min; detector temperature: 50 C). The pH and ORP were measured using the glass electrode method. The concentrations of TOC, IC, and TN were measured using a simultaneous TOC-TN analyzer (TOC-V CPH/CPN, Shimadzu Co.). BOD5 and CODMn analysis followed the methods of the Japan Society for Analytical Chemistry (JSAC, 1994). The concentration of NH4–N was measured spectrophotometrically using the indophenol method. The concentrations of chloride and sulfate were measured by ion chromatography (TOSOH
b d f c
a
e
Open h 1
2 3
g i
j
a: Mass cylinder b: Micro-tubing pump c: Distilled water injection port d: Nitrogen purge bag e: Gas sampling port for anaerobic column f: Gas volume measuring port g: Gas holder h: Glass wool and bead layer i: Glass wool and mesh layer j: Gas sampling ports for aerobic column k: Cock l: Silicon cap m: Leachate holder
k Open
l m
Anaerobic column
5
Aerobic column
Fig. 1. Experimental apparatus for column tests under anaerobic and aerobic conditions.
6
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
Table 1 Column settings BWM-A Packing material Atmosphere Bed volumea (L) Weight of packing material (kg-wet) Packing density (kg/m3) Moisture content (wt%) a
BWM-An
BWC-A
BWC-An
BWI-A
BWI-An
Shredded residue of bulky waste collected from households Aerobic Anaerobic 2.67 2.67 1.20 1.20
Carbonization residue derived from BWM Aerobic Anaerobic 2.67 2.67 1.24 1.24
Incineration ash obtained from BWM Aerobic Anaerobic 2.67 2.67 4.93 4.04
449 20
464 20
1846 20
449 20
464 20
1513 20
D = 10.8 cm; H = 20.9 cm.
reduction conditions. The yields of carbonization residue (BWC) and incineration ash (BWI) obtained from BWM were about 43% and 36.5%, respectively (Table 2).
Drying at 60oC for 24 h of shredded residue of bulky waste collected from households
Mechanicallytreated waste (BWM)
Incineration at 600˚C for 2 h in air
Carbonization residue (BWC)
Incineration ash (BWI)
Batch test Proximate analysis/Ignition loss Metal content Leaching test
Column test
Aerobic condition
Anaerobic condition
Generation amount of gas and leachate Composition of gas and leachate
Fig. 2. Entire experimental scheme.
CM-8). The concentrations of Cd, Cr, and Pb were analyzed by graphite furnace atomic absorption spectrometry (AAS; Z-8200, Hitachi Co.), whereas the concentrations of Zn, Cu, Mg, Ca, and Na were analyzed using an inductively coupled plasma emission spectrometer (ICP-AES, Shimadzu Co.) Finally, K was measured using flame AAS (Z-8200, Hitachi Co.). 3. Results and discussion 3.1. Characteristics of BWM, BWC, and BWI As previously mentioned, BWM mainly consisted of slowly- or non-decomposable components such as wood and plastics, implying that BWM is significantly less biodegradable than other organic wastes found in MSW. Fig. 3 shows that a large amount of organic matter was consumed by thermal treatment under either oxidation or
Weight percent (wt%-dry)
100
Carbonization at 500˚C for 1 h in nitrogen
Ash Fixed carbon Volatile matter Ignition loss
80
60
40
20
0
BWM
BWC
BWI
Fig. 3. Yield and composition of shredded residue of bulky waste (BWM), carbonization residue (BWC) and incineration ash (BWI).
Table 2 Total yield, organic matter, carbon, hydrogen, nitrogen, sulfur, and metal content of shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI) a
Total yield (wt%) Organic matter (wt%-dry) C (wt%-dry) H (wt%-dry) N (wt%-dry) S (wt%-dry)
BWM
BWC
BWI
100.0 73.4b 40.4 5.0 19.9 0.2
43.0 28.2b 22.4 1.8 0.6 0.1
36.5 11.4c – – – –
14.85 8.42 6299 4559 1792 4854 15,509 10,701 3582
29.60 36.55 7693 7562 7234 6446 27,499 6841 2750
Metal content (mg/kg-dry sample) Cd 26.47 Cr 9.91 Pb 4518 Zn 1884 Cu 2746 Ma 1752 Ca 10,878 Na 2182 K 1101
a Determined based on the weight of shredded residue of bulky waste (BWM). b Determined by summation of fixed carbon and volatile matter. c Determined by ignition loss at 600 C for 1 h.
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
The organic matter content of BWM, BWC, and BWI was 73.4%, 28.2%, and 11.4%, respectively, based on dried sample weights (Table 2). If the variation in ash amount with thermal treatment is negligible, approximately 83% and 94% of the organic matter in BWM was degraded by carbonization and incineration, respectively. 3.2. Batch leaching test 3.2.1. Release of organic matter The concentration of TOC released from BWM, BWC, and BWI during leaching tests at L/S = 10 (JLT-13) corresponded to 339.4, 58.4, and 27.9 mg/L, respectively, as presented in Table 3, indicating that the release of organic carbon from carbonization residue was at a remarkably low level, even though the level was twice that of incineration ash. 3.2.2. pH and metal releasing The pH and metal concentrations of the filtrate obtained from the leaching test at L/S = 10 (JLT-13) are shown in Table 3. The pH of BWM and BWC was 7.5 and 8.4, respectively; however, the pH of BWI was 10.4. Carbonization and incineration suppressed the leaching of heavy metals from their residues (Table 3). Heavy metals seemed to be well suppressed in BWC and BWI. Cd and Pb were not detected in BWI filtrate and Cr was not detected in BWC filtrate. Cd, Pb, and Zn leaching from BWC measured 0.055, 1.032, and 10.22 mg/kg-dry BWC, respectively; these levels are considerably lower compared to those from BWM. On the other hand, the alkali and alkali earth metals, except Mg, showed a tendency to release readily, regardless of pretreatment. Mg leaching level was conspicuously low in BWI. The high acid-buffering pH of incineration ash and the surface properties of carbonization residue could be factors in the reduction of metal release.
Table 3 The pH, TOC, and metal leaching content of filtrate obtained by the application of JLT-13 to shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI)
pH TOC (mg/L)
BWM
BWC
BWI
7.5 339.4
8.4 58.4
10.4 27.9
0.055 ND 1.032 10.22 0.15 129 1464 1022 316
ND 0.187 ND 0.07 0.22 7.108 2106 1012 142
Metal leaching content (mg/kg-dry sample) Cd 0.685 Cr 0.198 Pb 7.198 Zn 55.48 Cu 4.65 Mg 195 Ca 517 Na 1111 K 637 ND: not detected.
7
3.3. Column tests 3.3.1. Gas emissions and composition The O2 concentrations of three sampling ports averaged 18.6 ± 0.2% and 20.6 ± 0.0% in the aerobic BWM and BWC columns, respectively, without significant variation among sampling points. In the aerobic BWI column, however, O2 concentrations at ports 1, 2, and 3 were 19.3%, 17.6%, and 14.8%, respectively, even though both the upper and lower parts of the column were open to the atmosphere. Leachate naturally migrated downward, which might fill apertures in the lower part of the bed and obstruct air penetration to inside layers. The generation of CO2 was most active in the BWM column but it was not detected in the BWI column. Generated CO2 may have been absorbed into the BWI bed and formed calcite (CaCO3) by reacting with calcium ions (Kim et al., 2005). Even if microbial activity existed, it may have been inhibited by the high pH and the lack of substrate in incineration ash. In the anaerobic BWM column, H2 and CO2 were actively generated in the initial stage, and acid fermentation continued for 6 mo (Fig. 4a). CH4 emission was observed from the seventh month since the start of the column experiments. The primary reason for delayed methane fermentation may have been related to the physical composition of BWM, which consisted of slowly- or non-biodegradable components. The shortage of readily available substrate might bring about retardation of methane fermentation in the anaerobic BWM column. On the other hand, weak acid fermentation was maintained in the anaerobic BWC column over 10 mo without CH4 generation (Fig. 4b). This phenomenon appeared to be caused by substrate deficiency too. In the BWI anaerobic column, a very small, insignificant amount of CO2 was generated after 1 mo (Fig. 4c) and then it was hardly detected. H2 was continuously emitted; however, it may have resulted from hydration reactions of alkaline metals, such as aluminum and iron, rather than acid fermentation (Kim et al., 2005). Carbon emissions in the forms of CO2 and CH4 were 907 mg C/kg-dry BWM, 138 mg C/kg-dry BWC, and 4.1 mg C/kg-dry BWI under anaerobic conditions. These results indicated that gas generation amount from BWC and BWI was not much compared to that from BWM under anaerobic conditions. Because the aerobic columns were designed to be open to the atmosphere, the quantitative analysis of gas emissions was not possible although CO2 concentration was measured at three gas sampling ports. 3.3.2. Variation in pH, ORP, and concentrations of organic and inorganic constituents versus L/S Figs. 5–7 show the variation in pH, ORP, and releasing of carbon, nitrogen, chloride, and sulfate from BWM, BWC, and BWI under aerobic or anaerobic conditions versus L/S ratios, which were plotted on semi-logarithm graphs over the entire experimental period.
8
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
a
6
Gas generation amount (mmol/kg-dry)
H2 CO2
5
CH4 4
3
2
1
0 0
10
20
30
40
b
1.2
Gas genearation amount (mmol/kg-dry)
Week
1.0
H2 CO2
0.8
0.6
0.4
0.2
0.0 0
10
20
30
40
Week
c
4
Gas generation amount (mmol/kg-dry)
H2 CO2 3
2
1
0 0
10
20
30
40
Week Fig. 4. Generated gas amounts and composition in anaerobic columns. (a) shredded residue of bulky waste (BWM); (b) carbonization residue (BWC); (c) and incineration ash (BWI).
The variation in pH was not significant; pH did change slowly but steadily with packing sample and aerobic or anaerobic conditions. For BWM, the pH values for leachate obtained from the aerobic and anaerobic columns ranged from 7.3 to 8.4 and 6.1 to 7.6, respectively. IC concentration is deeply related to chemical equilibrium with pH. In the anaerobic BWM column, IC gradually decreased from 50 to 16 mg/L. In the same period, pH dropped slightly, and methane fermentation subsequently started slowly from L/S = 2.5. Owing to
an accumulation of organic acids, the pH of leachate obtained from anaerobic columns showed a gradual decreasing tendency. In BWC columns, the pH of leachate under aerobic and anaerobic conditions corresponded to 7.2–7.9 and 7.5–8.4, respectively. On the other hand, the pH of leachate collected from BWI columns ranged from 8.1 to 10.1 and 9.9 to 10.7 under aerobic and anaerobic conditions, respectively. As aerobic columns were always exposed to air, it seems that atmospheric CO2 affected the neutralization of highly alkaline incineration ash. The ORP of aerobic and anaerobic leachate differed by approximately 50–200 mV (Eh). As shown in Figs. 5–7, none of the anaerobic leachates could reach negative values despite efforts to develop anaerobic conditions excluding air permeation. Because ORP was measured 24 h after the start of water injection, the results may not represent the true redox conditions of the inner beds, especially in the anaerobic columns. In the aerobic BWM and BWC columns, BOD5 decreased below 10–30 mg/L around L/S = 1. Easily biodegradable organic matter is generally decomposed into CO2 by aerobic microorganisms, whereas it is first accumulated as organic acids in leachate under anaerobic conditions. Thus, TOC concentrations of leachate obtained from aerobic BWM and BWC columns were lower than those from anaerobic ones. In the BWM columns, TOC concentrations under anaerobic conditions were about 5– 10 times higher than those under aerobic conditions. However, such differences were not observed in aerobic and anaerobic BWI columns. Organic constituents seemed to be regularly washed out without rapid declines in BOD5 at the initial stages, unlike the patterns observed in the BWM and BWC aerobic columns. The TN concentration showed a similar declining trend as did TOC concentration. In particular, the NH4–N concentration rapidly decreased with reduced BOD5 for the initial 3–4 mo in the aerobic BWM and BWC columns. This may have been the result of assimilation by aerobic microorganisms or by nitrification of ammonia. However, the NH4–N concentration increased again after L/ S = 1.5, indicating that the latter explanation would not be responsible for the initial drop in NH4–N. With the exception of the above aerobic BWM and BWC columns, NH4–N concentrations showed a similar decreasing trend as TN concentration. Chloride decreased linearly with L/S ratio. Sulfate was discharged at a constant concentration from aerobic BWC and aerobic and anaerobic BWI columns, indicating that sulfate is dissolved in leachate under the given conditions. On the other hand, in the anaerobic BWM column, sulfate concentrations dropped sharply below 20 mg/L after L/S = 2. This may have represented a reduction of sulfate; however, the ORP value was too high relative to reference data. Generally, the reduction of sulfate occurs in the range of 50 to 100 mV (Eh) (Tchobanoglous et al., 1993).
450
8.0
400 350
7.5
300 250
7.0
200 150
6.5 pH
100
ORP
6.0
b
TOC CODMn BOD5 IC
450
ORP
400 350
7.5
300 250
7.0
200 150 100 50
105
TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
TN NH-4-N Cl 2SO4
104
Concentration (mg/L)
Concentration (mg/L)
pH
8.0
6.0
104
103
102
101
103
102
101
100
100
10-1 104
10-1 104
Cd Cr Pb
Zn Cu Mg
Ca Na K
Cd Cr Pb
103
103
102
102
Concentration (mg/L)
Concentration (mg/L)
8.5
6.5
50
105
9
ORP (mV, Eh)
8.5
pH
pH
a
ORP (mV, Eh)
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
101
100
10-1
101
100
10-1
10-2
10-2
10-3
10-3
10-4
Ca Na K
Zn Cu Mg
10-4
0
1
2
3
4
5
L/S
0
1
2
3
4
5
L/S
Fig. 5. Variation in quality of leachate obtained from shredded residue of bulky waste (BWM). (a) Aerobic; (b) anaerobic.
3.3.3. Variation in metal leaching versus L/S As shown in Figs. 5–7, alkali and alkaline earth metals such as Na, K, Mg, and Ca were washed out at a constant rate by regular water injection. However, heavy metals such as Cd, Cr, Pb, Zn, and Cu showed different leaching trends depending on packing material, pH, and aerobic or anaerobic conditions. In BWI columns, Cd was not detected after L/S = 0.4 under either aerobic or anaerobic conditions under
highly alkaline conditions of pH 9–10. In BWM and BWC columns, Cd release rapidly decreased, together with sulfate, under anaerobic conditions. From the abrupt drop in sulfate concentrations in the anaerobic BWM column, it was assumed that Cd precipitated by forming insoluble metal sulfate as follows: Cd2+ + S2 M CdS (s). In BWM and BWI columns, the leaching concentration of Cr was higher as pH of leachate became lower regardless
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
b
450 400
8.0
pH
350 300
7.5
250 pH
200
ORP
7.0
8.5
400 350 300
7.5
250 200
150
7.0
150
105 TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
TOC CODMn BOD5 IC
104
TN NH-4-N Cl SO42-
104
Concentration (mg/L)
Concentration (mg/L)
ORP
8.0
105
103
102
101
103
102
101
100
100
10-1 104
10-1 104 Cd Cr Pb
Ca Na K
Zn Cu Mg
Cd Cr Pb
103
103
102
102
Concentration (mg/L)
Concentration (mg/L)
450 pH
ORP (mV, Eh)
8.5
ORP (mV, Eh)
a
pH
10
101
100
10-1
101
100
10-1
10-2
10-2
10-3
10-3
10-4
Ca Na K
Zn Cu Mg
10-4 0
1
2
3
4
5
L/S
0
1
2
3
4
5
L/S
Fig. 6. Variation in quality of leachate obtained from carbonization residue (BWC). (a) Aerobic; (b) anaerobic.
of aerobic or anaerobic conditions. Unlike the leaching test at L/S = 10, Cr was detected in leachate from BWC columns. In particular, Pb and Zn were sensitive to pH variation, so that they were detected at high concentrations under conditions of relatively low pH. In the highly alkaline BWI column of pH 9–10, the leaching concentrations
of both metals were remarkably low relative to other columns; moreover, Pb was not detected from L/S = 0.6. However, the release of Cu was clearly greater under aerobic conditions in the BWM and BWC columns. In the BWI columns, the release of Cu declined constantly because of regular washing out. The concentration of Cu was slightly
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
200 8
150 pH
ORP
7
200
ORP
7
104
104
103
103
Concentration (mg/L)
105
100
102
101
100 TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
10-1 104
103
103
102
102
Concentration (mg/L)
10-1 104
101
100
10-1
10-2
101
100
10-1
10-2
10-3
10
150 pH
100
Concentration (mg/L)
250
8
100
101
300
9
105
102
350
ORP (mV, Eh)
pH
250 9
11 10
pH
300
10
Concentration (mg/L)
b
350
11
ORP (mV, Eh)
a
11
10-3 Cd Cr Pb
-4
0.0
0.2
Ca Na K
Zn Cu Mg
0.4
0.6
Cd Cr Pb
10-4 0.8
1.0
1.2
L/S
0.0
0.2
0.4
Ca Na K
Zn Cu Mg
0.6
0.8
1.0
1.2
1.4
L/S
Fig. 7. Variation in leachate quality obtained from incineration ash (BWI). (a) Aerobic; (b) anaerobic.
higher in the anaerobic column, but this difference appears to have been due to differences in initial Cu contents. 3.3.4. Comparison of washing-out rate of pollutants in column tests As illustrated in Figs. 5–7, logarithmic concentrations of non-reactive components such as Cl, Na, K, Mg, and Ca
decreased at constant rates, except for the initial unsaturated period. Tanaka and Koyama (1987) noted that the logarithmic concentration of non-reactive constituents was linearly decreased with cumulative leachate amount when a constant amount of water was injected into the column at regular intervals. Therefore, the future concentration of a pollutant removed at a constant rate by regular
12
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
washing out can be estimated by determining the washingout ratio from the slope. The concentrations of TOC, CODMn, BOD5, and TN also exhibited a constant decreasing trend versus cumulative L/S ratio. However, some constituents occasionally had zero or positive slopes, implying that their concentrations are governed by solubility under some conditions or that certain generation reactions occur. For example, there were sulfates in the BWI columns (Fig. 7a and b) and NH4–N in the aerobic BWM columns (Fig. 5a). The washing out of organic constituents appears to occur later than that of chloride. On the other hand, the release of heavy metals such as Cd, Cr, Pb, and Cu could not simply be described by the washing-out effect. 3.3.5. Comparison of metal leaching patterns between batch and column tests Table 4 shows the total amount of pollutants leached from each column during the 10 mo experiment period. Comparing the metal leaching amounts obtained by batch tests (JLT-13) in Table 3 with the cumulative amount of metal leaching from column tests in Table 4, the metal leaching characteristics were classified into the four following patterns: (1) pattern 1 (Batch > Column): the cumulative metal leaching amount in the column is relatively small and not expected to reach the leaching amount of the batch test at L/S = 10, even if the cumulative L/S in the column test reaches 10; (2) pattern 2 (Batch Column): the cumulative metal leaching amount is expected to converge with the leaching amount of the batch test when the cumulative L/S becomes 10; (3) pattern 3 (Batch < Column): the cumulative metal leaching amount considerably exceeds the leaching amount of the batch test at L/S = 10; and (4) pattern 4: metal is not detected in the batch test but is detected in the column test. A batch leaching test in the condition of L/S = 10 (JLT13) is generally performed as a standard method to determine stability of material in landfill. However, this kind of batch test is done without consideration of microbial activity,
aerobic or anaerobic conditions in landfill, etc. Thus, it is necessary to compare metal releasing results from batch test and column test depending on BWM, BWC, and BWI and aerobic or anaerobic conditions. Table 5 shows the four kinds of metal leaching patterns between the batch and column tests. Release of Cd, Cr, and Pb was characterized depending on material rather than aerobic or anaerobic conditions; their releasing from BWM was grouped into pattern 1 except that Cr under anaerobic conditions was classified into pattern 2. In case of BWC, Cd and Pb showed pattern 1. Cr was not detected in batch test but it released in column test, accordingly, it was classified into pattern 4. Opposite leaching patterns were observed for BWI, that is, Cd and Pb were classified into pattern 4 but Cr presented pattern 1. On the other hand, the releasing amount of Zn and Cu seems to vary with aerobic or anaerobic conditions even though same materials (Table 5). Finally, alkali or alkali earth metals were mainly classified in pattern 2 excepting four cases: Mg of BWC and BWI (pattern 1), Ca of BWM (pattern 3), and K of BWI (pattern 3). 3.4. Quantitative comparison of environmental loads into leachate depending on pretreatment methods Using the column data in Table 4, the total pollutant leaching amount of organic and inorganic constituents could be estimated according to the pretreatment method, as well as the aerobic and anaerobic conditions. Fig. 8 exhibits the relative potential loads of pollutants discharging into leachate assuming 1 t of bulky waste was disposed in landfills after shredding, carbonization, and incineration, respectively. The amounts of pollutant released from columns were normalized based on the result of anaerobic BWM, which were plotted on logarithm radar charts. As presented in Fig. 8a, the load of organic and inorganic constituents into leachate rapidly diminished under aerobic conditions. TOC, TN, CODMn, and NH4–N levels
Table 4 Cumulative leaching amounts of pollutants over the 10 months of column tests (unit: mg/kg-dry sample) Items
BWM-A
BWM-An
BWC-A
BWC-An
BWI-A
BWI-An
TOC IC CODMn TN NH4–N Cl SO2 4 Cd Cr Pb Zn Cu Mg Ca Na K
916 150 941 124 14.7 1508 1531 0.021 0.056 0.146 11.1 0.881 89.6 806 785 362
5337 106 3793 588 128 1774 1657 0.069 0.161 0.303 92.2 0.180 197 1938 928 548
266 27.7 181 30.3 8.73 4179 1660 0.043 0.016 0.114 18.5 1.02 87.9 1906 1081 399
645 156 249 59.7 22.8 4197 2509 0.004 0.018 0.115 6.23 0.045 114 2487 1087 484
494 9.97 235 70.3 32.5 5087 1564 0.001 0.017 0.022 0.023 0.202 1.13 1664 1981 705
548 12.5 299 76.6 39.4 5395 2246 0.004 0.013 0.069 0.299 0.738 1.97 2286 2492 730
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
13
Table 5 Four kinds of metal leaching patterns determined by batch and column tests
Cd Cr Pb Zn Cu Mg Ca Na K Pattern Pattern Pattern Pattern
1 2 3 4
BWM-A
BWM-An
BWC-A
BWC-An
BWI-A
BWI-An
s s s s s – – –
s – s s – – –
s * s – s – – –
s * s s s s – – –
* s * s – s – –
* s * s – –
(s): Batch > Column. (–): Batch Column. (): Batch < Column. (*): Not detected in batch test but detected in column test.
BWM-A BWC-A BWI-A
BWM-An BWC-An BWI-An
TOC 1 0
Sulfate
CODMn
0
Sulfate
CODMn
-1
-1
-2
-2
Chloride
TN
Chloride
TN
NH4-N
NH4-N
BWM-A BWC-A BWI-A
K
BWM-An BWC-An BWI-An
Cd 2
K
Cr
0
Cr
-2
-4
Pb
-4
Na
Pb
-6
-6
Ca
Zn
Mg
Cd 2 0
-2
Na
TOC 1
Cu
Ca
Zn
Mg
Cu
Fig. 8. Relative pollutant loads generated from shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI) under aerobic or anaerobic condition. (a) Organic and inorganic constituents under aerobic conditions; (b) organic and inorganic constituents under anaerobic conditions; (c) metals under aerobic condition; (d) metals under anaerobic condition.
in leachate were lowest in the aerobic BWC column. Considering that the organic matter of BWC was higher than in BWI, pollutant loads into leachate was fairly reduced in the
BWC column due to microbial activity. The total release of sulfate and chloride was similar in all columns regardless of aerobic or anaerobic conditions.
14
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
As illustrated in Fig. 8b, the leaching of heavy metals from BWC showed a different tendency between aerobic and anaerobic conditions. In previous research on metal leaching by JLT-13 (Hwang et al., in press), carbonization had a remarkable suppression effect on the leaching of heavy metals such as Cr, Cd, and Pb. However, the release of Cd from carbonization residue obviously increased under the aerobic conditions. The leaching of Cd from BWC measured 43 lg/kg in the aerobic column and 4 lg/ kg in the anaerobic column (Table 4), i.e., a ten-fold difference; however, both amounts were actually very low. Under anaerobic conditions, the amount of metal released from BWC was nearly equal to that released from BWI, except for Zn and Mg. These results indicate that metal immobilization of carbonization residue can be obtained by physical adsorption onto the surface or pore structure but such effect may vary in some extent depending on landfill environment such as aerobic or anaerobic conditions. As shown in Fig. 8, environmental load from carbonization into leachate was so low as to be comparable to that of incineration ash assuming that 1 t of shredded residue was disposed in landfill after thermal treatment. From these results, carbonization can be considered as one of the thermal pretreatment methods for organic wastes. 4. Conclusions Carbonization was considered as a thermal pretreatment method for landfilling. Approximately 83% of organic matter included in the shredded residue of bulky waste was decomposed by carbonization. Releasing of soluble organic carbon into filtrate was decreased by carbonization in batch leaching tests at L/S = 10, even though the level was twice that of incineration ash. Moreover, carbonization showed a remarkable effect on the immobilization of metals such as chromium, cadmium, and lead. In column tests, the discharge of organic constituents into leachate was lowest in the aerobic column of carbonization residue (BWC-A) owing to microbial activity. The release of ammonia nitrogen was also minimized in the BWC-A column. However, leaching characteristics of heavy metals varied with aerobic and anaerobic conditions. The leaching of heavy metals such as Cd, Cr, Pb, and Cu appeared to be suppressed under anaerobic conditions; however, suppression effect of Cd became weaker under aerobic conditions. Metal immobilization can be obtained by physical adsorption onto the surface or pore structure of carbonization residue but such effect is considered to vary depending on landfill environment such as aerobic or anaerobic conditions. The potential releasing of organic and inorganic constituents from organic wastes could be reduced by carbonization. Furthermore, the total releasing amount of organic and inorganic constituents from carbonization residue is
so low that it is nearly equal to that of incineration ash. This result shows carbonization can take a role as one of the thermal pretreatment methods for landfilling. Acknowledgments This work was performed with financial support from the Ministry of Environment, Japan.
References Bertolini, G., 2003. Worldwide variations in municipal waste combustion and the reasons for them. Waste Management World, 31–37, November–December. Caballero, J.A., Front, R., Marcilla, A., Conesa, J.A., 1997. Characterization of sewage sludges by primary and secondary pyrolysis. Journal of Analytical and Applied Pyrolysis 40–41, 433–450. Environment Agency of Japan, 1973. Environment Agency Notification No. 13 for measuring method for metals in industrial wastes, Japan. Fricke, K., Santen, H., Wallman, R., 2005. Comparison of selected aerobic and anaerobic procedures for MSW treatment. Waste Management 25, 799–810. Honda, I., Seno, M., Ochiai, Y., Shibata, A., Nozaki, M., 1993. Production of activated carbon from waste ion exchange resin. In: Proceedings of Fourth Annual Conference of the Japan Society of Waste Management Experts, pp. 285–288. Hwang, I.H., Matsuto, T., Tanaka, N., Sasaki, Y., Tanaami, K. Characterization of char derived from various types of solid wastes from the standpoint of fuel recovery and pretreatment before landfilling. Waste Manaement, in press, doi:10.1016/j.wasman.2006.05.013. Inguanzo, M., Domı´nguez, A., Mene´ndez, J.A., Blanco, C.G., Pis, J.J., 2002. On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions. Journal of Analytical and Applied Pyrolysis 63, 209–222. JSAC, 1994. Water Analysis, fourth ed. The Japan Society for Analytical Chemistry, Hokkaido Branch, Japan. Kim, S.-Y., Tanaka, N., Matsuto, T., Tojo, Y., 2005. Leaching behaviour of elements and evaluation of pre-treatment methods for municipal solid waste incinerator residues in column leaching tests. Waste Management and Research 23, 220–229. Kistler, R.C., Widmer, F., Brunner, P.H., 1987. Behavior of chromium, nickel, copper, zinc, cadmium, mercury, and lead during the pyrolysis of sewage sludge. Environmental Science and Technology 21, 704–708. Lu, G.Q., Low, J.C.F., Liu, C.Y., Lua, A.C., 1995. Surface area development of sewage sludge during pyrolysis. Fuel 74, 344–348. Sainz-Diaz, C.I., Griffiths, A.J., 2000. Activated carbon from solid wastes using a pilot-scale batch flaming pyrolyzer. Fuel 79, 1863–1871. Tanaka, N., Koyama, K., 1987. Study on production process of soluble pollutants in anaerobic landfill layer of municipal solid waste. Journal of the Japan Society of Civil Engineers 381, 217–225. Tchobanoglous, G., Theisen, H., Vigil, S.A., 1993. Integrated Solid Waste Management. McGraw-Hill, p. 386. USEPA, 1996. Test Methods for Evaluating Solid Waste, Physical/ Chemical Methods SW-846 Method 3052: Microwave assisted acid digestion of siliceous and organically based matrices. Office of Solid Waste’s manual of EPA-approved analytical methods, Washington, DC. Van der Sloot, H.A., 1996. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. Waste Management 16, 65–81.
Waste Management 28 (2008) 3–14 www.elsevier.com/locate/wasman
Evaluation of carbonization as a thermal pretreatment method for landfilling by column leaching tests I.H. Hwang, T. Matsuto
*
Laboratory of Solid Waste Disposal Engineering, Graduate School of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan Accepted 8 November 2006 Available online 30 January 2007
Abstract To evaluate carbonization as a thermal pretreatment method for landfilling, the releasing characteristics of organic and inorganic constituents from carbonization residue derived from shredded residue of bulky waste was investigated by means of batch and column leaching tests. Shredded residue of bulky waste itself and its incineration ash were tested together to compare pretreatment methods. In batch leaching tests at a liquid/solid ratio of 10, the release of organic carbon from carbonization residue was at a remarkably low level. Besides, carbonization contributed to immobilize heavy metals such as chromium, cadmium, and lead within its residue. In column tests, the discharges of organic constituents were lowest from carbonization residue under aerobic conditions due to microbial activity. The leaching of Cd, Cr, Pb, and Cu from carbonization residue was suppressed under anaerobic conditions; however, this suppression effect tended to be weaker under aerobic conditions. From the results showing that the total releasing amounts of organic and inorganic constituents from carbonization residue are so low as to be comparable to that of incineration ash, carbonization can be considered as one of the thermal pretreatment methods of organic wastes. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The early stabilization of landfill waste is one of the most important issues in solid waste management. In general, mechanical, biological, and thermal pretreatment methods are considered feasible ways to hasten landfill stabilization. The objectives of those pretreatment methods are summarized as follows (Fricke et al., 2005): (1) the reduction of waste volume and mass to secure landfill capacity; (2) the control of biological and chemical reactions to restrain gas and leachate generation; and (3) the immobilization of contaminants to prevent pollutants from migrating to the environment. Most countries in the European Union have recommended the mechanical–biological treatment of biodegradable waste prior to landfilling. On the other hand, several *
Corresponding author. Tel./fax: +81 11 706 6828. E-mail address: [email protected] (T. Matsuto).
0956-053X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2006.11.007
countries such as Switzerland, Japan, and Sweden have introduced municipal solid waste incinerators (MSWIs) for the thermal treatment of combustible waste. Incineration is considered to be remarkably effective at reducing waste volume and increasing sanitation. In particular, it can be one of the best options for countries having difficulties in securing proper sites for final disposal. Nevertheless, critical issues such as the emission of secondary pollutants and the high cost for their control still remain following incineration. Bertolini (2003) noted that the gross national product of countries treating more than 50% of waste at MSWIs corresponded to more than USD 10,000 per capita. Carbonization is a thermal process that produces carbonaceous materials commonly called ‘‘char’’. At actual carbonization plants for the treatment of solid wastes, rotary kilns or furnaces are operated in the range of 400–500 C. During the heating process under an inert atmosphere, moisture and volatiles are exhausted in their gas phases, whereas fixed carbon, ash, and some volatile matter form
4
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
residual materials with porous surfaces. In some facilities, evolved gas is recirculated in the carbonization system to be used as a subsidized fuel or atmospheric gas. Kistler et al. (1987) concluded that carbonization might have a positive effect on the immobility of heavy metals due to the high H+ buffering capacity and the alkaline nature of the slaked char. Honda et al. (1993) reported that an iodine adsorptive capacity of 680–720 mg/g was achieved by carbonization of waste ion exchange resin. Lu et al. (1995) noted that heavy metals, except mercury, are safely enclosed in the solid residues by pyrolysis. Caballero et al. (1997) and Inguanzo et al. (2002) suggested the pyrolysis of sludge be an alternative to incineration because it concentrated the heavy metals in the final residue and avoided the formation of toxic organic compounds. Sainz-Diaz and Griffiths (2000) reported that straw and furniture waste chars produced at 500 C had 50 m2/g and 40 m2/g of BET surface, respectively. In our previous research (Hwang et al., in press), chars derived from various municipal and industrial wastes were subjected to batch leaching tests. The release of heavy metals such as chromium, cadmium, and lead was significantly reduced by carbonization regardless of waste type. Furthermore, carbonization could degrade a considerable amount of organic matter in raw waste. Based on these results, we are going to investigate carbonization residue as a thermally pretreated material for landfilling using column tests. Column leaching tests have been used to simulate the leaching behavior of contaminants over the short, medium, and long term by relating the release, expressed as mg/kg leached, to the liquid/solid (L/S) ratio (Van der Sloot, 1996). Moreover, column tests provide information on the successive release characteristics of pollutants under the presence of microbial activity or given landfill conditions. Shredder residue of bulky waste discharged from households was chosen as the study material because of its high organic matter and metal content (Hwang et al., in press). ‘‘Char’’ will be referred to as ‘‘carbonization residue’’ in this study, because the objective of carbonization is not to recover fuel but to treat waste thermally before final disposal. In order to compare carbonization to other pretreatment methods, we also tested shredded residue of bulky waste itself as a mechanically treated waste and ash obtained from the incineration of shredded residue of bulky waste in parallel. The discharge of organic and inorganic constituents to leachate, as well as gas emissions, was measured to quantify environmental loads from shredded waste, incineration ash, and carbonization residue in an attempt to evaluate carbonization as a thermal pretreatment method. 2. Materials and methods 2.1. Materials The shredded residue of bulky waste collected from households was obtained from a shredding plant in Sap-
poro, Japan, in July 2004. Loaded bulky wastes were shredded by a hammer mill and then transported to a magnetic separator for ferrous metal recovery. The remaining residuals were subsequently divided into two fractions by a 50 mm screen. The fraction that passed through the screen was sampled as mechanically pretreated bulky waste (BWM). The composition of BWM was as follows: wood, 45%; plastics, 14.6%; paper, 1.6%; textiles, 0.6%; glass and porcelain, 0.2%; metals, 1.7%; and miscellaneous materials smaller than 2 mm, 36.3%. Approximately 50 kg of BWM were collected for the experiments and dried at 60 C until the moisture content reached a stable minimum. 2.2. Experiments 2.2.1. Carbonization and incineration About 10 kg of dried BWM were put into a rotary kiln type of reactor (B150 mm · 1200 mm) by a screw feeder to produce carbonization residues. The slope angle and rotation rate of the kiln were adjusted to 0.8 and 2 rpm, respectively, to maintain a retention time of 1 h. Temperature was 500 C and nitrogen was used as a carrier gas at a rate of 11 L/min to maintain a reducing atmosphere. On the other hand, about 25 kg of dried BWM was incinerated in the rotary kiln for 2 h to obtain incineration ash. The slope angle and rotation rate were set at 0.8 and 1 rpm, respectively. Incineration temperature was 600 C and air was introduced to the kiln at the rate of 50 L/min, taking a theoretical excess air ratio into consideration. After carbonization and incineration, the weight of carbonization residue (BWC) and incineration ash (BWI) was measured to estimate yields. 2.2.2. Composition of samples BWM was reduced in size to less than 1 mm using a laboratory-scale cutting mill. Carbonization residue (BWC) and incineration ash (BWI) were pulverized using a ball mill for 30 min to ensure homogeneity in the batch tests, and a sample of the fraction smaller than 1 mm was taken. The organic matter content was determined by proximate analysis; however, for the incineration ash, it was measured by ignition loss at 600 C for 3 h. Carbon, hydrogen, and nitrogen were measured by elementary analyzer (CHN recorder MT-5, Yanaco Co.). Sulfur was absorbed in hydrogen peroxide solution during the incineration of sample and its concentration was measured by ion chromatography (DX-500, Dionex Co.). The microwave-assisted acid digestion method was used to decompose the solid matrix for measuring the metal content of samples (USEPA, 1996). 2.2.3. Batch leaching test The release of organic matter and metals from the sample was investigated using the Japanese leaching test No. 13 (Environment Agency of Japan, 1973). We immersed 10 g of homogenized sample in a flask containing 100 mL of distilled water, which was capped and shaken horizontally for 6 h at 200 rpm. The mixture was filtered using 1 lm pore filter paper, and the filtrate was prepared for measuring
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
pH and the concentration of total organic carbon (TOC) and metals such as cadmium, chromium, lead, zinc, copper, magnesium, calcium, sodium, and potassium. 2.2.4. Column test As illustrated in Fig. 1, two types of columns were designed to simulate aerobic and anaerobic conditions. Transparent acrylic plastic columns were packed with shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI). To facilitate microbial activity under the given aerobic or anaerobic conditions, supernatant obtained from a mixture of soil and distilled water was seeded into the aerobic columns, whereas supernatant from anaerobically digested sludge was added to the anaerobic columns. The initial moisture content of the packed samples was adjusted to 20% and the temperature was maintained at 30 C. Table 1 outlines the conditions for each column. The gas composition, i.e., H2, CO2, O2, and CH4, was checked through gas sampling ports, but the gas generation amount was measured only at anaerobic columns. Every two weeks, 200 mL of distilled water were injected into each column to simulate rainfall. To prevent air permeation, micro-tubing pumps were used to sprinkle distilled water into the anaerobic columns. After 24 h, leachate was collected in a flask and weighed using an electric balance to confirm leachate quantity. The upper and lower parts of the anaerobic column were purged by nitrogen to drive out any air that may have permeated the column during the sampling procedure.
The pH and oxidation–reduction potential (ORP) of the leachate were checked as quickly as possible. Each leachate was filtered using a 1 lm pore filter paper prior to measuring TOC, inorganic carbon (IC), 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (CODMn), total nitrogen (TN), ammonia nitrogen (NH4–N), chloride (Cl), and sulfate ðSO2 4 Þ. To measure metal concentrations (Cd, Cr, Pb, Zn, Cu, Mg, Ca, Na, and K), the leachate was filtered through a 0.45 lm pore filter paper and acidified by nitric acid. The entire experimental scheme is presented in Fig. 2. 2.3. Analytical methods and instruments The concentrations of H2, CO2, O2, and CH4 were analyzed using gas chromatography with a thermal conductivity detector (MTI M200D for H2, column type: WG-100, flow rate of Ar: 33 mL/min; detector temperature: 50 C; Hitachi Type 164 for O2, CO2, and CH4, column type: WG-100, flow rate of He: 33 mL/min; detector temperature: 50 C). The pH and ORP were measured using the glass electrode method. The concentrations of TOC, IC, and TN were measured using a simultaneous TOC-TN analyzer (TOC-V CPH/CPN, Shimadzu Co.). BOD5 and CODMn analysis followed the methods of the Japan Society for Analytical Chemistry (JSAC, 1994). The concentration of NH4–N was measured spectrophotometrically using the indophenol method. The concentrations of chloride and sulfate were measured by ion chromatography (TOSOH
b d f c
a
e
Open h 1
2 3
g i
j
a: Mass cylinder b: Micro-tubing pump c: Distilled water injection port d: Nitrogen purge bag e: Gas sampling port for anaerobic column f: Gas volume measuring port g: Gas holder h: Glass wool and bead layer i: Glass wool and mesh layer j: Gas sampling ports for aerobic column k: Cock l: Silicon cap m: Leachate holder
k Open
l m
Anaerobic column
5
Aerobic column
Fig. 1. Experimental apparatus for column tests under anaerobic and aerobic conditions.
6
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
Table 1 Column settings BWM-A Packing material Atmosphere Bed volumea (L) Weight of packing material (kg-wet) Packing density (kg/m3) Moisture content (wt%) a
BWM-An
BWC-A
BWC-An
BWI-A
BWI-An
Shredded residue of bulky waste collected from households Aerobic Anaerobic 2.67 2.67 1.20 1.20
Carbonization residue derived from BWM Aerobic Anaerobic 2.67 2.67 1.24 1.24
Incineration ash obtained from BWM Aerobic Anaerobic 2.67 2.67 4.93 4.04
449 20
464 20
1846 20
449 20
464 20
1513 20
D = 10.8 cm; H = 20.9 cm.
reduction conditions. The yields of carbonization residue (BWC) and incineration ash (BWI) obtained from BWM were about 43% and 36.5%, respectively (Table 2).
Drying at 60oC for 24 h of shredded residue of bulky waste collected from households
Mechanicallytreated waste (BWM)
Incineration at 600˚C for 2 h in air
Carbonization residue (BWC)
Incineration ash (BWI)
Batch test Proximate analysis/Ignition loss Metal content Leaching test
Column test
Aerobic condition
Anaerobic condition
Generation amount of gas and leachate Composition of gas and leachate
Fig. 2. Entire experimental scheme.
CM-8). The concentrations of Cd, Cr, and Pb were analyzed by graphite furnace atomic absorption spectrometry (AAS; Z-8200, Hitachi Co.), whereas the concentrations of Zn, Cu, Mg, Ca, and Na were analyzed using an inductively coupled plasma emission spectrometer (ICP-AES, Shimadzu Co.) Finally, K was measured using flame AAS (Z-8200, Hitachi Co.). 3. Results and discussion 3.1. Characteristics of BWM, BWC, and BWI As previously mentioned, BWM mainly consisted of slowly- or non-decomposable components such as wood and plastics, implying that BWM is significantly less biodegradable than other organic wastes found in MSW. Fig. 3 shows that a large amount of organic matter was consumed by thermal treatment under either oxidation or
Weight percent (wt%-dry)
100
Carbonization at 500˚C for 1 h in nitrogen
Ash Fixed carbon Volatile matter Ignition loss
80
60
40
20
0
BWM
BWC
BWI
Fig. 3. Yield and composition of shredded residue of bulky waste (BWM), carbonization residue (BWC) and incineration ash (BWI).
Table 2 Total yield, organic matter, carbon, hydrogen, nitrogen, sulfur, and metal content of shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI) a
Total yield (wt%) Organic matter (wt%-dry) C (wt%-dry) H (wt%-dry) N (wt%-dry) S (wt%-dry)
BWM
BWC
BWI
100.0 73.4b 40.4 5.0 19.9 0.2
43.0 28.2b 22.4 1.8 0.6 0.1
36.5 11.4c – – – –
14.85 8.42 6299 4559 1792 4854 15,509 10,701 3582
29.60 36.55 7693 7562 7234 6446 27,499 6841 2750
Metal content (mg/kg-dry sample) Cd 26.47 Cr 9.91 Pb 4518 Zn 1884 Cu 2746 Ma 1752 Ca 10,878 Na 2182 K 1101
a Determined based on the weight of shredded residue of bulky waste (BWM). b Determined by summation of fixed carbon and volatile matter. c Determined by ignition loss at 600 C for 1 h.
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
The organic matter content of BWM, BWC, and BWI was 73.4%, 28.2%, and 11.4%, respectively, based on dried sample weights (Table 2). If the variation in ash amount with thermal treatment is negligible, approximately 83% and 94% of the organic matter in BWM was degraded by carbonization and incineration, respectively. 3.2. Batch leaching test 3.2.1. Release of organic matter The concentration of TOC released from BWM, BWC, and BWI during leaching tests at L/S = 10 (JLT-13) corresponded to 339.4, 58.4, and 27.9 mg/L, respectively, as presented in Table 3, indicating that the release of organic carbon from carbonization residue was at a remarkably low level, even though the level was twice that of incineration ash. 3.2.2. pH and metal releasing The pH and metal concentrations of the filtrate obtained from the leaching test at L/S = 10 (JLT-13) are shown in Table 3. The pH of BWM and BWC was 7.5 and 8.4, respectively; however, the pH of BWI was 10.4. Carbonization and incineration suppressed the leaching of heavy metals from their residues (Table 3). Heavy metals seemed to be well suppressed in BWC and BWI. Cd and Pb were not detected in BWI filtrate and Cr was not detected in BWC filtrate. Cd, Pb, and Zn leaching from BWC measured 0.055, 1.032, and 10.22 mg/kg-dry BWC, respectively; these levels are considerably lower compared to those from BWM. On the other hand, the alkali and alkali earth metals, except Mg, showed a tendency to release readily, regardless of pretreatment. Mg leaching level was conspicuously low in BWI. The high acid-buffering pH of incineration ash and the surface properties of carbonization residue could be factors in the reduction of metal release.
Table 3 The pH, TOC, and metal leaching content of filtrate obtained by the application of JLT-13 to shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI)
pH TOC (mg/L)
BWM
BWC
BWI
7.5 339.4
8.4 58.4
10.4 27.9
0.055 ND 1.032 10.22 0.15 129 1464 1022 316
ND 0.187 ND 0.07 0.22 7.108 2106 1012 142
Metal leaching content (mg/kg-dry sample) Cd 0.685 Cr 0.198 Pb 7.198 Zn 55.48 Cu 4.65 Mg 195 Ca 517 Na 1111 K 637 ND: not detected.
7
3.3. Column tests 3.3.1. Gas emissions and composition The O2 concentrations of three sampling ports averaged 18.6 ± 0.2% and 20.6 ± 0.0% in the aerobic BWM and BWC columns, respectively, without significant variation among sampling points. In the aerobic BWI column, however, O2 concentrations at ports 1, 2, and 3 were 19.3%, 17.6%, and 14.8%, respectively, even though both the upper and lower parts of the column were open to the atmosphere. Leachate naturally migrated downward, which might fill apertures in the lower part of the bed and obstruct air penetration to inside layers. The generation of CO2 was most active in the BWM column but it was not detected in the BWI column. Generated CO2 may have been absorbed into the BWI bed and formed calcite (CaCO3) by reacting with calcium ions (Kim et al., 2005). Even if microbial activity existed, it may have been inhibited by the high pH and the lack of substrate in incineration ash. In the anaerobic BWM column, H2 and CO2 were actively generated in the initial stage, and acid fermentation continued for 6 mo (Fig. 4a). CH4 emission was observed from the seventh month since the start of the column experiments. The primary reason for delayed methane fermentation may have been related to the physical composition of BWM, which consisted of slowly- or non-biodegradable components. The shortage of readily available substrate might bring about retardation of methane fermentation in the anaerobic BWM column. On the other hand, weak acid fermentation was maintained in the anaerobic BWC column over 10 mo without CH4 generation (Fig. 4b). This phenomenon appeared to be caused by substrate deficiency too. In the BWI anaerobic column, a very small, insignificant amount of CO2 was generated after 1 mo (Fig. 4c) and then it was hardly detected. H2 was continuously emitted; however, it may have resulted from hydration reactions of alkaline metals, such as aluminum and iron, rather than acid fermentation (Kim et al., 2005). Carbon emissions in the forms of CO2 and CH4 were 907 mg C/kg-dry BWM, 138 mg C/kg-dry BWC, and 4.1 mg C/kg-dry BWI under anaerobic conditions. These results indicated that gas generation amount from BWC and BWI was not much compared to that from BWM under anaerobic conditions. Because the aerobic columns were designed to be open to the atmosphere, the quantitative analysis of gas emissions was not possible although CO2 concentration was measured at three gas sampling ports. 3.3.2. Variation in pH, ORP, and concentrations of organic and inorganic constituents versus L/S Figs. 5–7 show the variation in pH, ORP, and releasing of carbon, nitrogen, chloride, and sulfate from BWM, BWC, and BWI under aerobic or anaerobic conditions versus L/S ratios, which were plotted on semi-logarithm graphs over the entire experimental period.
8
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
a
6
Gas generation amount (mmol/kg-dry)
H2 CO2
5
CH4 4
3
2
1
0 0
10
20
30
40
b
1.2
Gas genearation amount (mmol/kg-dry)
Week
1.0
H2 CO2
0.8
0.6
0.4
0.2
0.0 0
10
20
30
40
Week
c
4
Gas generation amount (mmol/kg-dry)
H2 CO2 3
2
1
0 0
10
20
30
40
Week Fig. 4. Generated gas amounts and composition in anaerobic columns. (a) shredded residue of bulky waste (BWM); (b) carbonization residue (BWC); (c) and incineration ash (BWI).
The variation in pH was not significant; pH did change slowly but steadily with packing sample and aerobic or anaerobic conditions. For BWM, the pH values for leachate obtained from the aerobic and anaerobic columns ranged from 7.3 to 8.4 and 6.1 to 7.6, respectively. IC concentration is deeply related to chemical equilibrium with pH. In the anaerobic BWM column, IC gradually decreased from 50 to 16 mg/L. In the same period, pH dropped slightly, and methane fermentation subsequently started slowly from L/S = 2.5. Owing to
an accumulation of organic acids, the pH of leachate obtained from anaerobic columns showed a gradual decreasing tendency. In BWC columns, the pH of leachate under aerobic and anaerobic conditions corresponded to 7.2–7.9 and 7.5–8.4, respectively. On the other hand, the pH of leachate collected from BWI columns ranged from 8.1 to 10.1 and 9.9 to 10.7 under aerobic and anaerobic conditions, respectively. As aerobic columns were always exposed to air, it seems that atmospheric CO2 affected the neutralization of highly alkaline incineration ash. The ORP of aerobic and anaerobic leachate differed by approximately 50–200 mV (Eh). As shown in Figs. 5–7, none of the anaerobic leachates could reach negative values despite efforts to develop anaerobic conditions excluding air permeation. Because ORP was measured 24 h after the start of water injection, the results may not represent the true redox conditions of the inner beds, especially in the anaerobic columns. In the aerobic BWM and BWC columns, BOD5 decreased below 10–30 mg/L around L/S = 1. Easily biodegradable organic matter is generally decomposed into CO2 by aerobic microorganisms, whereas it is first accumulated as organic acids in leachate under anaerobic conditions. Thus, TOC concentrations of leachate obtained from aerobic BWM and BWC columns were lower than those from anaerobic ones. In the BWM columns, TOC concentrations under anaerobic conditions were about 5– 10 times higher than those under aerobic conditions. However, such differences were not observed in aerobic and anaerobic BWI columns. Organic constituents seemed to be regularly washed out without rapid declines in BOD5 at the initial stages, unlike the patterns observed in the BWM and BWC aerobic columns. The TN concentration showed a similar declining trend as did TOC concentration. In particular, the NH4–N concentration rapidly decreased with reduced BOD5 for the initial 3–4 mo in the aerobic BWM and BWC columns. This may have been the result of assimilation by aerobic microorganisms or by nitrification of ammonia. However, the NH4–N concentration increased again after L/ S = 1.5, indicating that the latter explanation would not be responsible for the initial drop in NH4–N. With the exception of the above aerobic BWM and BWC columns, NH4–N concentrations showed a similar decreasing trend as TN concentration. Chloride decreased linearly with L/S ratio. Sulfate was discharged at a constant concentration from aerobic BWC and aerobic and anaerobic BWI columns, indicating that sulfate is dissolved in leachate under the given conditions. On the other hand, in the anaerobic BWM column, sulfate concentrations dropped sharply below 20 mg/L after L/S = 2. This may have represented a reduction of sulfate; however, the ORP value was too high relative to reference data. Generally, the reduction of sulfate occurs in the range of 50 to 100 mV (Eh) (Tchobanoglous et al., 1993).
450
8.0
400 350
7.5
300 250
7.0
200 150
6.5 pH
100
ORP
6.0
b
TOC CODMn BOD5 IC
450
ORP
400 350
7.5
300 250
7.0
200 150 100 50
105
TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
TN NH-4-N Cl 2SO4
104
Concentration (mg/L)
Concentration (mg/L)
pH
8.0
6.0
104
103
102
101
103
102
101
100
100
10-1 104
10-1 104
Cd Cr Pb
Zn Cu Mg
Ca Na K
Cd Cr Pb
103
103
102
102
Concentration (mg/L)
Concentration (mg/L)
8.5
6.5
50
105
9
ORP (mV, Eh)
8.5
pH
pH
a
ORP (mV, Eh)
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
101
100
10-1
101
100
10-1
10-2
10-2
10-3
10-3
10-4
Ca Na K
Zn Cu Mg
10-4
0
1
2
3
4
5
L/S
0
1
2
3
4
5
L/S
Fig. 5. Variation in quality of leachate obtained from shredded residue of bulky waste (BWM). (a) Aerobic; (b) anaerobic.
3.3.3. Variation in metal leaching versus L/S As shown in Figs. 5–7, alkali and alkaline earth metals such as Na, K, Mg, and Ca were washed out at a constant rate by regular water injection. However, heavy metals such as Cd, Cr, Pb, Zn, and Cu showed different leaching trends depending on packing material, pH, and aerobic or anaerobic conditions. In BWI columns, Cd was not detected after L/S = 0.4 under either aerobic or anaerobic conditions under
highly alkaline conditions of pH 9–10. In BWM and BWC columns, Cd release rapidly decreased, together with sulfate, under anaerobic conditions. From the abrupt drop in sulfate concentrations in the anaerobic BWM column, it was assumed that Cd precipitated by forming insoluble metal sulfate as follows: Cd2+ + S2 M CdS (s). In BWM and BWI columns, the leaching concentration of Cr was higher as pH of leachate became lower regardless
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
b
450 400
8.0
pH
350 300
7.5
250 pH
200
ORP
7.0
8.5
400 350 300
7.5
250 200
150
7.0
150
105 TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
TOC CODMn BOD5 IC
104
TN NH-4-N Cl SO42-
104
Concentration (mg/L)
Concentration (mg/L)
ORP
8.0
105
103
102
101
103
102
101
100
100
10-1 104
10-1 104 Cd Cr Pb
Ca Na K
Zn Cu Mg
Cd Cr Pb
103
103
102
102
Concentration (mg/L)
Concentration (mg/L)
450 pH
ORP (mV, Eh)
8.5
ORP (mV, Eh)
a
pH
10
101
100
10-1
101
100
10-1
10-2
10-2
10-3
10-3
10-4
Ca Na K
Zn Cu Mg
10-4 0
1
2
3
4
5
L/S
0
1
2
3
4
5
L/S
Fig. 6. Variation in quality of leachate obtained from carbonization residue (BWC). (a) Aerobic; (b) anaerobic.
of aerobic or anaerobic conditions. Unlike the leaching test at L/S = 10, Cr was detected in leachate from BWC columns. In particular, Pb and Zn were sensitive to pH variation, so that they were detected at high concentrations under conditions of relatively low pH. In the highly alkaline BWI column of pH 9–10, the leaching concentrations
of both metals were remarkably low relative to other columns; moreover, Pb was not detected from L/S = 0.6. However, the release of Cu was clearly greater under aerobic conditions in the BWM and BWC columns. In the BWI columns, the release of Cu declined constantly because of regular washing out. The concentration of Cu was slightly
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
200 8
150 pH
ORP
7
200
ORP
7
104
104
103
103
Concentration (mg/L)
105
100
102
101
100 TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
TN NH-4-N Cl 2SO4
TOC CODMn BOD5 IC
10-1 104
103
103
102
102
Concentration (mg/L)
10-1 104
101
100
10-1
10-2
101
100
10-1
10-2
10-3
10
150 pH
100
Concentration (mg/L)
250
8
100
101
300
9
105
102
350
ORP (mV, Eh)
pH
250 9
11 10
pH
300
10
Concentration (mg/L)
b
350
11
ORP (mV, Eh)
a
11
10-3 Cd Cr Pb
-4
0.0
0.2
Ca Na K
Zn Cu Mg
0.4
0.6
Cd Cr Pb
10-4 0.8
1.0
1.2
L/S
0.0
0.2
0.4
Ca Na K
Zn Cu Mg
0.6
0.8
1.0
1.2
1.4
L/S
Fig. 7. Variation in leachate quality obtained from incineration ash (BWI). (a) Aerobic; (b) anaerobic.
higher in the anaerobic column, but this difference appears to have been due to differences in initial Cu contents. 3.3.4. Comparison of washing-out rate of pollutants in column tests As illustrated in Figs. 5–7, logarithmic concentrations of non-reactive components such as Cl, Na, K, Mg, and Ca
decreased at constant rates, except for the initial unsaturated period. Tanaka and Koyama (1987) noted that the logarithmic concentration of non-reactive constituents was linearly decreased with cumulative leachate amount when a constant amount of water was injected into the column at regular intervals. Therefore, the future concentration of a pollutant removed at a constant rate by regular
12
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
washing out can be estimated by determining the washingout ratio from the slope. The concentrations of TOC, CODMn, BOD5, and TN also exhibited a constant decreasing trend versus cumulative L/S ratio. However, some constituents occasionally had zero or positive slopes, implying that their concentrations are governed by solubility under some conditions or that certain generation reactions occur. For example, there were sulfates in the BWI columns (Fig. 7a and b) and NH4–N in the aerobic BWM columns (Fig. 5a). The washing out of organic constituents appears to occur later than that of chloride. On the other hand, the release of heavy metals such as Cd, Cr, Pb, and Cu could not simply be described by the washing-out effect. 3.3.5. Comparison of metal leaching patterns between batch and column tests Table 4 shows the total amount of pollutants leached from each column during the 10 mo experiment period. Comparing the metal leaching amounts obtained by batch tests (JLT-13) in Table 3 with the cumulative amount of metal leaching from column tests in Table 4, the metal leaching characteristics were classified into the four following patterns: (1) pattern 1 (Batch > Column): the cumulative metal leaching amount in the column is relatively small and not expected to reach the leaching amount of the batch test at L/S = 10, even if the cumulative L/S in the column test reaches 10; (2) pattern 2 (Batch Column): the cumulative metal leaching amount is expected to converge with the leaching amount of the batch test when the cumulative L/S becomes 10; (3) pattern 3 (Batch < Column): the cumulative metal leaching amount considerably exceeds the leaching amount of the batch test at L/S = 10; and (4) pattern 4: metal is not detected in the batch test but is detected in the column test. A batch leaching test in the condition of L/S = 10 (JLT13) is generally performed as a standard method to determine stability of material in landfill. However, this kind of batch test is done without consideration of microbial activity,
aerobic or anaerobic conditions in landfill, etc. Thus, it is necessary to compare metal releasing results from batch test and column test depending on BWM, BWC, and BWI and aerobic or anaerobic conditions. Table 5 shows the four kinds of metal leaching patterns between the batch and column tests. Release of Cd, Cr, and Pb was characterized depending on material rather than aerobic or anaerobic conditions; their releasing from BWM was grouped into pattern 1 except that Cr under anaerobic conditions was classified into pattern 2. In case of BWC, Cd and Pb showed pattern 1. Cr was not detected in batch test but it released in column test, accordingly, it was classified into pattern 4. Opposite leaching patterns were observed for BWI, that is, Cd and Pb were classified into pattern 4 but Cr presented pattern 1. On the other hand, the releasing amount of Zn and Cu seems to vary with aerobic or anaerobic conditions even though same materials (Table 5). Finally, alkali or alkali earth metals were mainly classified in pattern 2 excepting four cases: Mg of BWC and BWI (pattern 1), Ca of BWM (pattern 3), and K of BWI (pattern 3). 3.4. Quantitative comparison of environmental loads into leachate depending on pretreatment methods Using the column data in Table 4, the total pollutant leaching amount of organic and inorganic constituents could be estimated according to the pretreatment method, as well as the aerobic and anaerobic conditions. Fig. 8 exhibits the relative potential loads of pollutants discharging into leachate assuming 1 t of bulky waste was disposed in landfills after shredding, carbonization, and incineration, respectively. The amounts of pollutant released from columns were normalized based on the result of anaerobic BWM, which were plotted on logarithm radar charts. As presented in Fig. 8a, the load of organic and inorganic constituents into leachate rapidly diminished under aerobic conditions. TOC, TN, CODMn, and NH4–N levels
Table 4 Cumulative leaching amounts of pollutants over the 10 months of column tests (unit: mg/kg-dry sample) Items
BWM-A
BWM-An
BWC-A
BWC-An
BWI-A
BWI-An
TOC IC CODMn TN NH4–N Cl SO2 4 Cd Cr Pb Zn Cu Mg Ca Na K
916 150 941 124 14.7 1508 1531 0.021 0.056 0.146 11.1 0.881 89.6 806 785 362
5337 106 3793 588 128 1774 1657 0.069 0.161 0.303 92.2 0.180 197 1938 928 548
266 27.7 181 30.3 8.73 4179 1660 0.043 0.016 0.114 18.5 1.02 87.9 1906 1081 399
645 156 249 59.7 22.8 4197 2509 0.004 0.018 0.115 6.23 0.045 114 2487 1087 484
494 9.97 235 70.3 32.5 5087 1564 0.001 0.017 0.022 0.023 0.202 1.13 1664 1981 705
548 12.5 299 76.6 39.4 5395 2246 0.004 0.013 0.069 0.299 0.738 1.97 2286 2492 730
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
13
Table 5 Four kinds of metal leaching patterns determined by batch and column tests
Cd Cr Pb Zn Cu Mg Ca Na K Pattern Pattern Pattern Pattern
1 2 3 4
BWM-A
BWM-An
BWC-A
BWC-An
BWI-A
BWI-An
s s s s s – – –
s – s s – – –
s * s – s – – –
s * s s s s – – –
* s * s – s – –
* s * s – –
(s): Batch > Column. (–): Batch Column. (): Batch < Column. (*): Not detected in batch test but detected in column test.
BWM-A BWC-A BWI-A
BWM-An BWC-An BWI-An
TOC 1 0
Sulfate
CODMn
0
Sulfate
CODMn
-1
-1
-2
-2
Chloride
TN
Chloride
TN
NH4-N
NH4-N
BWM-A BWC-A BWI-A
K
BWM-An BWC-An BWI-An
Cd 2
K
Cr
0
Cr
-2
-4
Pb
-4
Na
Pb
-6
-6
Ca
Zn
Mg
Cd 2 0
-2
Na
TOC 1
Cu
Ca
Zn
Mg
Cu
Fig. 8. Relative pollutant loads generated from shredded residue of bulky waste (BWM), carbonization residue (BWC), and incineration ash (BWI) under aerobic or anaerobic condition. (a) Organic and inorganic constituents under aerobic conditions; (b) organic and inorganic constituents under anaerobic conditions; (c) metals under aerobic condition; (d) metals under anaerobic condition.
in leachate were lowest in the aerobic BWC column. Considering that the organic matter of BWC was higher than in BWI, pollutant loads into leachate was fairly reduced in the
BWC column due to microbial activity. The total release of sulfate and chloride was similar in all columns regardless of aerobic or anaerobic conditions.
14
I.H. Hwang, T. Matsuto / Waste Management 28 (2008) 3–14
As illustrated in Fig. 8b, the leaching of heavy metals from BWC showed a different tendency between aerobic and anaerobic conditions. In previous research on metal leaching by JLT-13 (Hwang et al., in press), carbonization had a remarkable suppression effect on the leaching of heavy metals such as Cr, Cd, and Pb. However, the release of Cd from carbonization residue obviously increased under the aerobic conditions. The leaching of Cd from BWC measured 43 lg/kg in the aerobic column and 4 lg/ kg in the anaerobic column (Table 4), i.e., a ten-fold difference; however, both amounts were actually very low. Under anaerobic conditions, the amount of metal released from BWC was nearly equal to that released from BWI, except for Zn and Mg. These results indicate that metal immobilization of carbonization residue can be obtained by physical adsorption onto the surface or pore structure but such effect may vary in some extent depending on landfill environment such as aerobic or anaerobic conditions. As shown in Fig. 8, environmental load from carbonization into leachate was so low as to be comparable to that of incineration ash assuming that 1 t of shredded residue was disposed in landfill after thermal treatment. From these results, carbonization can be considered as one of the thermal pretreatment methods for organic wastes. 4. Conclusions Carbonization was considered as a thermal pretreatment method for landfilling. Approximately 83% of organic matter included in the shredded residue of bulky waste was decomposed by carbonization. Releasing of soluble organic carbon into filtrate was decreased by carbonization in batch leaching tests at L/S = 10, even though the level was twice that of incineration ash. Moreover, carbonization showed a remarkable effect on the immobilization of metals such as chromium, cadmium, and lead. In column tests, the discharge of organic constituents into leachate was lowest in the aerobic column of carbonization residue (BWC-A) owing to microbial activity. The release of ammonia nitrogen was also minimized in the BWC-A column. However, leaching characteristics of heavy metals varied with aerobic and anaerobic conditions. The leaching of heavy metals such as Cd, Cr, Pb, and Cu appeared to be suppressed under anaerobic conditions; however, suppression effect of Cd became weaker under aerobic conditions. Metal immobilization can be obtained by physical adsorption onto the surface or pore structure of carbonization residue but such effect is considered to vary depending on landfill environment such as aerobic or anaerobic conditions. The potential releasing of organic and inorganic constituents from organic wastes could be reduced by carbonization. Furthermore, the total releasing amount of organic and inorganic constituents from carbonization residue is
so low that it is nearly equal to that of incineration ash. This result shows carbonization can take a role as one of the thermal pretreatment methods for landfilling. Acknowledgments This work was performed with financial support from the Ministry of Environment, Japan.
References Bertolini, G., 2003. Worldwide variations in municipal waste combustion and the reasons for them. Waste Management World, 31–37, November–December. Caballero, J.A., Front, R., Marcilla, A., Conesa, J.A., 1997. Characterization of sewage sludges by primary and secondary pyrolysis. Journal of Analytical and Applied Pyrolysis 40–41, 433–450. Environment Agency of Japan, 1973. Environment Agency Notification No. 13 for measuring method for metals in industrial wastes, Japan. Fricke, K., Santen, H., Wallman, R., 2005. Comparison of selected aerobic and anaerobic procedures for MSW treatment. Waste Management 25, 799–810. Honda, I., Seno, M., Ochiai, Y., Shibata, A., Nozaki, M., 1993. Production of activated carbon from waste ion exchange resin. In: Proceedings of Fourth Annual Conference of the Japan Society of Waste Management Experts, pp. 285–288. Hwang, I.H., Matsuto, T., Tanaka, N., Sasaki, Y., Tanaami, K. Characterization of char derived from various types of solid wastes from the standpoint of fuel recovery and pretreatment before landfilling. Waste Manaement, in press, doi:10.1016/j.wasman.2006.05.013. Inguanzo, M., Domı´nguez, A., Mene´ndez, J.A., Blanco, C.G., Pis, J.J., 2002. On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions. Journal of Analytical and Applied Pyrolysis 63, 209–222. JSAC, 1994. Water Analysis, fourth ed. The Japan Society for Analytical Chemistry, Hokkaido Branch, Japan. Kim, S.-Y., Tanaka, N., Matsuto, T., Tojo, Y., 2005. Leaching behaviour of elements and evaluation of pre-treatment methods for municipal solid waste incinerator residues in column leaching tests. Waste Management and Research 23, 220–229. Kistler, R.C., Widmer, F., Brunner, P.H., 1987. Behavior of chromium, nickel, copper, zinc, cadmium, mercury, and lead during the pyrolysis of sewage sludge. Environmental Science and Technology 21, 704–708. Lu, G.Q., Low, J.C.F., Liu, C.Y., Lua, A.C., 1995. Surface area development of sewage sludge during pyrolysis. Fuel 74, 344–348. Sainz-Diaz, C.I., Griffiths, A.J., 2000. Activated carbon from solid wastes using a pilot-scale batch flaming pyrolyzer. Fuel 79, 1863–1871. Tanaka, N., Koyama, K., 1987. Study on production process of soluble pollutants in anaerobic landfill layer of municipal solid waste. Journal of the Japan Society of Civil Engineers 381, 217–225. Tchobanoglous, G., Theisen, H., Vigil, S.A., 1993. Integrated Solid Waste Management. McGraw-Hill, p. 386. USEPA, 1996. Test Methods for Evaluating Solid Waste, Physical/ Chemical Methods SW-846 Method 3052: Microwave assisted acid digestion of siliceous and organically based matrices. Office of Solid Waste’s manual of EPA-approved analytical methods, Washington, DC. Van der Sloot, H.A., 1996. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. Waste Management 16, 65–81.