Environ Monit Assess (2007) 135:41–48 DOI 10.1007/s10661-007-9706-2
Solid waste characteristics and their relationship to gas production in tropical landfill C. Chiemchaisri & W. Chiemchaisri & Sunil Kumar & J. P. A. Hettiaratchi
Received: 26 June 2006 / Accepted: 15 November 2006 / Published online: 26 April 2007 # Springer Science + Business Media B.V. 2007
Abstract Solid waste characteristics and landfill gas emission rate in tropical landfill was investigated in this study. The experiment was conducted at a pilot landfill cell in Thailand where fresh and two-year-old wastes in the cell were characterized at various depths of 1.5, 3, 4.5 and 6 m. Incoming solid wastes to the landfill were mainly composed of plastic and foam (24.05%). Other major components were food wastes (16.8%) and paper (13.3%). The determination of material components in disposed wastes has shown that the major identifiable components in the wastes were plastic and foam which are resistant to biodegradation. The density of solid waste increased along
C. Chiemchaisri (*) : W. Chiemchaisri Department of Environmental Engineering, Faculty of Engineering, Kasetsart University, 50 Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand e-mail:
[email protected] S. Kumar Solid Waste Management Division, National Environmental Engineering Research Institute (NEERI), Nehru Marg, Nagpur, India J. P. A. Hettiaratchi Department of Civil Engineering, Faculty of Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
the depth of the landfill from 240 kg m−3 at the top to 1,260 kg m−3 at the bottom. Reduction of volatile solids content in waste samples along the depth of landfill suggests that biodegradation of solid waste has taken place to a greater extent at the bottom of the landfill. Gas production rates obtained from anaerobic batch experiment were in agreement with field measurements showing that the rates increased along the depth of the landfill cell. They were found in range between 0.05 and 0.89 l kg−1 volatile solids day−1. Average emission rate of methane through the final cover soil layer was estimated as 23.95 g−2day−1 and 1.17 g−2day−1 during the dry and rainy seasons, respectively. Keywords Gas production . Landfill gas . Methane emission . Tropical climate . Waste characteristics
Introduction Landfill gas created from decomposition processes of solid wastes is mainly composed of methane and carbon dioxide. A wide range of gas production rates of between 0.187 and 0.424 m3 kg−1 wet wastes or between 0.009 and 0.02 m3 kg−1 wet wastes per year has been reported (Rees 1985). The produced gases, if not properly managed, could create several adverse effects, such as health risks or global warming consequences. The amount of landfill gas generated
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Environ Monit Assess (2007) 135:41–48
Fig. 1 Layout of pilot landfill with monitoring locations (V ventilation pipe, E close flux chamber)
78.5 m
E2 26.8 m V1 (1.5m)
V2 (3m)
V3 (4.5m) E1
38.8 m
V4 (6m) E3
60.8 m.
E4
47 m
Entrance
depends very much on climatic conditions, site geography, waste characteristics and other local factors. Higher gas production rates were usually obtained in landfill simulators operated under controlled conditions of, e.g, pH, moisture content and nutrient addition (Kinman et al. 1987; Lay et al. 1998). In actual landfills, several factors, such as low moisture content in wastes and lateral gas migration, may be associated with lower gas yield compared to controlled experimental studies. This study deals with landfill gas production from actual landfill in developing countries under tropical climatic environmental conditions. Usually, municipal solid wastes in developing countries share some common characteristics of
Fig. 2 Gas collection chamber
high food wastes and moisture content (Visvanathan et al. 2004). In the tropics, a clear distinction in rainfall pattern between dry (summer and winter) and wet (rainy) seasons also provides a unique environmental condition. The objective of this study was to determine solid waste characteristics and their relationship to gas production and emission rates by determining gas flow through ventilation pipes and diffusive emission through final cover soil layer. The results can be employed to identify appropriate control measures or to evaluate alternative options for landfill gas management, such as utilization as an energy source or landfill cover design, to minimize the emission of greenhouse gases.
Gas collecting pipe 40 cm
25 cm
Gas counter
Gas ventilation pipe
Environ Monit Assess (2007) 135:41–48
43
waste in the landfill cell was deposited about 2 years prior to this investigation. The study was conducted by excavating the wastes in the landfill cell to various depths of 1.5, 3, 4.5 and 6 m. and analyzing physical components and chemical characteristics, i.e. bulk density, material components, moisture, total solids, volatile solids, ash content and chemical compositions. After the excavation, perforated pipes and a gas collection chamber (Fig. 2) was installed to each depth for the measurement of landfill gas volume (locations V1 to V4 in Fig. 1). The gas was collected in chambers made of PVC with a diameter of 40 cm. The chambers were covered with acrylic plate to which collecting ports were connected. Collected gas was then transferred into a gas measurement device connected to an electromagnetic counter. The gas was sampled once a week for analysis of its composition using gas chromatography (Shimadzu, GC-14A). In order to determine specific gas production rates of buried solid waste, solid waste samples excavated
Gas sampling port
Thermometer
Incline manometer Acrylic plate Stainless steel body Insulating fiber
25 cm.
Stainless steel base 10 cm
49 cm
50.5 cm Fig. 3 Schematic of close flux chamber
Materials and methods This research was conducted at a pilot landfill in Nakhonpathom municipality, Thailand. The pilot landfill (Fig. 1) covers an area of about 8,000 m2 with an approximate effective depth of 6 m. Solid
Table 1 Physical and chemical characteristics of fresh and buried waste samples Parameters
Sample/depth Fresha
Physical characteristics Density (kg m−3 ) Waste composition (% w/w) Food/vegetable wastes Paper Plastic and foam Other organics Inorganic materials (glass, metals, stone) Others (hazardous, Unidentifiable fraction) Chemical characteristics Moisture content ( %w/w) Total solids (%w/w) Volatile solids (% TS) Ash (% TS) Chemical composition (% TS) Carbon Hydrogen Oxygen Nitrogen Phosphorus Sulfur
1.5 mb
3 mb
4.5 mb
6 mb
250 (139)
240
840
1,360
1,260
54.6 8.9 17.1 10.2 5.3 3.9
(16.5) (4.3) (6.1) (6.9) (5.1) (4.3)
6.9 ND 69.1 0.1 2.2 21.7
9.6 3.8 43.6 4.0 ND 39.0
4.1 ND 13.5 2.5 2.7 77.2
1.1 ND 26.6 3.1 4.2 65.0
65.2 34.8 80.5 19.5
(10.1) (10.1) (7.9) (7.9)
39.6 60.4 46.0 54.0
60.1 39.9 44.1 55.9
57.1 42.9 29.8 70.2
54.2 45.8 22.1 77.9
25.56 3.06 15.80 1.24 0.15 0.18
24.51 2.94 15.09 1.23 0.16 0.19
16.57 1.98 9.88 1.07 0.14 0.19
12.28 1.47 6.90 1.05 0.18 0.24
44.7 (4.4) 5.1 (0.5) 29.37 (3.7) 1.0 (0.7) 0.2 (0.1) 0.05 (0.04)
a
Average (SD) from 6 samples taken during 2001–2003
b
Average from 3 samples taken in 2003
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Environ Monit Assess (2007) 135:41–48
concentration in the chamber using the following equation.
12 1.5 m
Gas production (m3/d)
10
3m
J¼
4.5 m
8
6m
6
V dC A dt
ð1Þ
where
4
J
2 0 1
5
9
13
17
21
25
29
V A dC/dt
Time (weeks) Fig. 4 Gas production at various depths in the landfill cell
from different depths of the landfill cell were collected and subjected to anaerobic batch experiments in the laboratory. The samples were kept in a 10-cm diameter PVC cylinder, 0.5 m in length. The produced gas volume was continuously monitored for 50 days. The results from the anaerobic batch experiments were then compared with gas production rates determined from field measurements at the gas ventilation pipes. Gas emission rate through the cover soil was estimated by the close flux chamber method (Reinhart et al. 1992). The chambers were made of stainless steel 0.50 cm in diameter and 0.25 m in height (Fig. 3). They were installed at different locations over the surface of the pilot landfill (E1 to E4 in Fig. 1). The chambers were covered with heat insulators and acrylic sheets at the top where a thermometer and an incline manometer of temperature and pressure measurement were installed. Gas samples were taken from the chambers through a sampling port at the chamber cover and subjected to composition analysis. Methane emission rate was determined by the increasing rate of
Methane emission rate through cover soil (g m−2 day−1) Volume of chamber (m3) Cover area of chamber (m2) Increasing rate of methane concentration in chamber (g m−3 day−1)
Gas samples were also collected at different depths in the cover soil at 0 (soil surface), 0.6, 0.9 and 1.2 m (surface of solid waste layer) to determine a gas composition profile for the estimation of diffusive vertical gas migration. The cover soil was also sampled and analyzed for its density, soil texture, porosity, pH and moisture content according to standard methods (Soil and Plant Analysis Council 1999). Using the concentration profile of methane along the depth of cover soil, an emission rate of methane gas was estimated based on a diffusive gas transport equation (Tchobanoglous et al. 1993) as follows. Da4=3 Catm Cfill NA ¼ L
ð2Þ
where Gas flux of methane, g m−2 day−1 Effective diffusion coefficient, m2 day−1 gas filled porosity of soil, m3 m−3 Methane concentration at surface of landfill cover, g m−3
NA D α Catm
Table 2 Composition of landfill gas at different depths into the waste layer Depth into waste layer (m)
Gas compositiona (%v/v) CH4
1.5 3 4.5 6 a
54.70 64.11 62.21 60.11
CO2 (4.22) (1.96) (2.45) (2.72)
Average (SD) from 11 samples taken during 2003
28.09 31.08 32.39 31.38
O2 (2.98) (2.16) (2.82) (2.37)
3.12 0.96 1.12 1.74
N2 (0.87) (0.35) (0.56) (0.72)
14.07 (4.21) 3.82 (1.52) 4.26 (2.22) 6.76 (2.61)
Environ Monit Assess (2007) 135:41–48
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Table 3 Observed gas production in landfill cell and specific gas production rates of excavated samples from anaerobic batch experiment Depth into waste layer (m)
Average gas productiona m3 day−1
1.5 3 4.5 6
Average (SD) from 30 weeks monitored in 2003
b
Average from 3 excavated waste samples
c
% of gas production rate at 6 m depth
d
VS Volatile solids
Cfill L
%c
0.079 1.962 4.308 4.879
a
Specific gas production rateb
2.2 69.4 84.9 100
Methane concentration at bottom of landfill cover, g m−3 Thickness of landfill cover, m
Results and discussion Characteristics of fresh and buried solid waste samples Physical and chemical characteristics of fresh and buried (2-year-old) solid waste samples at depths of 1.5, 3, 4.5 and 6 m. are shown in Table 1. It can be seen that fresh solid waste was mainly composed of food wastes (54.6%) and plastic and foam (17.1%). Further major components were other organics (10.2%) and paper (8.9%). Average moisture content in solid wastes was 65.2% and volatile solids content was 80.5% of total solids. Determination of material components in solid waste samples, which have been
m3 kg–1day−1 VSd 0.5 5.7 7.1 9.4
× × × ×
%c
10−4 10−4 10−4 10−4
5.3 60.6 75.5 100
buried at different depths of 1.5, 3, 4.5 and 6 m, suggested that major identifiable components in the wastes were plastic and foam, which are resistant to biodegradation. By comparing their physical and chemical characteristics, it can be seen that the density of solid waste increased along the depth of landfill and ranged from 250 kg m−3 at the top to 1,260 kg m−3 at the bottom. Reduction of volatile solids content in waste samples along the depth of landfill suggested that biodegradation of solid waste had taken place to a greater extent at the bottom of the landfill. Moisture content of solid wastes at depths of 3–6 m was in the range of 54.2–60.1%, which is higher than that at 1.5 m (39.6%). Landfill gas production and its compositions at various depths The monitoring results of landfill gas volume from gas ventilation pipes laid to different depths in the
Table 4 Average methane emission rates through the cover soil during dry and wet period Location
Methane emission rate (g m−2 day−1) Dry period (January–June 2004) Range
E1 E2 E3 E4 Average
0.09–10.69 0.53–83.12 0.02–94.04 0.09–69.79 23.95
Wet period (July–October 2004) Avg.
Range
Avg.
2.52 14.92 55.00 23.34
ND 0.82–1.21 2.72–4.39 ND–0.02 1.17
ND 1.02 3.65 0.01
No. of measurementswere 8 during the wet period and 5 during the dry period ND Not detected
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Environ Monit Assess (2007) 135:41–48 0
Distance from soil surface (m)
-0.1
10
20
30
40
50
60
moisture (wet period) methane (wet period) moisture (dry period) methane (dry period)
-0.3
-0.5
-0.7
-0.9
moisture, methane (%) Fig. 5 Moisture and methane content at various depths of cover soil
pilot landfill over 30 weeks are shown in Fig. 4. It was found that volume of landfill gas fluctuated significantly with time during the monitoring period. It was highest at a depth of 6 m, varying between 1 and 10 m3 day−1 with an average value of 4.879 m3 day−1. Average gas volume of 4.308, 1.962 and 0.079 m3 day−1 was obtained at 4.5, 3 and 1.5 m, respectively. Gas volume tended to increase during the first 8–9 weeks. Afterwards, the gas volume declined and remained relatively constant after 18 weeks except for that at 3.0 m depth which was greatly reduced after 21 weeks, caused by the release of gas through the final cover soil. Moisture content of solid waste could be one of the major causes for the fluctuation of landfill gas. It is greatly influenced by rainfall precipitation and water infiltration through the final cover soil. From the meteorological data in the study area, weekly rainfall amounts in the area during the first 8 weeks ranged between 0 and 66 mm with an average of 12.0 mm. It averaged 42.8 mm between weeks 9 and 19 when higher gas production rate was observed. During the last 10 weeks, the rainfall intensity dropped to 21.5 mm and gas volume gradually declined. Reduction in moisture content of the final cover soil could be another factor affecting the amount of gas monitored. As soil moisture content dropped, cracking of the cover soil took place, promoting the emission of gas through the layer. Average gas compositions from different depths in the landfill cell are shown in Table 2. It can be seen that major components of landfill gas were
methane and carbon dioxide. Other components were nitrogen and oxygen. It was found that the methane content in gas from a depth of 3–6 m was relatively constant between 60.1 and 64.1%, whereas it was found to be lowest, at 54.7%, at a depth of 1.5 m. On the other hand, oxygen content was relatively high (3.12%) at the depth of 1.5 m compared to the others (0.96–1.74%). Carbon dioxide content was found to be relatively constant at 28.09–32.39% along the depth of the landfill. The composition of gas suggested that anaerobic conditions could be better developed in the deeper zone of the landfill (3–6 m). Due to the cracking of the cover soil, air could penetrate to some extent into the landfill down to a depth of 1.5 m. Gas production rate was also studied by anaerobic batch experiments of solid waste samples excavated from different landfill depths. Gas volume produced from each sample was measured in a 50-day incubation period. The obtained results are shown as specific gas production rates in Table 3. The production rates of gas were found to be relatively constant throughout the incubation. The gas generation rate expressed in terms of gas volume per initial weight of volatile solid in solid waste samples was found to be 9.4 × 10−4, 7.1 × 10−4, 5.7 × 10−4 and 0.5 × 10−4 m3kg−1 day−1 of volatile solids for the samples obtained at landfill depths of 6, 4.5, 3 and 1.5 m, respectively. These results were in agreement with field measurements (Table 3). The percentages of gas production at depths of 1.5, 3 and 4.5 m were 5.3%, 60.6% and 7.5.5%, respectively, of that at 6 m. The analysis of gas compositions showed that methane content in landfill gas was not very different among the waste samples and remained relatively constant throughout the experiment. Methane emission rate through final cover soil Determination of gas emission rate through the final cover soil was performed by using close flux chamber method. The experimental results are shown in Table 4. It was found that methane emission rate from landfill varied widely from a non-detectable level up to 94.0 g m−2 day−1. The emission rate during the dry period (January–June) was significantly higher than that of the wet period (July–October) due to the presence of cracks in the landfill surface with average emission rates of 23.95 and 1.17 g m−2 day−1 during the dry and wet seasons, respectively. The emission
Environ Monit Assess (2007) 135:41–48
rate during the dry season in this study was in accordance with 20.3 and 35.4 g m−2 day−1 reported for the same and nearby solid waste disposal sites during 2000–2004 (Chomsurin 1997; Chiemchaisri et al. 2006). The gas compositions at different depths of cover soil, i.e. 0 (soil surface), 0.6, 0.9 and 1.2 m (surface of solid waste layer), were also determined for the estimation of vertical diffusive methane flux. During the high rainfall period (weeks 9–19), emission of gas through the cover soil layer was non-detectable. Average methane compositions monitored during the wet (weeks 9–19) and dry (weeks 20–30) seasons are shown in Fig. 5. It was found that the compositions varied along the depth of the cover soil layer. At a depth of 0.9 m, the composition of gas was similar to that produced in the landfill containing methane and carbon dioxide as the major components. As the gas transported upward to the soil surface, methane and carbon dioxide content decreased whereas nitrogen and oxygen content increased significantly. This was caused by the dilution of gas with penetrated air in the cover soil layer. At the soil surface, methane content in the gas was reduced to about 19.1 and 25.5% during the dry and wet seasons, respectively. The analysis of cover soil properties suggested that clay was the main component (64%), followed by silt (20%) and sand (16%), with porosity of 40%. Moisture content of soil at the landfill surface varied from 7.5% during the dry period to 16% during the high rainfall period. As a result, the methane flux was estimated to be 82.2 g/m2 day−1. These values are considerably higher than the measured value obtained from the close flux chamber method. As the calculation method did not take actual variation of soil moisture content (or gas-filled porosity) into account, it tended to over-estimate the gas emission as gasfilled porosity is assumed to be equal to total porosity in the soil. The variation of soil moisture content, one of the controlling factors of landfill gas emission, was investigated. Figure 5 shows the profile of soil moisture along the depth of cover soil during the dry and wet periods. It was found that moisture content in soil at the surface dropped below the shrinkage limit (14.8%) during the dry period resulting in the cracking of the topsoil layer. During the wet period, soil moisture remained above the limit and the layer became relatively impermeable for gases.
47
Conclusions From the measurement of gas volume through ventilation pipes laid to different depths of the tropical landfill, it was found that gas volume increased along the depth of the landfill. Highest gas production was observed at a depth of 6 m with an average value of 4.879 m3 day−1 compared to 4.308, 1.962 and 0.079 m3 day−1 at depths of 4.5, 3 and 1.5 m, respectively. The observed gas compositions also varied along the depth of the landfill. Methane content in gas from depths of 3–6 m ranged between 60.1 and 64.1% and dropped to 54.7% at 1.5 m. Gas production rates from solid waste samples in anaerobic batch tests were in agreement with the field observations and ranged from 0.5 × 10−4 to 9.4 × 10−4 m3kg−1 d−1 volatile solids. Surface emission rates of methane gas as determined by the close flux chamber method also varied significantly with time. The emission rates during the dry period were considerably higher than that during the wet period due to the presence of cracks in the landfill surface. Average emission rates during the dry and wet seasons were 23.95 and 1.17 g m−2 day−1, respectively. The analysis of gas composition for the estimation of the gas emission rate through the final cover soil showed that methane and carbon dioxide content in the gas decreased as the gas migrated upward to the soil surface mainly due to the dilution of gas with penetrated air into the cover soil. The methane emission rate was estimated by using vertical diffusive gas transport equation as 82.2 g m−2day−1, considerably higher than the measured value from the close flux chamber method due to unaccounted soil moisture content.
Acknowledgements The authors gratefully acknowledge financial support from Kasetsart University Research and Development Institute (KURDI) and Swedish International Development Cooperation Agency (SIDA) under Asian Regional Research Program on Environmental Technology (ARRPET) for this study.
References Chiemchaisri, C., Chiemchaisri, W., & Sawat, A. (2006). Mitigation of methane emission from solid waste disposal site in the tropics by vegetated cover soil. Asian Journal of Water Environment and Pollution, 3(2), 29–33.
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