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Journal of the Air & Waste Management Association Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uawm20

Methane Balance of a Bioreactor Landfill in Latin America a

a

a

a

Jenny Sanderson , Patrick Hettiaratchi , Carlos Hunte , Omar Hurtado & Alejandro Keller a

b

University of Calgary, Calgary, Alberta, Canada

b

Kiasa and DeMarco Sociedad Anónima, Santiago, Chile Published online: 24 Jan 2012.

To cite this article: Jenny Sanderson , Patrick Hettiaratchi , Carlos Hunte , Omar Hurtado & Alejandro Keller (2008) Methane Balance of a Bioreactor Landfill in Latin America, Journal of the Air & Waste Management Association, 58:5, 620-628, DOI: 10.3155/1047-3289.58.5.620 To link to this article: http://dx.doi.org/10.3155/1047-3289.58.5.620

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TECHNICAL PAPER

ISSN:1047-3289 J. Air & Waste Manage. Assoc. 58:620 – 628 DOI:10.3155/1047-3289.58.5.620 Copyright 2008 Air & Waste Management Association

Methane Balance of a Bioreactor Landfill in Latin America Jenny Sanderson, Patrick Hettiaratchi, Carlos Hunte, and Omar Hurtado University of Calgary, Calgary, Alberta, Canada

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Alejandro Keller Kiasa and DeMarco Sociedad Anónima, Grupo Urbaserkiasa, Santiago, Chile

ABSTRACT This paper presents results from a methane (CH4) gas emission characterization survey conducted at the Loma Los Colorados landfill located 60 km from Santiago, Chile. The landfill receives approximately 1 million metric tons (t) of waste annually, and is equipped with leachate control systems and landfill gas collection systems. The collected leachate is recirculated to enable operation of the landfill as a bioreactor. For this study, conducted between April and July 2000, a total of 232 surface emission measurements were made over the 23-ha surface area of the landfill. The average surface flux rate of CH4 emissions over the landfill surface was 167 g 䡠 m⫺2 䡠 day⫺1, and the total quantity of surface emissions was 13,320 t/yr. These values do not include the contribution made by “hot spots,” originating from leachate pools caused by “daylighting” of leachate, that were identified on the landfill surface and had very high CH4 emission rates. Other point sources of CH4 emissions at this landfill include 20 disconnected gas wells that vent directly to the atmosphere. Additionally, there are 13 gas wells connected to an incinerator responsible for destroying 84 t/yr of CH4. The balance also includes CH4 that is being oxidized on the surface of the landfill by methanotrophic bacteria. Including all sources, except leachate pool emissions, the emissions were estimated to be 14,584 t/yr CH4. It was estimated that less than 1% of the gas produced by the decomposition of waste was captured by the gas collection system and 38% of CH4 generated was emitted to the atmosphere through the soil cover. INTRODUCTION The vast majority of biodegradable organic solid waste produced in Latin America ends up in landfills. Anaerobic

IMPLICATIONS There have been recent efforts to accurately estimate anthropogenic global methane emissions, of which landfill emissions are a major contributing factor. So far, most of the work on estimating landfill methane emissions has taken place in developed countries, with a dearth of accurate estimates for developing countries. Methane balance calculations at a landfill allows methane emission rates to be inventoried and characterized but are often difficult to carry out if engineered structures such as gas and leachate collection systems are not present. This methane balance on an engineered landfill in Chile fills the information gap in this area.

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decomposition of biodegradable organic solid waste in landfills is known to produce large quantities of methane (CH4) and carbon dioxide (CO2). The produced gases, if not captured for beneficial purposes, usually escape into the atmosphere. Although both CH4 and CO2 are greenhouse gases, atmospheric emission of CH4 is more of a concern because of its higher global warming potential (GWP). CH4 is now believed to have 23 times the GWP of CO2 over a 100-yr period.1 Emissions from sanitary landfills around the world are estimated to account for almost 10% of the worldwide anthropogenic CH4 emissions into the atmosphere and are therefore a major source of greenhouse gas emissions.2 To date, most of the research on landfill CH4 emissions has been performed at landfills in North America and Europe, and little is known of landfill CH4 emissions in developing countries. Usual CH4 budget estimations for these countries rely upon gross assumptions on the rate of waste generation and biodegradation kinetics. Recently, there have been efforts by researchers to more accurately estimate and predict emissions from landfills in developing countries. An example of this is emission inventories from municipal landfills carried out in India.3 A preliminary type of gas estimations have been attempted in Thailand.4 Quality assurance/quality control (QA/QC) and uncertainty estimates in national greenhouse gas (GHG) emission inventories have been carried out in some European countries and the United States.5–7 Such studies are necessary to provide reliable landfill CH4 emission estimates, but are usually not available for developing country situations. The high ambient temperatures and high organic content associated with solid waste streams in Latin America result in high CH4 generation rates. Most of the existing landfills in Latin American countries are not engineered, and controlling gas emissions is one of the least considered factors in design of landfills. Therefore, there is a high potential for Latin American landfills to emit significant quantities of greenhouse gases into the atmosphere. Yet, there are very little data available in the literature on actual emissions, and GHG estimations are based on theoretical models and various assumptions. It is important to undertake field studies to ascertain the validity of these assumptions and to obtain an accurate estimate of CH4 emissions from landfills in this region. For this reason, this emission characterization survey was conducted at Loma Los Colorados landfill in Chile during a 4-month period between April and July 2000. The CH4 Volume 58 May 2008

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balance was conducted both through theoretical calculations of gas generation and by field investigation to determine the amount of CH4 escaping into the atmosphere from various sources. The specific objectives of this study were to provide a detailed analysis of sources of CH4 emissions at an active landfill in Latin America and to evaluate this on the basis of on-site characteristics that differ from those of developed countries. This particular landfill was chosen because it is an engineered and lined landfill so that lateral migration of gas is not a significant factor and also because the sources of emission were relatively easy to distinguish when compared with the traditional open dumps typical of this region. SITE DETAILS The Loma Los Colorados landfill is located in Montenegro, 60 km northwest of Santiago, the capital city of Chile (Figure 1). Landfill operations began at Loma Los Colorados in March 1996 and it is expected to receive waste for at least 70 yr. At the time of this study, Loma Los Colorados was receiving approximately 3500 metric tons (t)/ day of municipal solid waste, which represents about half of the output originating from Santiago, a city with a population of over 6 million. In May 2000, Loma Los Colorados contained 4.4 million t of waste that was compacted to an approximate density of 0.9 t/m3. The area of the landfill was 23-ha and it had a maximum height of approximately 45 m.8 The landfill cross-sectional details are shown in Figure 2. The landfill had a state-of-the art gas collection and leachate collection system. A gas incinerator was used to burn landfill gas (LFG) originating from landfill extraction wells, which reduces CH4 output to the atmosphere. Leachate was recirculated at a rate of 140 –170 m3/day by pre-wetting the waste before disposal (120 –140 m3/day) and through surface irrigation (20 –30 m3/day). Thus, the landfill was being operated as a bioreactor where gas production was maximized. This practice has since been discontinued because of lateral migration and escape of

leachate from side slopes of the landfill, or “daylighting” of leachate. A daily cover of 15–20 cm was used in the central area of the landfill, and 20 –30 cm on the landfill’s slopes. In addition, a variable intermediate soil cover of 30 – 40 cm was provided in the areas of the landfill not actively receiving waste (Figure 3). There were 33 gas wells, spaced at a distance of 80 m and distributed evenly in a star pattern over the surface (Figure 4). The gas wells extended to a maximum depth of 2 m above the bottom highdensity polyethylene (HDPE) liner of the landfill. Thirteen of the 33 wells were connected to the incinerator. The remaining 20 wells were vented directly to the atmosphere and represent point sources of CH4 emissions. METHODOLOGY The CH4 balance at Loma Los Colorados landfill was carried out using a combination of empirical measurements as well as theoretical calculations. The CH4 emissions were partitioned into several distinct pathways. The mass balance is given by: CH 4 Generated ⫽ CH4 Emission from Landfill Surface ⫹ Passive Venting through Gas Wells ⫹ CH4 Recovered and Incinerated ⫹ Microbial CH4 Oxidation to CO2 ⫹ CH4 escaping with Leachate ⫹ CH4 unaccounted for and Stored

(1)

Subsurface CH4 migration is not included in the mass balance because the landfill is lined with a composite liner (HDPE located above a 60-cm thick layer of compacted clay of 1 ⫻ 10⫺6 cm/sec hydraulic conductivity), and therefore subsurface migration is assumed to be negligible.

Figure 1. Location of Loma Los Colorados landfill. Volume 58 May 2008

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Sanderson et al.

80 m distance between gas collection wells

Approximate height of landfill in May 2000-55m

Landfill Liner and Leachate Collection

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Figure 2. Cross-sectional view of Loma Los Colorados landfill.

CH4 Generated The amount of CH4 generated within the landfill is calculated from the Scholl Canyon model, which is an empirical model that uses field-derived parameters to estimate potential CH4 generation.9 The Scholl Canyon model employs first-order reaction kinetics and a negligible lag time for anaerobic conditions to develop and gas to be produced. At Loma Los Colorados, high gas emissions are observed in waste that has been deposited only few weeks prior. Under these conditions, the Scholl Canyon model is the most suitable for estimation of theoretical gas yield.

冘

CH4 Emissions from Landfill Surface A flux chamber was used to perform field measurement of CH4 surface emissions. The closed chamber technique has been the most widely used in landfill settings.10 –13 The flux chamber used at Loma Los Colorados consisted of a stainless steel cylinder (30-cm diameter, 16.5-cm height) with an acrylic top. Gas concentrations in the chamber were measured using a portable gas analyzer. The chamber was first inserted approximately 1 cm into the ground and a small amount of moistened soil mounded up around the interface between the chamber and the ground to achieve a gas-tight seal. The flux rate was calculated using the following equation:

n

Q T ⫽ kLo

Mie⫺kti

(2)

CH 4 Flux ⫽

i⫽1

where QT is the volumetric rate of gas production (m3/yr), k is the gas production rate constant (year⫺1), Lo is the gas production potential (m3/t), Mi is the sub-mass landfilled in year i (tons), ti is the age of sub-mass i (years), and n is the number of years of landfilling.

␳CH4 dCCH4 V ⫻ 100 dt A

(3)

where dC/dt is the change in the percentage of CH4 in the chamber with time (%/min), V is the chamber volume (m3), and A is the chamber area (m2). The dC/dt value is calculated from linear regression of the CH4 concentration in four or five headspace samples

6353100

6353000 Phase 1B

Phase 1A

Phase 1C

6352900

6352800

Phase 2A1

Phase 2B1

Phase 2C1

6352700

6352600 331600

331700

331800

331900

332000

332100

332200

Figure 3. Topographic map of Loma Los Colorados showing active and completed portions of the landfill surface as of May 2000. Notes: Phase 1AB and 2AB1 are completed phases. Phase 1C and 2C1 are active. 622 Journal of the Air & Waste Management Association

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4 6353000

#2

11

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#3 6352900

#1

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15 12 6

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22 19

10

2

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23 20 #4

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24

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7 14

6352800 25

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21 31 30

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6352700 Leachate Pools Connected Gas Wells Unconnected Gas Wells

6352600 331600

331700

331800

331900

332000

332100

332200

Figure 4. Location of gas wells at Loma Los Colorados landfill.

taken over 20 min. The sampling duration is limited to 20 min to prevent significant accumulation of gas inside the chamber, which would reduce the rate of gas diffusion into the chamber, resulting in a negative bias in the flux measurements.14 Surface CH4 emission rates were obtained between April 7, 2000 and July 19, 2000. This time span was during the winter months in the southern hemisphere. It is recognized that the study provided a snapshot of emissions during this period and does not consider seasonal variations. Nevertheless, it should be pointed out that this landfill was in its active phase of waste landfilling, and therefore, the best information can be obtained as snapshots, Flux measurements were taken over the entire area of the landfill except within the working face. Hence, the emissions via the various surface locations were evaluated. The landfill was divided into a grid with points spaced 30-m apart. Gas flux measurements were taken at each of the grid points, excluding 16 points that fell on areas of active landfilling (within a 0.18-ha working face) and on access roads. This resulted in a total of 232 flux measurements. Passive Venting through Gas Wells The locations of the vented wells are shown on Figure 4. To determine the gas output from these wells, tubing was inserted into the wells and gas concentrations were measured every meter, to a depth of 5 m, using a gas analyzer. The gas flow was measured by first sealing the wells with plastic and duct tape, leaving only the end of the inner tube of the gas well open and then immediately timing the filling of a plastic bag of known volume. The weight of the bag used was 28.8 g and its volume was 50 L. It must be stressed that there are limitations to the use of this technique because of the pressurization of the bag as it fills toward its capacity. The filling of the bag is linear until it reaches a point close to its capacity when it becomes curvilinear. The slope of the linear portion gives an Volume 58 May 2008

indication of the gas flux. This information is used in a manner similar to the gas flux determinations from the flux chamber technique. A representative sample of wells (8 of 20) unconnected to the gas collection system was selected to acquire an estimate of uncontrolled atmospheric emissions from these wells. The percentage gas output with respect to depth and the gas flow rate was profiled in these 8 wells. These 8 wells represent a 40% sample of the total number of unconnected wells. The CH4 and CO2 compositions were profiled at every 1 m to a maximum depth of 5 m. The composition used in emission estimation was taken for the maximum LFG concentration reading in the profile, because this represented the reading with the least air infiltration. The emission of CH4 from each well was then determined by:

冘 20

QT ⫽

共Q i ⫻ Ci/100兲

(4)

i⫽1

where QT is the kg/yr of gas (CH4), Qi is the flow of LFG from well (kg/yr), and Ci is the CH4 concentration in well (CH4 as a fraction of total LFG). CH4 Recovered and Incinerated The amount of CH4 incinerated from the 13 wells connected to the gas collection system was determined using recorded data reports at the landfill. The total hours of operation and the volume of gas incinerated was taken from these reports. Microbial CH4 Oxidation to CO2 A significant amount of CH4 can be oxidized by a soil cover applied at a landfill before LFG can reach the atmosphere.15 CH4 oxidation in soil takes place because of the presence of methanotrophic bacteria. These metabolize Journal of the Air & Waste Management Association 623

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Sanderson et al. CH4 into CO2 through various complex biological pathways. Factors that affect the metabolism of these CH4fixing bacteria, such as moisture content, organic fraction, nutrient level, and temperature, have a profound influence on oxidation rates. The cover soil used at this site is typically a clay soil with a maximum dry density of 1.83 t/m3, 13.45% moisture content, and 6.35% total volatile solids (dry wt.). These soil conditions, along with the average ambient temperature of 14 °C and average annual rainfall of 369 mm/yr, are sufficient to allow methanotrophic activity in soils. Both surface emissions and depth concentrations of CH4 below the surface were used to calculate CH4 oxidation in the landfill cover soil. The depth readings were taken below the surface through gas sampling and surface emissions were measured using the flux chamber. The equipment used for measuring depth concentrations was the Gas Hammer Depth Sampler, designed at the University of Calgary.16 The apparatus was made of a sliding hammer to drive a hollow pointed steel rod with a diameter of 12 mm into the soil. Tubing extends down into the hollow rod, the end of which is attached to a gas analyzer at the surface. Once the sampler has been driven into the ground, a handle at the top of the sampler is used to open a gap below the surface, which permits the entry of soil into the tubing. The gas is drawn up through the tubing to the attached gas analyzer, which displays the percentage gas at that depth. The sampler is used for gas detection, and is a quick method to detect gas below the surface. Driving the rod into the ground provides an excellent seal against atmospheric air intrusion at 33-cm depth. The concentrations of CH4, CO2, and oxygen gas (O2) were measured at 33-cm depth and flux chamber measurements were conducted on the surface. Nitrogen was not measured directly but estimated as the balance gas present in samples. These were used in conjunction to calculate the CH4 oxidation rate in the intermediate soil cover. The intermediate soil cover’s thickness varies from 30 to 40 cm and researchers have shown that most CH4 oxidation occurs in the top 30 cm of a soil cover, with maximum CH4 uptake occurring between 20 and 30 cm.17 This suggests that gas concentrations at 33-cm depth in combination with surface data can be used to calculate CH4 oxidation in the soil cover. The values obtained for CH4 and CO2 flux at the surface were measured and the fraction of CH4 in the total LFG calculated assuming that the LFG was only made up of CH4 and CO2. In total, 21 flux measurements were taken. If no CH4 oxidation occurs, the percent CH4 remains constant as it migrates from a depth of 33 cm to the soil surface. Otherwise, CH4 oxidation will be the difference between the surface value and depth value, and can be calculated using the equation:

冘 冉 冊 n

1⫺

ox ⫽

i⫽1

CH4Si CH4Ti

n

(5)

where ox is the average CH4 oxidation, CH4S is the fraction of CH4 at surface, CH4T is the theoretical fraction 624 Journal of the Air & Waste Management Association

of CH4 surface, and n is 21, or the number of flux measurements. There is a primary confounding factor and that is slow migration of CO2 to the surface due to (1) slow diffusivity of CO2, (2) high solubility of CO2 in water, and (3) density differences of CH4 and CO2 relative to air. These factors may lead to increases in CH4 concentration if there is no oxidation. This is substantiated by various researchers and field studies. In very wet landfills, Henry’s Law of partitioning of CO2 to the aqueous phase may promote increases in CH4 concentration.13 As LFG migrates, CO2 may be sorbed.18 Long transport pathways through water-filled pores can result in fractionation of gas through dissolution of CO2 into the aqueous phase, whereas less soluble CH4 remains in the gaseous phase.19 The heterogeneous nature of flow within a landfill should result in areas dominated by both diffusion and advection. However, for purposes of our assessment we assumed negligible advection and diffusion as the underlying cause of migration. A considerable pressure gradient should not exist as an advective driving force for gas migration. Because the landfill is not a pressurized system, diffusion is the main mechanism of gas migration. The theoretical fraction of CH4 that occurs at the surface can be calculated using a diffusivity ratio of CO2 relative to CH4 as given by the equation:

CH 4T ⫽

CH4D (CH4D) ⫹ 共b ⫻ CO2D)

(6)

where CH4T is the theoretical fraction of CH4 at surface, CH4D is the fraction CH4 of LFG at origin, CO2D is the fraction CO2 of LFG at origin, and b is MCO2/MCH4. MCO2 is the fraction of migration of CO2 relative to the fraction of migration of CH4. The theoretical value of b ⫽ 0.65 represents the maximum CH4 oxidation that could occur according to the relative diffusivity of CO2 and CH4 with negligible advection.9 The b value was calculated experimentally using a plot of CH4 oxidation and LFG flux.16 The steps for determination of CH4 oxidation using relative migration of CO2 to CH4 are: (1) using eq 6, a theoretical value, CH4T, is obtained for the fraction of CH4 of the total LFG at the surface; (2) CH4T is input into eq 5 and the average CH4 oxidation is calculated; and (3) the value of b is adjusted until the average CH4 oxidation equals zero. CH4 Escaping with Leachate Gas transport in a landfill is a complex process because of the presence of moisture in the landfill and the active transport of moisture through advective and diffusive flow. As can be expected, there would be a significant amount of CH4 that can be transported with the leachate system both as a separate gas phase as well as dissolved in the leachate. The amount of CH4 that escaped via the leachate collection system was difficult to quantify. At Loma Los Colorados landfill, at the time of this field assessment, there was another source of gas escape associated with leachate. CH4 gas also escaped from the landfill with daylighting leachate, or the leachate that migrates laterally resulting in the formation of leachate Volume 58 May 2008

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Sanderson et al. pools or thin layers of liquid accumulating on the side slopes of the landfill. Leachate migrating downwards may encounter a low permeable zone within the landfill and travel laterally toward the slopes, and “daylights” creating leachate pools. At this landfill, the aggressive leachate recirculation tried by the operators before the time of this investigation has resulted in this leachate daylighting incidences. Because it was observed that there were significant gas emissions from these leachate pools, an investigation was carried out to quantify the emissions from these pools. Leachate pool emission of CH4 is one component that is not normally included in a CH4 mass balance. Daylighting of leachate from the sides of a landfill is not a usual occurrence in most landfills. Therefore, there is no standard procedure to assess the gas escape via this route. CH4 emissions associated with leachate pools are in the form of bubbles of CH4 emitted to the atmosphere. An attempt was made to estimate this source of emissions by placing the flux chamber on the surface of the pool with an insertion depth of 2.4 –2.8 cm, and thereafter, the closed flux chamber gas measurement procedure was followed. These measurements resulted in extremely high CH4 emission rates, but the results were not included in the overall CH4 balance because the accuracy of these measurements could not be verified.16 CH4 Unaccounted For There is a significant amount of CH4 generated that was not accounted for. The CH4 escape via the leachate collection system could not be determined accurately. Also, the temporary internal storage of gas could not be determined. It is clear that there will be unaccounted and stored CH4 in the landfill due to complex transport mechanisms and unknown pathways. RESULTS CH4 Generation An understanding of waste composition and site conditions is needed to realistically apply the Scholl Canyon model. Because the initial moisture content of the waste deposited at this landfill was high (approximately 50%) and because it is also pre-wetted with recirculated leachate, a waste half-life of 5 yr was used in the CH4 generation model giving a decay constant, k, of 0.1385.20 The waste composition data were obtained from a waste composition survey undertaken by Universidad Catolı´ca de Valparaı´so.21 This study revealed that the organic content in the waste in Chile was approximately 75%, of which 49% was food waste. The biodegradable fractions (BFs) of waste components were taken from literature9 and are presented in Table 1 along with waste composition data. The gas composition typically measured at the landfill was 57% CH4 and 43% CO2. Using this data, the CH4 potential of waste, Lo, was calculated and found to be 105 m3/t. On the basis of these values for k and Lo, the Scholl Canyon model predicts that the CH4 generation rate as of July 2000 would be 52,519,400 m3/yr or approximately 6000 m3/hr. Using 0.665 kg/m3 as the density of CH4 at 20 °C gives an estimated CH4 generation as 34,925 t/yr. Volume 58 May 2008

Table 1. Waste composition and biodegradable components of municipal solid waste in Chile. Waste Component Food waste Paper Cardboard Textiles Plastic Wood Leather and rubber Metal Glass Dirt and Ashes Others

Waste Composition (%)

Biodegradable Fraction (%)

48.73 13.69 5.00 4.35 9.90 0.47 2.45 2.99 2.75 3.11 6.54

98 58.5 47 22 1 22 1

CH4 Emissions from Landfill Surface The slope of the landfill was 3:1 and the flux measurements were taken on various areas of the landfill. Of the 232 points measured, 191 points occurred on the landfill slope, 30 of which fell on the border of the landfill. Measurements were not corrected for possible errors resulting from factors such as temperature changes within the chamber during measurement and changes in the concentration gradient in the soil, but these errors were minimized by the short duration of measurement and proper selection of chamber height.14 Average values were calculated both as arithmetic mean and as a weighted average. The weighted average was calculated on the basis of representative area, because there was variation in the superficial distance between the points of measurement, that is, the measurements on the slopes were further apart than the measurements on the upper, flat area of the landfill. On a planar basis, points were equidistant. Points were assumed to be central to the area they represented. The area represented by points that fell on the border was only half of the grid planar area because emissions are assumed to end at the border of the landfill. The various points taken and their representative areas are shown in Table 2. This table also shows that the grid design gave 16 points on the road and 16 points on the active phase. Measurements were not taken at these points but emission was accounted for in the active phase using the weighted average flux. The weighted average CH4 flux from the landfill surface was 167 g 䡠 m⫺2 䡠 day⫺1. The soil surface emissions were high relative to landfills in North America and other temperate climates. Typical average emission ranges of CH4 for landfills in the northeastern United States are 9 to 130 g 䡠 m⫺2 䡠 day⫺1.22 The value of 130 g 䡠 m⫺2 䡠 day⫺1 approaches that at Loma Los Colorados, and was measured at a partially covered site with no active gas recovery system. The weighted average values for LFG emissions were used in the determination of overall CH4 emissions from the landfill surface and shown in Table 2. Total surface emissions at the landfill were calculated as 13,320 t/yr. Passive Venting from Unconnected Gas Wells Total LFG flow rates and gas concentrations from the unconnected wells were estimated on the basis of that of Journal of the Air & Waste Management Association 625

Sanderson et al. Table 2. LFG emissions of the landfill surface as weighted values on the basis of area.

Sections Border Slope Central Road Active face Total

No. of Points

m2/pta

30 161 41 16 16

593.8 948.7 900.0 948.7 948.7

Surface Area

% Area

17,814 152,740.7 36,900 15,179.2 15,179.2 237,813.1

7.5 64.2 15.5 6.4 6.4

1,085 9,303 2,247 0 682.4 13,320

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the 8 wells taken as a representative sample. It is estimated that the 20 wells emit a total of 120 m3/hr of CH4. Therefore, the total CH4 mass emitted from the unconnected gas wells was calculated as 1180 t/yr. CH4 Recovered and Incinerated The average volume of CH4 gas incinerated from the 13 wells connected to the gas collection system during the study period was 575.25 m3/day, and the average mass of CH4 incinerated was 30.25 kg/day. The CH4 gas incinerated was calculated on the basis of the known values of CH4 concentration and the volume of LFG extracted. The daily volume of LFG and mass of CH4 incinerated is shown in Table 3. Using this data over these three months as a representative sample, the total CH4 incinerated on a yearly basis was calculated to be 84.05 t/yr. CH4 Oxidation in Cover Soil The fraction of CO2 migration relative to CH4 migration, b, was calculated to be 0.78. The theoretical value of b, as already mentioned, is 0.65 and represents the maximum CH4 oxidation that could occur according to the relative diffusivity of CO2 and CH4. This could underestimate the influence of migration because the negative values may represent greater than 0% CH4 oxidation, but provides a more realistic basis for calculation of CH4 oxidation than calculations based purely on diffusion or advection alone. The percent CH4 oxidation is shown in Table 4 for no diffusivity effect and using the calculated and theoretical b values. The CH4 oxidation is calculated on the basis of percent CH4 oxidation and the calculated CH4 surface emission. The amount of CH4 oxidized in the soil cover was estimated to be 1081.56 t/yr. Table 3. Gas Incinerated at Loma Los Colorados.

Month

Volume of LFG (m3/day)

CH4 (kg/day)

April May June July Mean

200 556 1314 231 575.25

80 223 526 92 230.25

626 Journal of the Air & Waste Management Association

Relative Migration

% CH4 Oxidation

CH4 Oxidized (t/yr)

No diffusivity effecta b ⫽ 0.78368b b ⫽ 0.65c

4.66 7.51 11.9

651.05 1081.56 1799.18

CH4 (t/yr)

Notes: aBorder surface area is represented as 593.8 m2 rather than 474.3 m2 to account for irregularities in the overall border of the landfill. Total surface area of the landfill is 237,810 m2.

Notes: Source, KDM, April to July 2000 reports.

Table 4. CH4 oxidation in the soil cover.

Notes: aNo diffusivity taken into account represents advection or advection/ diffusion, in which the ratio of migration rate of CO2 to CH4 is 1 in the absence of oxidation. bUsing experimental b ⫽ 0.78368 represents slower migration of CO2 due to sorption, diffusivity differences, and density. cUsing theoretical b ⫽ 0.65, represents the maximum CH4 oxidation that could occur according to the relative diffusivity of CO2 to CH4, with the influence of advection, zero, or negligible.

CH4 Escaping with Leachate Four leachate pool locations emitting LFG were measured. The leachate pools account for a small portion of the total landfill area; however, substantially higher flux occurred from leachate pools than from points located on the soil surface because of the preferential pathway for CH4 transport. For example, the range of individual CH4 flux from the soil surface was 0 –1421 g 䡠 m⫺2 䡠 day⫺1 (average 167 g 䡠 m⫺2 䡠 day⫺1), however, the flux from the leachate pools varied from 2908 to 34,886 g 䡠 m⫺2 䡠 day⫺1 (average 13,231 g 䡠 m⫺2 䡠 day⫺1). This is a substantial difference and indicates that the flux from leachate pools is almost 80 times that from the soil surface. However, the area of leachate pools is small and highly variable. Because of the smaller and variable areas of leachate pools and the unreliability of emission estimations, this source of CH4 escape is not included in the overall CH4 balance. As noted earlier, the total volume of CH4 escaping with leachate via the leachate collection system was not determined. These leachate pools encountered in the Loma Los Colorados landfill highlight the importance of effective bioreactor design. CH4 Unaccounted for and Stored The CH4 unaccounted for and stored is the balance of the CH4 generated that cannot be accounted for by the identified escape routes. Note that the gas that escapes with the leachate extraction system is included in this value. Additionally, because the gas emitted from the leachate pools was not calculated it is also included in this total. The CH4 unaccounted for and stored value was calculated to be 19,259 t/yr. DISCUSSION OF RESULTS The overall CH4 mass balance for the Loma Los Colorados landfill in t/yr is shown in Table 5. Overall, when all sources of emissions at Loma Los Colorados, except leachate-associated losses were considered, the soil cover oxidized 3% of the CH4 produced, 4% was collected by unconnected gas wells, and 0.2% was collected and burned at the incinerator. Among the accounted emission sources, the vast majority of emissions (38%) were associated with surface emissions. The low efficiency of the gas collection system may be due to several reasons. Most importantly, the connected wells were operating as passive wells without an Volume 58 May 2008

Sanderson et al. Table 5. Mass balance and landfill gas composition data.

CH4

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Generation Scholl Canyon output Emission Soil surface Unconnected gas wells Incinerator Subtotal CH4 oxidation Unaccounted for and stored

CH4 (t/yr)

% Total

CH4 (% volume)

34,925

100

57.0

13,320 1,180 84 14,584 1,082 19,259

38 4 ⬍1

53.4 46.2 60.2 57.0

3 55

32.3

active gas extraction system. The operators are reluctant to pump the gas out because of the relatively high potential for ingression of atmospheric air via the permeable intermediate covers during pumping. As the data show, unconnected wells account for higher amounts of gas than the connected wells. Since the study was completed, the operators have connected most of the high producing unconnected wells to the gas collection network. The low efficiency of the gas collection system is not surprising considering the adopted practices of landfill construction and operation at Loma Los Colorados. Although the landfill is operated as a bioreactor, it is not constructed to ensure proper leachate recirculation or gas collection. The use of low permeable intermediate cover layers impedes vertical flow of leachate and restricts the flow of gas toward capturing wells. The intermediate cover being used at the landfill has somewhat promoted the growth of methanotrophic bacteria. The intermediate cover has oxidized CH4 at a rate of approximately 7.5% of surface emissions (and 3% of total). Considering the low levels of organics and moisture in the soil, this relatively low CH4 oxidation efficiency is reasonable. It is important to note that it has been reported that there is a 10% CH4 oxidation efficiency of typical final cover soils used in U.S. landfills.23 The study found that a large fraction (approximately 55%) of the produced gas is not being emitted via wellknown routes such as surface emissions and collection well system. Because the landfill operators used low permeable soil as intermediate cover, one would expect a high potential for gas storage within the landfill matrix. Nevertheless, the reported value of 19,259 t/yr (or 55% of total) is high. This high value could be the result of the Scholl Canyon model to calculate gas generation potential. It is possible that the k and Lo values used for the calculation are higher than actual. The Lo and k values were calculated using field data, including waste characteristics and climatic conditions. However, the construction practice of using low permeable intermediate cover was not considered in the selection of these two key parameter values. Use of the low intermediate cover reduced the efficiency of leachate recirculation, and thereby prevented the landfill from being operated as a true bioreactor. The flux measurements taken at the leachate pools indicate that a significant amount of CH4 gas escapes via this route. The use of low permeability cover Volume 58 May 2008

contributes to the formation of these pools and hence provision of a preferred route for gas migration and escape. Unfortunately, the quantity of CH4 gas escaping by this route could not be reliably estimated. Although the CH4 balance may have limitations because of the selection of key parameter values for the Scholl Canyon model, the study provides interesting information in relation to the relative importance of surface CH4 emissions and the efficiency of the gas collection system at Loma Los Colorados. This study revealed serious limitations in the design and operation of bioreactor landfills in Latin America. Although heavy emphasis is placed on leachate collection and recirculation systems, very little attention is directed toward daily and intermediate cover composition and gas collection system design. The unexpectedly high surface emission rates point to a disturbing trend of CH4 emissions from mega landfills being built in developing countries. The increasing trend in Latin America toward high efficiency leachate capture (with the use of synthetic liner systems) and recirculation to increase biodegradation of organic waste and concomitant gas production is a concern if the gas capturing systems are not efficient. Such changing practices and the specificity of waste management practices should be considered in estimation of CH4 emissions from Latin American landfills. The site characteristics at Loma Los Colorados differ significantly from those found in the developed countries of Europe and North America. The conditions at Loma Los Colorados indicate that several factors need to be considered for each country or region to accurately model worldwide LFG emissions. These are: • Climate and moisture conditions; • Waste composition; • Relative numbers of active and closed landfill sites; • Differences between active and closed landfill sites; • Prevailing landfill construction and operation practices (e.g., soil cover, gas recovery systems, efficiency of recovery, time delay until placement of final soil cover); and • Rate of CH4 oxidation in cover soil. Landfills in developing nations can be expected to have higher gas production and emission rates than those typically found in developed nations. This is primarily due to a higher organic content of waste, climatic factors (e.g. higher average temperature, higher rainfall), and differing landfill practices. Considering the favorable conditions, such as high organic content of waste, high rainfall and temperature, and the practice of leachate recirculation, the Loma Los Colorados landfill in Chile produced and emitted higher gas quantities over a short time period than a similar sized landfill in North America and Europe. The results indicated that most of the gas emissions come from a small percentage of the landfill surface area having high flux rates. The existence of these outliers, or emission “hot spots”, underscores the need for a sufficiently large sample size when using the flux chamber technique to determine a landfill’s total gas emissions. Journal of the Air & Waste Management Association 627

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Sanderson et al. CONCLUSIONS This paper presents an indicative CH4 balance for a bioreactor landfill in Latin America. Several CH4 emission sources were considered. Among the sources considered, surface emissions was the largest single source with approximately 38% of the total. It is worthwhile to note that unaccounted for CH4 amounted to more than 50%, thus indicating that CH4 is either stored or lost in some other manner. The results show that the design and construction of the gas extraction system in Loma Los Colorados is inadequate and does not sufficiently remove CH4. However, if CH4 can be managed through better design of a gas extraction system and biological oxidation, bioreactors can contribute to mitigating climate change. Better understanding of CH4 storage and losses through the leachate systems in landfills is necessary to conduct a proper CH4 balance. Bioreactor landfills are being lauded as a more environmentally acceptable method of waste disposal because they require less time to stabilize waste, thereby eliminating long-term environmental and human health risks. However, the CH4 balance study at the Loma Los Colorados landfill illustrates potential shortcomings of bioreactor landfills when improperly designed or operated. For example, if the gas collection system is inadequately designed and constructed, achieving a collection efficiency of less than 5%, large amounts of GHGs could escape into the atmosphere in a short period of time, potentially contributing significantly to global warming. Part of the poor collection efficiency may be due to a potential shortcoming of bioreactor landfill design, namely their rapid gas production rates resulting in the escape of gas before placement of the final impermeable covers. ACKNOWLEDGMENTS The work described in this manuscript was supported by grants from the Canadian International Development Agency (CIDA) and NSERC. The authors acknowledge collaborators from the Universidad Catolı´ca de Valparaı´so (UCV) in Chile, Dr. Juan Palma, Dr. Marcel Szanto´, Dr. Rau´l Espinace, Manuel Cerda, Dr. Jose´ Torres and Dr. Jorge Santana; and from the University of Calgary, Vince Stein and Dr. Ida Soh. The authors also acknowledge the numerous people from Loma Los Colorados landfill who contributed to make the project a success.

7. Emission of Greenhouse Gases in the United States, 1998; DOE/EIA0573(98) Energy Information Administration; U.S. Department of Energy: Washington, DC, 1999. 8. Kiasa and DeMarco Sociedad Anónima. Various Monthly Reports on the Operation and Functioning of the Landfill Loma Los Colorados. Operacio´n y Funcionamiento del Relleno Sanitario Loma Los Colorados. Presentacio´n: Servicio de Salud Metropolitano del Ambiente (SESMA). Loma Los Colorados, Montenegro, Chile, March 1996 to July 2000. 9. Tchobanoglous, G.; Theisen, H.; Vigil, S. Integrated Solid Waste Management: Engineering Principles and Management Issues; McGraw-Hill: New York, 1993. 10. Whalen, S.C.; Reeburgh, W.S.; Sandbeck, K.A. Rapid Methane Oxidation in a Landfill Cover Soil; Appl. Environ. Microbiol. 1990, 56, 34053411. 11. Nozhevnikova, A.; Lifshitz, A.B.; Lebedev, V.S.; Zavarzin, G.A. Emission of Methane into the Atmosphere from Landfills in the Former USSR; Chemosphere 1993, 26, 401-417. 12. Czepiel, P.; Mosher, B.; Shorter, J.H.; McManus, J.B.; Kolb, C.E.; Allwine, E.; Lamb, B.K.; Harriss, R.C. Landfill Methane Emissions Measured by Enclosure and Atmospheric Tracer Methods; J. Geophys. Res. 1996, 101, 16711-16719. 13. Bogner, J.; Meadows, M.; Czepiel, P. Fluxes of Methane between Landfills and the Atmosphere: Natural and Engineered Controls; Soil Use Manage. 1997, 13, 268-277. 14. Perera, M.D.N., Hettiarachi, J.P.A.; Achari, G. A Mathematical Modeling Approach to Improve the Point Estimation of Landfill Gas Surface Emissions Using the Flux Chamber Technique; J. Environ. Eng. Sci. 2002, 1, 451-463. 15. Stein, V.B.; Hettiaratchi, J.P.A. Methane Oxidation in Three Alberta Soils: Influence of Soil Parameters and Methane Flux Rates; Environ. Technol. 2001, 22, 101-111. 16. Sanderson, J.N. Quantitative Assessment of Methane Emissions from Landfills. M.Sc. Thesis, Department of Civil Engineering, University of Calgary, Alberta, Canada, 2001. 17. Kightley, D.; Nedwell, D.B.; Cooper, M. Capacity for Methane Oxidation in Landfill Cover Soils Measured in Laboratory-Scale Soil Microcosms; Appl. Environ. Microbiol. 1995, 61, 592-601. 18. Ward, R.S.; Williams, G.M.; Hills, C.C. Changes in Major and Trace Components of Landfill Gas during Subsurface Migration; Waste Manage. Res. 1996, 14, 243-261. 19. Boltze, U.; De Freitas, M.H. Monitoring Gas Emissions from Landfill Sites; Waste Manage. Res. 1997, 15, 463-476. 20. Cossu, R.; Andreottila, G.; Muntoni, A. Modeling Landfill Gas Production in Landfilling of Waste: Biogas; Christensen, T.H., Cossu, R., and Stegmann, R., Eds.; and E&FN SPON: London, U.K., 1996. 21. Espinace, R.; Schiappacasse, M.C. Characterization of Open Dumps “Lajarilla” (Vina´⬘del Mar) and “El Molle (Valparaiso). Application of the Treatment of Leachate from Urban Solid Waste, Evaluation of the Effect of Velocity in the Biodegradation Process and Settlement. Faculty of Construction Engineering and Faculty of Biochemical Engineering, Vin ˜ a del Mar, Chile, 1997. 22. Mosher, B.W.; Czepiel, P.M.; Harriss, R.C. Methane Emissions at Nine Landfill Sites in the Northeastern United States; Environ. Sci. Technol. 1999, 33, 2088-2094. 23. Czepiel, P.; Mosher, B.; Shorter, J.H.; McManus, J.B.; Kolb, C.E.; Allwine, E.; Lamb, B.K.; Harriss, R.C. Landfill Methane Emissions Measured by Enclosure and Atmospheric Tracer Methods; J. Geophys. Res. 1996, 101, 16711-16719.

REFERENCES 1. Intergovernmental Panel on Climate Change. Technical Summary. In Climate Change 2001: The Scientific Basis. Joos, F., Ramirez-Rojas, A., Stone, J.M.R., Zillman, J., Eds.; Cambridge University: Cambridge, U.K., 2001. 2. Bogner, J.; Meadows, M.; Repa, E. A New Perspective of Measuring and Modeling of Landfill Methane Emissions; Waste Age 1998, 29 118-130. 3. Kumar, S.; Mondal, A.N.; Gaikwad, S.A.; Devotta, S.; Singh, R. N. Qualitative Assessment of Methane Emission Inventory from Municipal Solid Waste Disposal Sites: a Case Study; Atmos. Environ. 2004, 38, 4921-4929. 4. Chomsurin, C. Evaluation of Gas Migration and Methane Oxidation in Domestic Solid Waste Landfills. Masters Thesis, Asian Institute of Technology, Bangkok, Thailand, 1997. 5. Charles, D.; Jones, B.M.R.; Salway, A.G.; Eggleson, H.S.; Milne, R. Treatment of Uncertainties for National Estimates of Greenhouse Gas Emissions; AEAT-2688-1; Prepared for DETR Global Atmosphere Division, AEA Technology, Culham, U.K., 1988. 6. Rypdal, K.; Zhang, L.C. Uncertainties in the Norwegian Greenhouse Gas Inventory; Report 2000/13, Statistics Norway, Oslo–Kongsvinger, Norway, 2000.

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About the Authors Jenny Sanderson is a research associate, Patrick Hettiaratchi is an associate professor, Carlos Hunte is a Ph.D. candidate, and Omar Hurtado is a Ph.D. candidate with the Department of Civil Engineering, Engineering Research and Education (CEERE) at the University of Calgary. Alejandro Keller is a landfill engineer with Kiasa and DeMarco Sociedad Anónima, Grupo Urbaserkiasa, Santiago, Chile. Please address correspondence to: Patrick A. Hettiaratchi, University of Calgary, CEERE, 2500, University Drive Northwest, Calgary, Alberta, Canada T2N 1N4; phone: ⫹1-403-2205503; fax: ⫹1-403-282-7026; e-mail: [email protected].

Volume 58 May 2008