May 10, 2008
Methane Oxidation in Compost-Based Landfill Cover With Vegetation During Wet and Dry Conditions in the Tropics
By Tanthachoon, Nathiya Chiemchaisri, Chart; Chiemchaisri, Wilai; Tudsri, Sayan; Kumar, Sunil
ABSTRACT The effect of compost and vegetation on methane (CH^sub 4^) oxidation was investigated during wet and dry conditions in a tropical region. A laboratory-scale experiment was conducted to examine the performance of nonvegetated and vegetated landfill cover systems in terms of CH^sub 4^ oxidation efficiency. Two types of landfill cover materials (compost and sandy loam) and two species of tropical grasses (Sporobolus virginicus and Panicum repens) were studied for their effect on the CH^sub 4^ oxidation reaction. It was found that the use of compost as cover material could maintain a high methane oxidation rate (MOR) of 12 mol CH^sub 4^/m^sup 3^ * day over a 250-day period. Leachate application showed a positive effect on promoting methanotrophic activity and increasing MOR. A high MOR of 12 mol CH^sub 4^/m^sup 3^ * day was achieved when using compost cover with P. repens during wet and dry seasons when leachate irrigation was practiced. In dry conditions, a lower MOR of 8 mol CH^sub 4^/m^sup 3^ * day was observed for 80 days.
Landfilling is a conventional technique for municipal solid waste management in many developing countries. However, significant environmental impacts such as leachate and gas production can be created if the landfills are not designed and operated properly. In many developing countries, the gases produced from biodegradation of solid waste in landfills are released directly to the atmosphere without any treatment. Methane (CH^sub 4^) and carbon dioxide (CO2) are the major landfill gases that contribute to ambient temperature rising referred to as the "global warming effect" or "greenhouse effect." Furthermore, the global warming potential (GWP) of CH^sub 4^ is reported to be approximately 23 times higher than CO2 in a 100- yr time horizon.1 Therefore, appropriate landfill gas (LFG) management is one essential for landfill operation. To diminish CH^sub 4^ emissions from landfills, CH^sub 4^ can be utilized as an energy source in large landfills, but it should be eliminated when the utilization is not feasible, especially in small landfills.
One possible alternative for CH^sub 4^ mitigation is using landfill cover soil as a biofilter, in which soil microorganisms play an important role in consuming CH^sub 4^ via their metabolism (Figure 1). CH^sub 4^-oxidizing bacteria, or methanotrophs, are the specific microorganisms that have in common the ability to utilize CH^sub 4^ as a sole carbon and energy source under aerobic conditions.2 The microbiological process of oxidizing CH^sub 4^ to CO2 by methanotrophs is called "microbial CH^sub 4^ oxidation." Previous studies on landfill CH^sub 4^ oxidation in the cover layer have demonstrated the potential for reducing CH^sub 4^ in LFGs. However, the capacity of landfill cover soil to oxidize CH^sub 4^ depends on both the physical and the chemical properties of landfill cover materials, such as soil type, moisture content, density, organic and nutrient content, etc.3-7 Additionally, environmental conditions (e.g., temperature and oxygen availability) can also impact the performance of landfill cover soils. Provided that the proper cover material and the appropriate environment are optimized during landfill operation, the application of the CH^sub 4^ oxidation process in topsoil is preferable to control CH^sub 4^ emission from landfills with low cost association.
The aim of this work was to improve the efficiency of CH^sub 4^ oxidation in landfill cover soil using compost and vegetation with leachate irrigation practices. In addition, the effect of compost application on microbial CH^sub 4^ oxidation was investigated to determine optimum conditions for methanotrophic activity. The overall loose texture and high porosity of compost encourage air diffusion and moisture retention; in addition, further supplemental nutrients were also supplied for microbial activities.6,8,9
The provision of vegetation on the landfills could also help encourage the penetration of oxygen downward into the rhizosphere via plant lacunae or aerenchyma. 10,11 Previous reports reveal the beneficial effect of plants for stimulating microbial CH^sub 4^ oxidation in the rhizosphere.12,13 Local grasses in tropical regions were examined in this study. To keep the proper moisture content of cover soil at an optimum range of 10-15% for methanotrophic activity,4,7 leachate irrigation on landfill cover can compensate for water loss through evaporation and plant transpiration. When leachate irrigation was used for this purpose, its volume, which must be treated and discharged to the environment, could be substantially reduced. Irrigated leachate could also enhance soil microbial activities by providing supplemental nutrients for the microorganisms in soil. Figure 2 illustrates CH^sub 4^ oxidation in landfill cover soil with vegetation and leachate irrigation.
MATERIALS AND METHODS
Acrylic columns with diameters of 15 cm and heights of 100 cm were used to simulate a landfill cover system. Landfill cover materials were filled into the columns to a depth of 60 cm. All columns were purged at the bottoms with artificial LFGs (CH^sub 4^:CO2 = 60:40), a flow rate of 4 mL/min (equivalent to a CH^sub 4^ flux of 14 mol CH^sub 4^/ m3 * day). Air diffusion was supplied naturally from the top of the columns. Each column had 5 rubber septum ports for gas sampling along the depth of column (i.e., 5, 15, 30, 50 cm from soil surface and gas inlet). The columns were irrigated with 200 mL of either rainwater or leachate every 4 days to maintain soil water content and also to represent rainy or wet conditions in a tropical climate. This applied hydraulic loading rate (2.83 mm/ day) was set according to the average annual precipitation in Thailand.
Stabilized leachate with a low ratio of biochemical oxygen demand/ chemical oxygen demand was used in this study. The characteristics of leachate were continually examined in the time interval following the standard method for examination of water and wastewater.14 Table 1 shows the average value of leachate characteristics. The dry condition was simulated with no irrigation practice. Additionally, the moisture content of cover materials were also continuously monitored throughout the experimental period by soil moisture sensors (ECHO, model EC-10). These sensors were installed at 5- to 15-, 25- to 35-, and 45- to 55-cm depths from the soil surface, and moisture data were recorded on-line via a data recorder (ECHO, model Em5). The schematic of experimental system is illustrated in Figure 3.
CH^sub 4^ Oxidation in Landfill Cover Systems
Effect of Cover Materials. Two types of landfill cover materials, sandy loam and compost, were used in the column experiment for investigating the effect of their properties on CH^sub 4^ oxidation. Sandy loam consists of 80% sand, 8% silt, and 12% clay (wt basis), whereas compost was a commercial grade of leaf compost. The physical and chemical properties of these materials were determined according to a handbook for tropical soil methodology.15 Their characteristics are presented in Table 2. The initial moisture of sandy loam was prepared to have an optimum moisture content of 10-15% (dry wt basis) for methanotrophic activity, whereas compost was used with its original moisture content of 50-55%. Four columns were operated under either rainwater or leachate irrigation as previously mentioned.
Vegetation on Landfill Cover. Vegetation was conducted to evaluate the effect of plants on CH^sub 4^ oxidation. Compost was used as landfill cover material in this experimental section. Two tropical grasses (Figure 4), i.e., Sporobolus virginicus and Panicum repens, were selected according to their salt tolerant characteristics. All grasses were grown for approximately 2 wk in nursery pots before being planted into the experimental columns at the same initial height of 10 cm and the same number of plants for each column (two plants in each column, equivalent to a plant density of 113 plants/m2). Four vegetated columns were operated with rainwater or leachate irrigation similar to the previous experimental section. Moreover, a simulated sunlight condition was supplied for plant growth during the day (average light intensity of 35,000 luxes).
Comparison between Wet and Dry Conditions. In two previous experimental sections, CH^sub 4^ oxidation efficiency was evaluated under intermittent irrigation of rainwater and leachate in wet conditions. However, a dry condition without irrigation was also simulated to determine CH^sub 4^ oxidation efficiency in each of the cover materials (sandy loam and compost) and each of the vegetated cover materials (sandy loam with P. repens and compost with P. repens).
CH^sub 4^ Oxidation Rate. Gas samples were collected throughout the depth of the soil column via rubber septum ports. Subsequently, 300-[mu]L gas samples were analyzed by gas chromatography (GC) using a model 6890 series (Agilent). The GC analytical condition was set as follows: column model of CTR (Alltech Associates, Inc.) I; inlet temperature of 105 [degrees]C; column temperature of 35 [degrees]C; thermal conductivity detector temperature of 150 [degrees]C; and carrier gas (helium) flow rate of 65 mL/min. To determine CH^sub 4^ conversion efficiency of landfill cover, the methane oxidation rate (MOR) was calculated from the reduction of CH^sub 4^ concentration in landfill cover, as shown in the following equation: ... (1)
where Q is the gas flow rate (mL/day); (CH^sub 4^)in and (CH^sub 4^)out are the CH^sub 4^ concentration (mol/mL) of inflow and outflow, respectively; and V is the volume of landfill cover material (m3).
Methanotrophic Activity. The activity study was conducted to evaluate the potential for methanotrophic activity throughout the depth of landfill cover system and to examine performance of each cover material. Ten grams of soil samples from a bare column experiment were transferred to 188-mL serum bottles capped with rubber septa and aluminum rings. Each sample was examined in duplicate. Subsequently, 10 mL of pure CH^sub 4^ was added to the incubated bottle for a concentration of 9% CH^sub 4^ in the headspace. The actual initial gas concentration was determined 5 min after pure CH^sub 4^ was injected to ensure a homogeneous gas distribution inside the bottle. All bottles were incubated at room temperature (28-30 [degrees]C) and gas constituents in the headspace were investigated by GC at the time of initiation and every day for 10 days.
Production of Extracellular Polysaccharide and Methanotrophic Community. Soil samples, after the experimental period, were preserved at -20 [degrees]C for extracellular polysaccharide (EPS) determination in terms of D-glucose by using the "total and labile polysaccharide analysis of soils" method.16 Moreover, the population of methanotrophic bacteria throughout the experimental columns was determined by using a fluorescence in situ hybridization (FISH) technique. Soil samples were prepared by the following steps: (1) extraction with sodium chloride (NaCl); (2) fixation into paraformaldehyde and washing with phosphate buffer solution (PBS); (3) immobilization of fixative sample onto the gelatin coated slide; (4) hybridization with oligonucleotide probes (My84 +My705 and Malpha450 for type I and type II methanotrophic bacteria, respectively) and washing excess probes; and (6) staining with DAPI (4',6-diamidino-2-phenylindole) to observe total microorganisms. The numbers of methanotrophs were defined as the percentage of total cell numbers determined by DAPI counting.
Mass Balance of Carbon in Landfill Cover System. Four major components in the carbon mass balance were input, output, reaction, and change in accumulation. In the landfill cover system, carbon inflow consisted of carbon from CH^sub 4^ gas and leachate irrigation whereas carbon out- flow was carbon from direct emission of CH^sub 4^ to the atmosphere and from leachate percolation. Moreover, CH^sub 4^ was also oxidized via the CH^sub 4^ oxidation reaction in which carbon from CH^sub 4^ would be consumed and partially utilized for methanotrophic growth. The heterotrophic growth derived from organic carbon also increases biomass accumulation in soil. The carbon mass balance is illustrated in the following equation:
DeltaC = [C(CH4 in) - C(CH4 out) - C(CH4 oxidized)] + [Cirrigation - Cpercolation] (2)
where DeltaC is the change in soil carbon mass; C(CH^sub 4^ in) and C(CH^sub 4^ out) are carbon mass from the CH^sub 4^ inflow and out- flow, respectively; C(CH^sub 4^ oxidized) is the carbon mass from oxidized CH^sub 4^; and Cirrigation and Cpercolation represent the carbon mass from rainwater/leachate irrigation and percolation, respectively.
RESULTS AND DISCUSSION
Effect of Compost as Landfill Cover Material on CH^sub 4^ Oxidation
MORs in experimental columns with sandy loam and compost materials are shown in Figure 5. It was found that both materials provided high MOR (8 mol CH^sub 4^/ m3 * day) at the beginning of the experiment. The methanotrophic activity rapidly developed shortly after startup. Throughout the experimental period, the MOR in compost could be maintained at a higher value ( 12 mol CH^sub 4^/ m3 * day) and longer period (over 250 days) than in sandy loam ( ~8 mol CH^sub 4^/m3 * day over 200 days), especially in the case of leachate irrigation. Higher porosity materials such as leaf compost could allow more air to penetrate into their texture, thus resulting in higher oxygen availability for CH^sub 4^ oxidation as compared with sandy loam. The analysis of gas profile along the depth of both materials also confirmed the capability of oxygen penetration into compost deeper than in sandy loam (Figure 6). The 50-cm-depth active zone of methanotrophic activity was found in compost, whereas sandy loam presented only 15-30 cm. High porosity and water-holding capacity characteristics, which support adequate oxygen and moisture content, have also been reported to benefit CH^sub 4^ oxidation.6,8,17
In addition to the beneficial physical properties of compost, higher organic content in compost also affects CH^sub 4^ oxidation. The MOR increases with the increasing of organic content in landfill cover.18 Humer and Lechner19 also showed that high-organic material such as compost was very efficient in oxidizing CH^sub 4^; it provided high porosity as well as nutrients for methanotrophs. For these reasons, compost should exhibit higher CH^sub 4^ oxidation efficiency than sandy loam. However, from our experiment, compost could sustain high MOR throughout the experimental period only in the case of leachate irrigation, but not in rainwater irrigation. This could imply that leachate also had a positive effect on methanotrophic activity. The benefit of leachate on CH^sub 4^ oxidation was possibly because of its chemical properties in providing sufficient nutrients, which would alter the effect of maintaining moisture content. A similar observation was also reported by Maurice et al.20 However, long-term irrigation could also diminish methanotrophic activity. Watzinger et al. 21 showed that high-water accumulation caused clogging in soil pores and restricted gas diffusion. Our study showed that MOR in all columns rapidly declined at the end of the experiment, caused by the depletion of oxygen concentration below 2-3% by volume. This absence of sufficient oxygen supply was reported to critically affect the CH^sub 4^ oxidation reaction.22
EPS formation by methanotrophs was also considered in this study. In the experimental columns, the slime of EPS could be observed and was confirmed by the analysis of D-glucose concentration. At the end of the experiment, EPS in sandy loam was found to be approximately 5 mg D-glucose/g throughout the depth profile whereas in compost EPA was in the range of 15-20 mg D-glucose/g. The highest EPS concentration was found in the upper layer, where the highest methanotrophic activity took place. Hilger et al.23,24 and Chiemchaisri et al.25 proposed that EPS production contributes to the sustenance of methanotrophs from unsuitable conditions, such as desiccation, predation, heat, and a carbon-rich environment. The accumulation of EPS in landfill cover soil limited MOR by sealing in soil pores and limiting oxygen penetration. From our study, CH^sub 4^ oxidation could be achieved at a relatively constant rate over 250 days in compost cover with leachate contribution.
The activity of methanotrophs in sandy loam and compost was studied in a laboratory batch experiment. Figure 7 illustrates the CH^sub 4^ consumption activity of sandy loam and compost at various depths. For sandy loam (Figure 7, a and c), the CH^sub 4^ concentration gradually declined from 9 to 0% in 100 hr. A lower CH^sub 4^ consumption rate was observed from 50-60 cm. In compost material, the CH^sub 4^ consumption was completely accomplished within 20 hr (Figure 7, b and d). Higher CH^sub 4^ consumption in the active zone of compost (0-30 cm) confirmed its high MOR in the column experiment. These results indicated that compost throughout the entire depth provided high methanotrophic activity. Nevertheless, it should also be noted that the increase in the lag periods before CH^sub 4^ consumption, especially in the case of compost, was to a certain extent related to the depth of soil sampling. Deeper layers of soil were generally more anoxic for methanotrophs. Roslev and King26,27 reported that some methanotrophs can survive under anoxic conditions for several months because of resting cell formation (cysts or exospores) and then respond when CH^sub 4^ and oxygen once again become available. They also indicated that the duration of lag period increased with increasing starvation (absence of CH^sub 4^) time. On the contrary, these resting stages were not found in our experiment. This was determined by staining techniques, which indicated presence of the Gram- negative rod of CH^sub 4^-oxidizing bacteria, but not their cysts or exospore forms. As reported by Whittenbury et al.,28 resting cells would be formed because of the absence of CH^sub 4^, desiccation, and drying. Therefore, formation of methanotrophic resting cells did not occur in our experimental materials (compost or sandy loam), which were sampled from the CH^sub 4^-available environment. Moreover, the difference in the ratio of oxygen and CH^sub 4^ (O2:CH^sub 4^) was considered a major cause for the increase in the lag period. A high O2:CH^sub 4^ ratio ( ~1.8) in our batch experimental condition was close to that of ratio in the upper layer of compost (15 cm depth from surface), thus its methanotrophic activity could rapidly resuscitate when incubation was initiated. The much lower ratio of O2:CH^sub 4^ in its original surroundings was attributed to the extended period of time for adaptation of methanotrophic activity. The experimental results demonstrated that the activity of methanotrophs in compost was higher than in sandy loam and the active zone in compost was found deeper than that of sandy loam. Additionally, detection of methanotrophic bacteria at different depths was performed by using a FISH technique. The abundance of methanotrophs was found only in the topsoil, i.e., approximately 5% of the total microorganisms in the sandy loam column at a 5- to 15-cm depth, and 10-30% of the total microorganisms in the compost column at a 5- to 30-cm depth. From the results, the higher methanotrophic population found in compost confirmed higher methanotrophic activity and an increased methanotrophic active zone in compost.
Effect of Vegetation on CH^sub 4^ Oxidation
Compost material was used as final cover in this vegetated column experiment. The compost columns with two species of tropical grasses (S. virginicus and P. repens) were compared for their performance on stimulating CH^sub 4^ oxidation. Figure 8a shows MOR in the case of rainwater irrigation. It was found that the vegetated columns with S. virginicus and P. repens lost their CH^sub 4^ oxidation efficiency at the beginning period of the experiment due to water logging in the topsoil. However, when the grasses were recultivated, MOR subsequently recovered. Afterward the vegetated column with P. repens had a steady MOR of approximately 12 mol CH^sub 4^/m3 * day for over 330 days, whereas S. virginicus exhibited lower a MOR and shorter active period. As shown in Table 3, although the average MOR was not significantly different in the presence or absence of grasses, the active zone was significantly deeper in the existence of vegetation (P. repens).
In the case of leachate irrigation (Figure 8b), a similar trend of MOR was obtained in the experimental columns with and without vegetation. These two columns showed high methanotrophic activity, with a constant MOR of 12 mol CH^sub 4^/m3 * day for 250 days, whereas the column with S. virginicus had a fluctuating period of MOR before it gradually self-recovered to a constant rate of 11 mol CH^sub 4^/ m3 * day. These results clearly show that the leachate application practice could sustain high MOR in a long-term operation.
MORs at various depths in the experimental columns are shown in Table 3. The vegetated columns had higher MORs at deeper zones (15- 30 and 30-50 cm) compared with nonvegetated columns, due to the support from the plant root system and rhizosphere environment for methanotrophic bacteria. P. repens possessed a longer andwider root system compared with S. virginicus (Figure 4); hence, it could supply more oxygen into the deeper zones and enhanced MOR. Plant roots can also produce exudates and release them to the rhizosphere, which provides beneficial support as a nutrient supplement and moisture retention for soil microorganisms.13,29 Consequently, all vegetated columns contained a high amount of methanotrophs (10-30% of the total microorganisms) throughout almost the entire column depth (5-50 cm), especially at the rhizosphere, whereas the column without vegetation revealed the abundance of methanotrophs only in the topsoil. Root systems significantly responded to the abundance of methanotrophs in the lower portions of the column in comparison with the nonvegetated columns. The oxygen supplied in the deeper zones by root systems seemed to be an important factor in regulating methanotrophic growth, followed by the availability of nitrogen sources. The existence of grasses encouraged methanotrophic activity until the oxygen-deficient condition took place. In this study, P. repens clearly exhibited its advantage in enhancing CH^sub 4^ oxidation when rainwater or leachate was irrigated. It also revealed strong tolerance characteristics to leachate and LFGs.
As a constant CH^sub 4^ loading rate of 14 mol CH^sub 4^/ m3 * day was applied, a high efficiency of CH^sub 4^ removal at 86% (equivalent to MOR of 12 mol CH^sub 4^/m3 *day) was achieved in the compost cover layer (both vegetated and nonvegetated systems), whereas only a moderate removal of 57% (8 mol CH^sub 4^/m3*day) was achieved in sandy loam. Previous studies5,7 also found that MOR increased with increasing CH^sub 4^ loading rate, but the removal effi- ciencies decreased as the loading rate increased.
The declination of CH^sub 4^ oxidation capacity in longterm operation was caused by moisture and EPS accumulations at the top of the cover layer, resulting in limited oxygen penetration through soil pores and therefore suppressed methanotrophic activity. Moreover, the excess exudates from plant roots could also be the cause of oxygen deficiency for methanotrophic activity.
Effect of Moisture on CH^sub 4^ Oxidation
To simulate wet and dry conditions in the tropics, a wet- ondition experiment was conducted by intermittent application of rainwater or leachate. A dry-condition experiment was examined without irrigation practice. From the previous results, rainwater and leachate irrigation provided high CH^sub 4^ oxidation in both nonvegetated soil and vegetated conditions. The comparison of CH^sub 4^ oxidation efficiency between wet and dry conditions is shown in Table 3. The wet condition maintained a high MOR of 11 mol CH^sub 4^/ m3 *day throughout the experimental period of 100 days in the case of the nonvegetated compost column, and 330 days in the case of the compost column with P. repens. Leachate irrigation successfully sustained high MOR at 12 mol CH^sub 4^/m3 *day for over 240 days in nonvegetated and vegetated columns. Meanwhile, the dry condition had lower MOR of 8 mol CH^sub 4^/m3 * day in a shorter active period of approximately 80 days.
The moisture contents along the depth of the experimental columns under wet and dry conditions are demonstrated in Figure 9. Sandy loam exhibited periodical fluctuations in moisture content but maintained an appropriate range for methanotrophic activity (13-16% dry wt basis). The fluctuation was found higher in the top layer because of evaporative loss. In the compost case, the moisture content was much higher (33-36%), but the fluctuation was much less than sandy loam because of the higher water adsorptive capacity of compost. In the nonirrigation practice, moisture content noticeably changed along the depth in both the sandy loam and compost columns. A proper moisture content of 11-14% was only preserved in the middle and bottom part of the sandy loam column from the water-saturated LFG supply, whereas a gradual decrease in moisture content was revealed in the top part because of evaporation. Similarly, in the compost case, steady moisture content in the lower zone was also achieved and the decline in moisture content in the upper layer was even faster.
The results indicated that the irrigation frequency of 200 mL/4 days could maintain soil moisture fluctuation within the proper range (13-16%) for methanotrophs, and this fluctuation also encourages oxygen diffusion into the cover soil during the nonirrigated period to enhance CH^sub 4^ oxidation. Intermittent irrigation helped provide oxygen availability into the deeper zones of cover soil during the drying period and subsequently enhanced CH^sub 4^ oxidation.
Comparing the experimental results under wet and dry conditions indicated that moisture maintenance in cover soil was an important factor governing CH^sub 4^ oxidation. The water content should be maintained at the optimum level for CH^sub 4^-oxidizing bacteria and gas transport in soil. An increase in water content could increase water fill in the soil voids and then inhibit oxygen diffusion into the soil.4 Low water content, on the other hand, resulted in microbial desiccation and activity reduction.30 Intermittent irrigation of rainwater and leachate in wet conditions maintained appropriate water content for methanotrophic activity and supported plant growth in landfill cover materials. Nevertheless, leachate application also helped to stimulate MOR and a methanotrophic active period because of the provision of supplemental nutrients. However, the absence of moisture control in dry conditions produced moderate methanotrophic activity in both nonvegetated and vegetated columns. The CH^sub 4^ oxidation capacity of methanotrophs was reduced over a short period because of water loss to below critical point for microorganism survival. It can be concluded that the occurrence of CH^sub 4^ oxidation in landfills was successfully maintained over the year duration by application of leachate if rainfall is not available. Moreover, the operation without any irrigation in dry condition could also produce moderate MORs for 80 days.
The determination of water balance in sandy loam and compost are shown in Figure 10. Irrigation was continually applied every 4 days during the experimental period. Percolation of leachate from the experimental column was measured. Water production from the CH^sub 4^ oxidation reaction was omitted from this water balance calculation because it was previously determined to be an insignificant component. Loss of water through evaporation can be estimated. In sandy loam, it was found that approximately 40% of the irrigation evaporated during the 320-day applied period and the remaining approximate 60% percolated from the sandy loam medium. In the case of compost, the degree of percolation and evaporation was the same (50% each). This information is useful in determining the leachate application rate for CH^sub 4^ oxidation.
The carbon mass balance for landfill cover system was also determined (Table 4). There was a 23 and 44% increase in carbon CH^sub 4^ oxidation for cover soil as compared with sandy loam under rainwater and leachate irrigation, respectively. These improved reactions yielded a 32 and 56% reduction in CH^sub 4^ emission for compost as compared with sandy loam cover under rainwater and leachate irrigation. On the basis of this research, a practical design for landfill cover system for effective CH^sub 4^ removal can be proposed. High porosity and nutrient-rich material such as compost is a suitable material to be used as landfill cover, with effective depth of 0.60 m. However, this compost should be well stabilized with a low-oxygen uptake rate to reduce competitive oxygen consumption of heterotrophic and methanotrophic bacteria. Intermittent irrigation of mature leachate enhanced oxygen diffusion into landfill cover and promoted CH^sub 4^ oxidation capacity. Provision of vegetation (tropical grass such as P. repens) at an optimum density also helped sustain CH^sub 4^ oxidation in long- term operations.
From the experimental investigation of CH^sub 4^ oxidation in a compost-based landfill cover with vegetation, the following conclusions can be drawn:
(1) Compost was found to be effective landfill cover material for methanotrophic activity. It could provide high and constant MOR at 12 mol CH^sub 4^/ m3 * day over a 240-day experimental period with leachate irrigation. The utilization of compost allowed oxygen penetration into the deeper zones via its loose texture and supplied supplemental nutrients for methanotrophic activity.
(2) High MOR was achieved in a compost-based cover layer with vegetation (P. repens) over 330 days under rainwater application and 240 days under leachate irrigation (wet condition). The longer and wider root system of P. repens provided deeper oxygen diffusion into cover material and benefited methanotrophic activity.
(3) The operation of landfill under naturally dry conditions had a moderate MOR of 8 mol CH^sub 4^/m3 * d for 80 days.
The authors would like to acknowledge Swedish International Development Cooperation Agency (SIDA) for their financial support in this research work through Asian Regional Research Program on Environmental Technology (ARRPET).
A low-cost option for controlling CH^sub 4^ emission from landfills is the CH^sub 4^ oxidation process by methanotrophic bacteria in final landfill cover soil. This study aimed to improve landfill cover soil in the tropical region for an effective CH^sub 4^ oxidation. The findings suggest that compost is a suitable landfill cover material to encourage CH^sub 4^ oxidation and planting with tropical grasses also helps to sustain CH^sub 4^ oxidation. Results of this investigation provide another direction for improving landfill gas management in the tropical region.
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Nathiya Tanthachoon, Chart Chiemchaisri, and Wilai Chiemchaisri
Department of Environmental Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand
Department of Agronomy, Faculty of Agriculture, Kasetsart University, Bangkok, Thailand
National Environmental Engineering Research Institute (NEERI), Kolkata Zonal Laboratory, Kolkata, India
About the Authors
Nathiya Tanthachoon is currently a doctoral student in the Department of Environmental Engineering at Kasetsart University in Thailand and also working as a lecturer at the School of Engineering at Naresuan University in Thailand. Chart Chiemchaisri and Wilai Chiemchaisri are associate professors in the Department of Environmental Engineering at Kasetsart University. Sayan Tudsri is a professor in the Department of Agronomy at Kasetsart University. Sunil Kumar is a scientist in the National Environmental Engineering Research Institute (NEERI), Kolkata, India. Please address correspondence to: Chart Chiemchaisri, Department of Environmental Engineering, Faculty of Engineering, Kasetsart University, 50 Phaholyothin Road, Chatuchak, Bangkok, Thailand 10900; phone: +6- 2942-8555; fax: +6-2579- 0730; e-mail: [email protected]
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