Recently, a process based approach in landfill design and operation has been developed ... A bioreactor landfill is managed to accelerate decomposition of the.
OPERATING STRATEGIES FOR ENHANCING WASTE BIODEGRADATION AND INCORPORATION OF VEGETATED COVERSOIL TO MINIMIZE METHANE GAS EMISSION IN MSW LANDFILL C. Chiemchaisri1, W. Chiemchaisri1, S. Sittichoktam1,U. Yodsang1, K. Chittanukul1, N.Luknanulak1, T.Kornboonraksa1 and S.Tadsri2 1
2
Department of Environmental Engineering, Faculty of Engineering Kasetsart University, Bangkok 10900, Thailand
Department of Agronomy, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand
ABSTRACT Recently, a process based approach in landfill design and operation has been developed called bioreactor landfill. A bioreactor landfill is managed to accelerate decomposition of the organic wastes by controlling moisture content, recycling nutrients and seeding of microorganisms by circulating leachate back into the landfill cell. This research investigated the beneficial effect of using this concept in accelerating the biodegradation of wastes and simultaneously minimizes leachate problem in tropical landfill. In pilot scale experiment, four landfill lysimeters of 0.9 m. and 2.7 m. height were employed. The biodegradation of solid wastes in the lysimeters having different operating techniques, i.e. leachate circulation and leachate storage in the waste cells was compared with the conventional landfill operation. It was found that those operating techniques were beneficial to waste degradation and leachate management in tropical landfill. The water balance in those lysimeters was also verified. In order to control methane emission from these waste disposal sites, one of the low-cost alternatives is the utilization of microbial activity to oxidize methane and convert it to carbon dioxide in top cover soil provided that preferable environmental conditions are prevailed. Laboratory-scale experiment was conducted to examine the effect of leachate loading on methane oxidation in non-vegetated and vegetated cover soil. Sandy loam consists of 50% sand and 50% organic material (wt/wt) was used as cover soil. Two species of tropical grass, C. plectostachyus and C. dactylon was successfully grown in typical condition of landfill (exposed with landfill gas and rainwater). Cynodon dactylon, but not Cynodon plectostachyus, promoted methane oxidation in soil and leachate application also helped increasing methane oxidation rate in soil non-vegetated cover soil. At higher organic loading, however, adverse effect of organic loading on methane oxidation rate and plant growth was observed. This study suggests that selection of the appropriate plant species and the operation with optimum leachate loading were the key factors in order to achieve sustainable methane oxidation in landfill cover soil. INTRODUCTION Landfill has been widely used for municipal solid waste (MSW) disposal all over the world. Especially in developing countries, it is considered to be a reliable and cost effective method if adequate land is available. Many countries in Asia depend on landfill as their only sole
option for final waste disposal. However, improper management and operation of landfill could create severe environmental impacts such as groundwater pollution and nuisance odor. Due to advance knowledge of landfill behavior and decomposition process of MSW, there have been several researches focusing on upgrading existing landfill technology from a storage/containment (conventional landfill) to a process-based approach called bioreactor landfill. In contrary to conventional landfill, bioreactor landfill is designed to maximize the infiltration of water into the wastes in order to minimize leachate migration into subsurface environment and maximize landfill gas generation rates under controlled conditions (Reinhart and Townsend, 1998). A bioreactor landfill is managed to accelerate decomposition of organic wastes by controlling moisture content of wastes, recycling of nutrients and seeding of microorganisms by leachate circulation. Several researches on bioreactor landfill in laboratory, pilot and full scale have been conducted during past 30 years (Reinhart and Townsend, 1998). Most of them have agreed that leachate circulation could enhance waste stabilization, leachate quality and landfill gas production (Pohland, 1980; Titlebaum, 1982; Barlaz et al., 1987; Stegmann and Ehrig, 1989; Lay et al., 1998). However, some operational problems associated with leachate circulation were lack of appropriate circulation technique, leachate channeling and leachate ponding (Buivid et al., 1981). The experiments on bioreactor landfill were mostly conducted under semi-arid climatic condition using wastes with relatively low moisture content. In contrary, MSW from developing countries are mainly composed of easily biodegradable organic matter with high moisture content especially in tropical climate, therefore, providing different environmental conditions for microbial biodegradation. This study aimed to provide a better understanding of bioreactor landfill technology and their applicability in the tropical climate. In the aspect of gas emission control from landfill, the landfill cover soil plays an important role in controlling the emission of methane gas. Final cover may be consisted of multicomponents, which include surface layer (vegetative support), protection layer, drainage layer, hydraulic barrier layer, foundation layer, and gas collection (control) layer. However, it is not always possible to have completely of all these components, especially in small landfills of developing countries. In those countries, single cover layer using clay material is common and thus allow substantial gas emission from landfill through the final cover layer when the soil moisture content reduced to the shrinkage limit and the cracking took place. In order to minimize methane emission from those small landfills, there are several reports suggested the existence of methanotrophic bacteria in the final cover soil which help utilizing methane and converting them to carbon dioxide (Whalen et.al, 1990; Boeckx et.al, 1996; Roslev et.al, 1997). Application methane biodegradation via methanotrophic activity in topsoil is preferable to control of methane emission from landfill at low construction cost. Investigation on the proper design of landfill final cover layer to promote methanotrophic activity in landfill topsoil is essential for reduction of methane emission from landfill to the atmosphere. This technology will be attractive for small landfills in developing countries, as they require minimum operating and maintenance cost in post-closure period. Methane oxidation found in a final cover soil in different regions may associate with many factors such as type of soil, depths, temperature and moisture content (Boeckx et.al, 1996). Moreover, vegetation grown on the landfill surface was also shown to support this
biochemical reaction (Hilger et.al, 2000). In this study, methane oxidation in vegetated top cover soil with two species of tropical grass is studied. The effect of leachate irrigation on methane oxidation and plant growth is also investigated. MATERIALS AND METHODS Solid Waste Biodegradation Study in Pilot Scale Landfill Lysimeter In pilot scale study, four landfill lysimeters made of steel pipe of 0.90 m. diameter and 2.7-m height were used (Figure 1). The leachate drainage system was provided at the bottom of each lysimeter with a drainage pipe connected with 1-inch valve. Gravel layer (10-30 mm.) of 200 mm. thickness and geotextile sheet were provided at the bottom of lysimeter to prevent clogging of suspended solids at the outlet. Leachate was circulated back to the lysimeter in the gravel layer placed below cover soil at the top of waste layer. Distribution of leachate was accomplished through perforated PVC pipe of 1-inch diameter installed in the gravel layer. Solid waste was placed in the lysimeter at 200 – 250 mm. thickness each time and compacted to a density of about 600 kg / m3 until total height of 2 m was reached. Clay loam layer of 300 mm. was used to cover the top of each cell to prevent the release of gas through the cover soil layer. Gas collection system was installed at the top of waste layer. It was made from perforated 1-inch diameter PVC pipe placed into a gravel pack to prevent clogging from solid particles in solid waste. Different experimental conditions were applied in each lysimeter as follows. Lysimeter#1: Control cell: Only simulated rainwater was added. Lysimeter#2: Simulated rainwater and leachate circulation once a week. Lysimeter#3: Simulated rainwater and leachate circulation once a week and leachate storage in the lysimeter at 50 % of solid water layer height. Lysimeter#4: Simulated rainwater and leachate storage in the lysimeter at 50% of solid waste layer height. Rainwater was added into the lysimeters to simulate wet period (rainy season) for about 140 days. Afterwards, a dry period was simulated in which rainwater addition was terminated. The quantity of produced leachate was measured to verify the water balance in the lysimeter. Solid waste samples were analyzed for their temperature density, moisture content and chemical composition. Leachate samples from each lysimeter were analyzed for their chemical characteristics. Gas samples were analyzed for their composition by a gas chromatography. Methane Oxidation Study in laboratory Soil Column Six soil columns made of acrylic having 15-cm diameter and 100-cm height were used in laboratory scale experiment. The schematic of experimental unit is shown in Figure 2. The columns were purged with synthetic landfill gas containing 60% methane and 40% carbon dioxide at a flow rate of 3 mL/min. Sandy loam soil consists of 50% sand and 50% organic material (wt/wt) was used as cover soil. The plants used in the study are local grass found in tropical climate i.e. Cynodon plectostachyus and Cynodon dactylon. They were grown for about two weeks in the nursery pots before being planted into soil columns. A Light bulb for plant growing purpose is used to supply light for plant during daytime (average light intensity of 30,000 lux).
Figure 1 Schematic of Landfill Lysimeter for Solid Waste Biodegradation Study
Rainwater or Leachate
Flowmeter
Synthetic LFG
Effluent
Figure 2 Schematic of Soil Column for Methane Oxidation Experiment
Rainwater or synthetic leachate was irrigated to the soil column to control soil moisture content at 10-15%. The leachate was diluted with rainwater and final concentration of 1,880 mgCOD/L was obtained for the first experiment (day 0-154). The applied concentration was then increased to 3,760 mg/L and 9,400 mg/L at day 155 and 225 respectively. Two soil columns without plant are prepared as the control experiment. One of them was irrigated with rainwater and the other with leachate. Two plant species, i.e. Cynodon plectostachyus and Cynodon dactylon, were grown in others four columns (two columns for each plant). The columns were operated in the same manner as the controls. The effect of leachate irrigation on vegetation was studied by determining the following parameters; plant growth rate (height, number of leaves, root length and weight of dry mass) and plant damage (abnormal appearance). In addition, soil organic content and electrical conductivity were monitored to determine the degree of organic matter and salt accumulations in soil. Methane Oxidation Rate (MOR) was used for the determination of microbial activity. It can be calculated from following equation: MOR (CH4 mol/m3.d) = Q [(CH4) In – (CH4) out]/V Where
Q (CH4) In (CH4) out V
= Gas flow rate (ml/day) = Inflow methane concentration (moles/ml) = Outflow methane concentration (moles/ml) = Volume of soil (m3)
RESULTS AND DISCUSSION Enhancement of Solid Wastes Biodegradation in Pilot Scale Landfill Lysimeters In pilot scale study, four landfill lysimeters was constructed to investigate different operating strategies to enhance the degradation of municipal solid wastes (MSW). All lysimeters have been filled with municipal solid wastes taken from a waste transfer station in Bangkok and mixed with dewatered digested sludge from a sewage treatment plant at a ratio of 4:1 on weight basis. It was found that initial densities of solid wastes were 621 kg/m3 on average. Four different operating conditions have been applied to the lysimeters. Simulated rainfall (using actual rainwater) is applied daily on the top cover soil based on average at a rate of 70% that of average rainfall amount in Thailand. After the total amount of rainwater has been applied, the lysimeters will be operated without water addition to investigate landfill behavior during dry season. Temperature variation in each lysimeter was measured at different depth (30, 100 and 170 cm. from bottom of waste layer) in the lysimeters. It was found that the variations of temperature were mainly affected by ambient temperature and fluctuated within a range between 26 to 330 C (ambient temperatures were ranged between 30-35 0 C). The temperature was found to be slightly lower at the middle and bottom part of lysimeter#3 and #4 as the wastes are immersed under stored leachate in the lysimeters. Moisture content of wastes was also measured in each lysimeter as shown in Fig.3. It can be seen obviously that moisture content was higher at the bottom than the upper part. They were approximately 80% as compared to 50% in the upper part. However, there was not much
difference in moisture content of wastes among each lysimeter during rainfall period. This observation suggests that re-circulation and storage of leachate had little influence on moisture content during this period as they can be maintained by an infiltration of rainwater. As the operation reached the dry period, the lysimeters operated without re-circulation (Lysimeter#1 and #4) had their moisture content dropped down whereas those of the others could be maintained relatively constant showing that the re-circulation help maintaining moisture content during the absence of rainfall.
Figure 3 Moisture content variation in each lysimeter Settlement rates of waste layer in the lysimeters are also determined. It was found that the settlement rates in lysimeter#1 and #2 were higher than that of lyismeter#3 and #4. Highest settlement was found in control lysimeter (7.5 cm. after 195 days) whereas the lysimeter with stored leachate had lowest settlement rate (2 cm. after 195 days). Chemical characteristics of leachate including pH, VFA/Alkalinity, conductivity, BOD, COD and TKN in each lysimeter were determined. It was found that pH of leachate was maintained relatively constant at about 6 in all lysimeters while VFA/Alkalinity were maintaining in a range of 2-3. Leachate conductivity during rainfall period was slightly different among the lyismeter but maintained in the range of 30-40 mS/cm. In the latter dry period, a dropping trend in leachate conductivity was observed. BOD and COD concentration of leachate were found in a range of 40,000-60,000 mg/l and 40,000-70,000 mg/l respectively. High BOD/COD ratio suggested that the leachate was highly biodegradable which is normal for young landfill. TKN concentration in lysimeter #1 and #2 are slightly higher than that of lysimeter#3 and # 4 but they were all in the range between 2,000-3,500 mg/l.
Determinations of water balance in the lysimeters are shown in Fig.4. From the measurement of leachate amount being produced, re-circulated and stored in the lysimeter, a loss of water through evaporation can be estimated. It was found that leachate produced in the lysimeters were about 40% of rainwater added and the re-circulation and storage of leachate could lessen the volume of leachate produced and delayed leachate formation by about 30-40 days.
Figure 4 Water balance in the lysimeters Cumulative COD loading from leachate produced were shown in Fig.5. Total COD loading discharged from control lysimeter was 11,000 g after 170 days of operation whereas the almost the same amount was produced in lysimeter#4 with leachate storage though discharged period has been delayed. In lysimeter#2 and #3, they are totally re-circulated back into the waste cells without any discharge. TKN loading followed the same pattern, however, at much lower rate. It was also found that storage of leachate could reduce the discharged loading by about 20%. Methane contents in produced gas along the experimental period are shown in Fig.6. During the start-up period, they are gradually increased and were in the range of 5-20%. It was found that the lysimeter operated with leachate storage (lysimeter#3 and #4) has produced slightly higher methane content than the others. As the operation entered the dry period, the methane production in most of the lysimeters has been increased to about 30% but slightly higher in
lysimeter#3. It was found that leachate re-circulation in combination with leachate storage could enhance waste biodegradation as it could help retaining high moisture content of wastes, especially during dry period, and improving the biodegradation of solid wastes.
Figure 5 COD loading from the lysimeters
Figure 6 Methane content in gas from each lysimeter
Minimization of Methane Emission by Methane Oxidation in Vegetated Cover Soil The effects of leachate strengths on plant growth had been initially cultivated in pot study where two species of plants (C. plectostachyus and C.dactylon) were irrigated daily with 500 mL of rainwater and leachate at different concentrations. It was found that C. plectostachyus grew well when it was applied with rainwater or diluted leachate (1,880 mgCOD/L). At higher leachate concentrations of 3,760 and 9,400 mgCOD/L, the plant growth was retarded after 13th and the 7th weeks of cultivation period respectively. When comparing the growth rates of grass between irrigated with rainwater and diluted leachate, it was found that leachate application could promote the growth of both C. plectostachyus and C. dactylon. However, the growth of C. dactylon was also promoted at higher concentration of leachate (3,760 mgCOD/L) and it was found to be the most tolerant grass specie to leachate irrigation in this study. Nevertheless, all three species was severely damaged when being irrigated with higher concentration of leachate (9,400 mgCOD/L). During this plant cultivation study, the accumulations of salt (EC) and organic carbon (OC) contents in soils from leachate irrigation were not observed. The irrigation of diluted leachate had little influence on EC and OC contents in soil. Determination of soil pH showed that pH was slightly dropped down when leachate was applied to soil. The oxygen uptake rate (OUR) in soil increased when higher organic loading was supplied to the soil. The main reason for inhibition of plant growth when high strength leachate was applied could be the shortage of oxygen in soil as a result of too high organic loading. The growths of C. plectostachyus and C. dactylon in soil column were observed for almost 300 days. The result showed that both plants could be grown in typical condition of landfill (exposed with landfill gas and rainwater) when irrigated with rainwater and diluted leachate. Similar to the results obtained from batch experiment, it was found that the growth of both plants in soil column could be promoted when optimal supplement nutrients from leachate were supplied. Methane oxidation rate (MOR) was determined in soil column operated under different conditions. In case of non-vegetated cover soil irrigated with rainwater, MOR was found to be 10-12 mol/m3.d during the first 50 days and gradually declined to 4 mol/m3.d at 200 days of operation (Figure 7). In case of soil column with leachate irrigation, MOR could be maintained above 8 mol/m3.d throughout the experimental period. For soil column with C. dactylon, MOR was slightly reduced from 10-12 mol/m3.d to 8 mol/m3.d. Leachate irrigation was not significantly affected MOR in this vegetated cover soil. In case of C. plectostachyus, MOR was initially maintained at the same level as non-vegetated soil but rapidly dropped down to less than 2 mol/m3.d after only 80 days of operation. From the experimental results, methane oxidation was successfully maintained in non-vegetated soil with leachate irrigation and cover soil with C. dactylon. When leachate concentration has been increased to 20% and 50%, adverse effect of organic loading on MOR and plant growth was observed in all cases. At 20% of leachate concentration, MOR of soil column without plant dropped down to less than 6 mol/ m3.d while that of column with C. dactylon maintained in a range of 7-9 mol/ m3.d. As leachate concentration was increased to 50% after day 225, MOR in all columns dropped down to 1-4 mol/ m3.d. Most of methane oxidation took place at 5-15 cm. depth from soil surface. All plant could not survive at this high organic loading condition however a low rate of methane oxidation in cover soil was still detected.
methane oxidation rate(mol/m3.d)
14
1,880 mg/L leachate
3,760 mg/L leachate
9,400 mg/L leachate
12 rainw ater w /o plant
10
rainw ater C.dactylon
8
rainw ater C.plectostachyus
6
leachate C.plectostachyus
4
leachate C.dactylon
2
leachate w /o plant
0 0
50
100
150
200
250
tim e (da y)
Figure 7 Total Methane oxidation rate in soil columns under different operating conditions
CONCLUSION From the experimental results of solid waste biodegradation study in pilot scale landfill lysimeter and methane emission study in vegetated soil column, the following conclusions can be drawn. 1) In pilot scale experiment, leachate circulation in combination with leachate storage was found to be the most appropriate operating techniques in bioreactor landfill operated under tropical environment. They could help retaining high moisture content of wastes during dry period and improving the biodegradation of solid wastes. The methanogenic phase could be reached at 190 days in the lysimeter. 2) The application of bioreactor landfill concept could be used to accelerate biodegradation of solid wastes in the lysimeter. Moreover, proper water management in the landfill can significantly reduce the discharge of pollutant loading from the landfill. 3) Two species of tropical grass, i.e. C. plectostachyus and C. dactylon could be grown in typical condition of landfill when irrigated with rainwater and diluted leachate (1,880 mgCOD/lL). Application of diluted leachate could promote the growth of both plants. However, they were severely damaged when being irrigated with higher concentration of leachate (9,400 mgCOD/L). 4) In soil column study, C. dactylon, but not C. plectostachyus promoted methane oxidation. Irrigation of diluted leachate could also promote methane oxidation in non-vegetated cover soil. The study suggests that selection of the appropriate plant species and optimum concentration of leachate were important to achieve sustainable gas and leachate control by cover soil.
ACKLOWLEDGEMENT This study has been supported by Swedish International Development Cooperation Agency (SIDA) under Asian Regional Research Program on Environmental Technology (ARRPET). REFERENCES Barlaz, M.A., M.W. Milke and R.K. Ham (1987), Gas Production Parameters in Sanitary Landfill Simulators, J. Waste Manage. Res., 5, 27-39. Boeckx P. and Cleemput O.V. (1996), Methane Oxidation in a Neutral Landfill Cover Soil: Influence of Moisture Content, Temperature and Nitrogen Turnover, J. Environ.Qual., Vol.25, p.178-183. Buivid, M.G., D.L. Wise and M.J. Blanchet (1981), Fuel Gas Enhancement by Controlled Landfilling of Municipal Solid Waste, J. Resources Conserv., 6, 3-20. Hilger H.A., Wollum A.G. and Barlaz M.A. (2000), Landfill Methane Oxidation Response to Vegetation, Fertilization and Liming, J. Environ.Qual., Vol.29, p.324-334. Lay, J.J., Y.Y. Li and T. Noike (1998), Developments of Bacterial Population and Methanogenic Activity in a Laboratory Scale Landfill Bioreactor, Wat. Res., 32(12), 36733679. Pohland, F.G. (1980), Leachate Recycle as Landfill Management Option, J. of Env. Eng. Div. 106 (EE6), 1057-1069. Reinhart, D.R. and T.G. Townsend (1998), Landfill Bioreactor Design and Operation, Lewis Publishers, New York, 189 p. Roslev P., Iverson N. and Henriksen K. (1997), Oxidation and Assimilation of Atmospheric Methane by Soil Methane Oxidizers, Applied and Environmental Microbiology, Vol.63, No.3, p.874-880. Stegmann, R. and H.J. Ehrig (1989), Enhancement of Gas Production in Sanitary Landfill Sites- Experiences in West Germany, In Resource Recovery from Solid Waste, 425-434. Titlebaum, M.E. (1982), Organic Carbon Content Stabilization Through Landfill Leachate Recirculation, J. WPCF, 54(5), 428-433. Whalen S.C., Reeburgh W.S. and Sandbeck K.A. (1990), Rapid Methane Oxidation in a Landfill Cover Soil, Applied and Environmental Microbiology, Vol.5, No.11, p.3405-3411.
