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ACCELERATION OF SOLID WASTE BIODEGRADATION IN TROPICAL LANDFILL USING BIOREACTOR LANDFILL CONCEPT C. Chiemchaisri, W. Chiemchaisri, U. Nonthapund and S. Sittichoktam Department of Environmental Engineering, Faculty of Engineering 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. The study was conducted in laboratory and pilot scale experiments. The laboratory scale experiment was conducted using 70 liter digesters over 245 days having different leachate circulation of 4,8 and 15 times per month suggested that landfill cell with leachate circulation of 15 and 8 times per month yielded higher methane production than that of 4 times per month. The digester with 8 times per month of leachate circulation has reached methanogenic phase after 104 days. 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. KEYWORDS Landfill operation, bioreactor landfill, leachate circulation, tropical climate 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. Basic principle of conventional landfill design is to contain or store the wastes so that the exposure to human and environment could be minimized. This is done by the prevention of gas emission from landfill and infiltration of surface water. This type of conventional landfill is called “dry tomb” landfill. However, low moisture content can result in slow degradation rate extending landfill life. This results in additional financial burden for long term after care of landfill, low quality and quantity of landfill gas. Severe impacts are occasionally found from these landfills due to the failure of their containment systems.

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. MATERIALS AND METHODS This research work was conducted in both laboratory and pilot scale. The laboratory scale experimental system consists of 3 anaerobic digester made of PVC with a diameter of 30 cm. and 100 cm. height having total volume of 70.68 liter (Figure 1). Leachate was separated at the bottom of the digester and drained into 8-liter storage tank before circulating back to the top of digester with a 1/3 hp centrifugal pump. The frequency of circulation was set at 4, 8 and 15 times per month having in total the same volume of circulated leachate. Accordingly, the volume of circulated leachate was set at 8.1, 4.0 and 2.2 liter for each circulation respectively. Organic wastes (food, vegetable and fruit wastes) of 16 kg. were mixed with digested sludge from a sewage treatment plant at a ratio of 5:1 on dry weight basis and put into each digester. Physical and chemical characteristics of wastes, i.e. temperature, density, moisture content, volatile solids, chemical composition was determined. Leachate characteristics in terms of pH, chemical oxygen demand (COD), volatile fatty acid (VFA) and alkalinity were also analyzed. The waste degradation efficiency was studied in terms of organic waste reduction, leachate characteristics and gas production rate. The pH of circulated leachate was controlled in a neutral range by an addition of sodium bicarbonate (NaHCO3). In pilot scale study, four landfill lysimeters made of steel pipe of 0.90 m. diameter and 2.7-m height were used (Figure 2). 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

Figure 1 Schematic of laboratory scale experimental system

Figure 2 Schematic of landfill lysimeter 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 amount of rainwater was calculated from Hydrologic Evaluation of Landfill Performance (HELP) model using actual rainfall data in the central region of Thailand (Climatological Department, 2001). 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, field capacity, total solid, volatile solid and chemical composition. Leachate samples from each lysimeter were collected once a week and analyzed for pH, alkalinity, biochemical oxygen demand (BOD), COD, VFA, alkalinity, total kjeldahl nitrogen (TKN) and monitoring of leachate quantities. Gas samples were collected once a week and analyzed the gas composition (CO2, O2, N2, CH4 and H2) by gas chromatography with thermal conductivity detector (TCD). RESULTS AND DISCUSSION Biodegradation of Solid Wastes in Laboratory Scale Anaerobic Digesters Three anaerobic digesters (No.1, 2 and 3) were operated at different leachate circulation frequency of 15, 8 and 4 times per month for 245 days. From the determinations of physical characteristics of solid wastes in the digesters, initial densities of solid wastes was found to be between 1147-1171 kgm-3 and increased to 1454-1585 kgm-3 as a result of solid waste settlement during the digester operation with leachate circulation. The wastes had average initial moisture content of 83.4% and 14.8% of volatile solids on wet weight basis. The chemical compositions of wastes were 49.6% carbon, 5.6% hydrogen, 1.6% nitrogen, 0.2% phosphorus and 0.2% sulfur on dry weight basis. The ambient temperature was found to vary between 25-40oC and simultaneously the digester temperature fluctuated between 20-35oC. During the operation, it was found that moisture content of wastes in the upper part of the digester were between 75.7-77.8% as compared to 81.1-88.5% at the bottom. The average moisture content was highest in the digester operated at 8 times per month of leachate circulation (No.2) at 83.1% followed by 81.2% and 78.4% of the digesters having 15 and 4 times per month of leachate circulation (No.1 and 3) respectively. Leachate characteristics from those three digesters in terms of pH, COD, VFA, alkalinity during the operational period were determined. It was found that solid waste biodegradation has initially been in acidogenic phase for about 100 days where COD, VFA has gradually increased to 30,000 and 25,000 mgl-1 respectively. As a result of buffering chemical addition, pH of the leachate was initially dropped down slightly to 5.8 and gradually increased to 7.5 as the biodegradation entered methanogenic phase. In methanogenic phase, COD and VFA in the leachate gradually dropped down as majority of them were converted into methane gas while pH and alkalinity were maintained relatively constant without further addition of buffering reagent. There was not much difference in leachate characteristics among the digesters operated at different leachate circulation rate during acidogenic phase but the

reduction in COD and VFA in methanogenic phase were affected by leachate circulation rate and corresponded to methane gas production from each digester. Figure 3 shows cumulative production of methane gas in those three digesters. There was no significant difference on methane production among the digesters during acidogenic phase. As the methanogenic phase has been reached, however, methane production in digester No.2 has increased drastically after 104 days whereas those for digester No.1 and 3 required 124 and 145 days respectively. Total methane production was 496, 493 and 376 liter for the digester No.1, 2 and 3 at the end of the operation. It was found that the frequency of leachate circulation affected solid waste biodegradation as suggested by the methane production rate. The digesters No.1 having 15 times per month gave highest gas production whereas the digester No.2 required shortest time to reach methanogenic phase. These observations were in accordance with the determinations of chemical characteristics of solid wastes at the end of the experiment that suggested that the digester No.1 and 2 had higher volatile solid reduction as compared to the digester No.3.

Figure 3 Cumulative production of methane gas in anaerobic digesters Biodegradation of Solid Wastes 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. This amount of water is estimated to be equal to the water infiltrated through final cover soil according to Hydrologic Evaluation of Landfill Performance (HELP) model. 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.4. 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 4 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.5. 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 5 Water balance in the lysimeters

Cumulative COD loading from leachate produced were shown in Fig.6. 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%.

Figure 6 COD loading from the lysimeters Methane contents in produced gas along the experimental period are shown in Fig.7. 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 7 Methane content in biogas from each lysimeter CONCLUSION From the experimental results of solid waste biodegradation in laboratory scale anaerobic digester and pilot scale landfill lysimeter, the following conclusions can be drawn. 1) Leachate circulation was found to have positive effect on solid waste biodegradation as laboratory scale anaerobic digester with leachate circulation of 15 and 8 times per month yielded higher methane production than that of 4 times per month. The time required to reach methanogenic phase was also shortened. 2) The optimum circulation frequency was found to be 8 times per month at which highest methane production was observed. The minimum period of time required to reach methanogenic phase was 104 days at 8 times per month as compared to 124 and 145 days for 15 and 4 times per month of leachate circulation respectively. 3) 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. 4) 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. ACKLOWLEDGEMENT This study has been supported by Swedish International Development Cooperation Agency (SIDA) under Asian Regional Research Program on Environmental Technology (ARRPET).

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