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Impact of using high-density polyethylene geomembrane layer as landfill intermediate cover on landfill gas extraction
Waste Management 31 (2011) 1059–1064
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Impact of using high-density polyethylene geomembrane layer as landfill intermediate cover on landfill gas extraction Zezhi Chen a,b,d, Huijuan Gong a,c,⇑, Mengqun Zhang c, Weili Wu d, Yu Liu e, Jin Feng e a
State Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Nanjing 210093, PR China School of the Environment, Nanjing University, Nanjing 210093, PR China c Center of Materials Analysis, Nanjing University, Nanjing 210093, PR China d Jiangsu Engineering Research Center for Biomass Energy and Low Carbon Technology, Nanjing University, Nanjing 210093, PR China e Shanghai Qiyao Micropowers Technology Company, Shanghai 201203, PR China b
a r t i c l e
i n f o
Article history: Received 24 May 2010 Accepted 13 December 2010 Available online 12 January 2011
a b s t r a c t Clay is widely used as a traditional cover material for landfills. As clay becomes increasingly costly and scarce, and it also reduces the storage capacity of landfills, alternative materials with low hydraulic conductivity are employed. In developing countries such as China, landfill gas (LFG) is usually extracted for utilization during filling stage, therefore, the intermediate covering system is an important part in a landfill. In this study, a field test of LFG extraction was implemented under the condition of using high-density polyethylene (HDPE) geomembrane layer as the only intermediate cover on the landfill. Results showed that after welding the HDPE geomembranes together to form a whole airtight layer upon a larger area of landfill, the gas flow in the general pipe increased 25% comparing with the design that the HDPE geomembranes were not welded together, which means that the gas extraction ability improved. However as the heat isolation capacity of the HDPE geomembrane layer is low, the gas generation ability of a shallow landfill is likely to be weakened in cold weather. Although using HDPE geomembrane layer as intermediate cover is acceptable in practice, the management and maintenance of it needs to be investigated in order to guarantee its effective operation for a long term. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Landfilling is the primary method to treat municipal solid waste (MSW) in China. In 2008, the disposed MSW was treated by various methods at varying amounts; sanitary landfilling (54.6%), dumping (33.2%), incineration (10.2%) and compost (2.0%) (Xu, 2010). It is expected that burial of MSW in landfills will persist for at least one or two decades in China (Xu, 2010). Landfills are the major source of anthropogenic methane and carbon dioxide emissions. The landfill gas (LFG) is generated by anaerobic decomposition of organic waste. In addition to the main components of methane (50–65 vol.%) and carbon dioxide (30–40 vol.%), LFG also contains over a hundred of undesirable potentially toxic chemical compounds (Park and Shin, 2001; Chiriac et al., 2007). Due to the contribution to the greenhouse effect, the toxicological relevance, the danger of explosion, and the olfactory annoyance, collecting and combusting LFG to yield energy ⇑ Corresponding author at: State Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Nanjing 210093, PR China. Tel.: +86 25 83685262; fax: +86 25 83325180. E-mail addresses: [email protected], [email protected] (H. Gong). 0956-053X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2010.12.012
could solve both environmental pollution and energy shortage (Zamorano et al., 2007). Generally, a sanitary landfill is divided into many cells before filling waste and each of the cells covers hundreds of centiares. The waste was dumped into the cells, respectively. When one cell is filled with waste to a certain extent, another cell is then used for the sequential waste filling. The covering systems which include daily covers, intermediate covers and final covers are essential factors for the successful operation of a sanitary landfill. The daily cover is laid over the newly dumped waste in a certain cell day by day. After a period of time, the cell needs not to be filled with the waste continually for the moment, then the intermediate cover should be laid on the top of this cell, and the new waste will be dumped into another cell. After a cycle, the waste was dumped upon the former cell again. Before the landfill is fully filled, there will be several intermediate covers laying on it. When the landfill is closed, the final cover is constructed and laid over the whole landfill. The covering materials of the landfills should be low hydraulic conductive to avoid escape of LFG and infiltration of rainwater. Covering systems of landfills involve partial or complete isolation of waste from the surrounding environment. Previously,
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engineers and researchers were mainly concerned with the final covers of landfills, and many materials have been widely used, involving geosynthetic clay liners (GCLs), geomembranes (GMs), compacted soil liners (CSLs), compacted clay liners (CCLs), highdensity polyethylene (HDPE) geomembranes, nonwoven needlepunched geotextile (GT). The advantages of these materials are consistent physical properties, low leakage rates, and facile installation, etc. (Simon and Müller, 2004; Bouazzaa and Vangpaisal, 2006; Müller et al., 2008; Hwan and Chung, 2008). Other materials for substituting clay or soil have also been tried, such as deinking by-products (DBPs), fluvial clay deposit, silty soil excavated from highway or tunnel projects, soil blended with municipal solid waste compost, digested sewage sludge, mixture of incineration fly ash and sewage sludge, and sandy soil amended with hydrated lime (Elshorbagy and Mohamed, 2000; Göktürk, 2003; Kim et al., 2005; Bozbey and Guler, 2006; Samah et al., 2008; Herrmann et al., 2009; Xu et al., 2010). In recent years, in order to eliminate the uncaptured methane emission from landfills, active or passive aerated biocovers with methanotrophic ability have become a hotspot of research (Perdikea et al., 2008). In developing countries such as China, as there are more food waste components in MSW, and these components are easily degraded and can produce LFG in short time, therefore, a large amount of LFG is generated during the filling stage. On the other hand when such a landfill is closed the LFG generation would decrease sharply. At present, most of LFG projects in China are operating under the waste landfilling stage and collecting LFG as much as possible to accomplish better energy utilization as well as greenhouse gas (GHG) emission abatement is the goal for these projects. Therefore, in order to prevent the uncaptured LFG from escaping, the intermediate covers are more critical compared to final covers and daily covers for these landfills. As for the covering materials during the cell filling stage, clay is conventionally used for intermediate cover. In China, according to the Technical Code for Municipal Solid Waste Sanitary Landfills (CJJ10-2004) (Chinese Construction Ministry, 2004), for a filling sanitary landfill the depth of clay for intermediate cover should be over 30 cm. However this requirement is rarely implemented thoroughly in practice, as clay is increasingly costly and scarce, and it also reduces the storage capacity of landfills. Due to the scarce resource of clay, engineers have tried to use HDPE geomembranes instead of clay as the landfill intermediate cover. Compared with clay layer, the advantage of HDPE geomembrane layer is that it can be used repeatedly as a low hydraulic barrier and does not occupy the space of landfills, the disadvantage is that it is easily crinkled which causes adverse influence on sealing property. Furthermore using HDPE geomembrane is cost effective. Now in China, the cost of HDPE geomembrane layer per hectare is about 100 thousand RMB Yuans (around 14.9 thousand US dollars), and that of soil layer is about 120 thousands RMB Yuans (around 17.9 thousand US dollars), thus the HDPE geomembrane layer is cheaper than soil layer, and it would be much more cost saving if the HDPE geomembrane layer is reused. Accordingly the effectiveness of using the HDPE geomembrane layer as the landfill intermediate cover should be investigated before it is used in landfills. From the viewpoint of LFG extraction in the landfills, conventional clay layer is preferred as the intermediate cover since it has good compatibility with the vertical gas extraction wells. If the clay is substituted by the HDPE geomembranes, both positive and negative effects of the HDPE geomembrane layer on gas extraction performance may occur. This study was aimed at making investigation on the feasibility of using HDPE geomembranes as a substitute for intermediate cover in landfills through a gas extraction test in a landfill.
2. Material and methods 2.1. Test field and material A filling landfill near Ningbo which is located in the southeast China was selected as the test field in this study. It was constructed with total storage capacity of 647 million cubic meters, and put into use in 2006. The initial daily MSW disposal amount was 500 tons and the data increased to 800 tons in 2009. Before the test of this study, over 700,000 tons of wastes had been dumped in it. The landfill is valley shaped, the deepest height of waste layer was 20 m, and the height of the shallow layers was about 4 m. As the landfill is located in a shaly mountain, soil is difficult to obtain locally, thus HDPE geomembranes were used as the cover layer in the landfill. The HDPE geomembranes used as the covering materials are roll shaped. The thickness, width and mass per unit area of them are 0.5 mm, 4 m and 460 g m 2, respectively. The hydraulic conductivity of the HDPE geomembrane is lower than 1.0 10 7 cm s 1. The HDPE geomembranes were laid on the landfill surface. The adjacent HDPE geomembranes were overlapped without stitching or welding and many wasted tires were placed
Fig. 1. Overall view of the landfill covered by HDPE geomembrane layer.
Fig. 2. A vertical gas well constructed for the field test.
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Z. Chen et al. / Waste Management 31 (2011) 1059–1064 Sample Butterfly port valve
Orifice
Mobile flare
Branch pipe HDPE geomembrane layer
Open flare Waste
Bottom layer
W1
Blower
W2 General pipe
W3
Butterfly Sample port valve
Flowmeter
W4 Fig. 3. A schematic view of the test system.
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on the overlapped area to prevent the HDPE geomembranes from being blown away by wind. An overall view of the landfill is shown in Fig. 1. Four vertical gas extraction wells were constructed for the test in the middle zone of the landfill, where the wastes were separately dumped in therecent three years. The total depth of the waste in the test field was 12 m. The four wells were arranged at the vertices of a 25 m 25 m square region, and the depth of each well is the same as that of the waste. In order to ensure air sealing, the joints of the HDPE geomembranes and the gas wells were clamped by HDPE pipe flanges, thus there were no gaps between the HDPE geomembranes and the gas wells. A pneumatic water pump was installed at the bottom of each gas well to drain the water out. The photograph of a constructed gas well for the field test is shown in Fig. 2.
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30 17 Nov 18 Nov 19 Nov 20 Nov 21 Nov 22 Nov 23 Nov 24 Nov 25 Nov 26 Nov
2.2. Method
Fig. 4. Gas flow and the proportion of the main components in the early test.
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-0.8 Well vacuum / kPa
Four vertical gas wells (named as W1, W2, W3, and W4, respectively) were parallel connected to form a general pipe, and the latter was joined to a mobile flare. A schematic view of the test system is shown in Fig. 3. The mobile flare is an integrated equipment including a blower, an open flare, a gas flowmeter and other necessary components. The main function of the mobile flare is to pump LFG out of the wells and then burn it to solve air pollution as well as eliminate the hidden trouble of explosion. As the mobile flare was installed on a chassis with wheels, it could be easily moved by other machines such as a bulldozer or a grab. During the test, the four wells were all opened, and the gas extraction for the whole wells as well as for individual well was investigated. The total gas flow was read by a flowmeter in the mobile flare equipment, and at the same time the gas flow of each well was calculated by the pressure drop of an orifice plate which is mounted on the corresponding branch pipe. Sample ports on the pipes were used for measuring gas components (CH4, CO2, O2, and N2), gas pressure and temperature. The total gas flow was controlled by adjusting running speed of the blower through frequency conversion and the gas flow for each well was controlled by the butterfly valve on the corresponding branch pipe. During the field testing period, the blower ran continuously allday, and in the mean time the gas flow and gas components were monitored and adjusted timely to keep equilibrium between gas generation and extraction.
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Fig. 5. Volume percentages of the gas components and well vacuums versus gas flows of W1 well.
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Fig. 6. Appearance of the test field before and after the replacement of HDPE geomembrane layer.
3. Results At first the gas extraction test was carried out with the HDPE geomembrane layer being maintained in the original status. It was soon found that air was sucked into the gas wells and there were not only many broken holes on the HDPE geomembranes but also leakage gaps between them. Therefore, the HDPE geomembrane layer had to be replaced and the new HDPE geomembranes were welded together. After that the test resumed.
v) and oxygen (lower than 3.0%, v/v) was acceptable both for gas well operation and for combustion utilization in the engines. It was also noticed that the pipe vacuum was very low (about 1.5 kPa), which indicated serious air leakage into the gas wells. Air leakage phenomenon was obvious from the data of the individual well. Fig. 5 displays the gas flows and the volume percentages of the main components in the W1 well under different vacuum. It shows that oxygen and nitrogen concentration increased dramatically with slight increase in gas flow. The data of other wells were similar to that of W1 well.
3.1. Results in early test 3.2. Test results after the replacement of HDPE geomembrane layer As previously mentioned that in the early test period, although HDPE geomembranes were laid on the landfill and the gas wells had good airproof connection with HDPE geomembranes initially, many leakage points were found soon on the surface of the HDPE geomembranes which could impair gas extraction. The results of this gas extraction period are presented in Fig. 4. Fig. 4 displays 10-day-data recorded the gas flow and the proportion of the main gas components in the early period of the gas extraction test. The data were measured in the general pipe for the total gas flow of the four wells. During the test the gas flow was adjusted step by step, and finally it was found that if the gas flow was over 40 m3 h 1, the amount of oxygen would exceed 3.0% (v/v) which is the acceptable upper limit for oxygen concentration in gas wells. Thus the total gas extraction ability for the four wells was determined as 40 m3 h 1. Under this condition the proportion of the main gas components such as methane (over 40%, v/
Since serious air leakage was found in the test, effort to improve the sealing property of the HDPE geomembrane layer was made. The old HDPE geomembranes were replaced by new ones and they were heat welded together to guarantee the sealing property on the whole test field. Fig. 6 shows the appearance of the test field before and after the replacement of HDPE geomembrane layer. After amendment of the HDPE geomembrane layer, gas extraction test resumed for another month. Data measured in the general pipe for the total gas flow is displayed in Fig. 7. From the above results, the total gas flow increased to 50 m3 h 1 with acceptable methane and oxygen concentrations under long time running, also the vacuum (around 4.0 kPa) in the general pipe was much higher than before, that means the air sealing property of the covering layer improved greatly.
Components / % v/v
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60 50 40 30 9 Dec 11 Dec 13 Dec 15 Dec 17 Dec 19 Dec 21 Dec 23 Dec 25 Dec 27 Dec 29 Dec 31 Dec 2 Jan
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Fig. 7. Gas flows and volume percentages of the gas components after amendment of the HDPE geomembrane layer.
CH4
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Fig. 8. Volume percentages of the gas components and well vacuums versus gas flows of W1 well after the amendment of the HDPE geomembrane layer.
Z. Chen et al. / Waste Management 31 (2011) 1059–1064
4.1. Necessity for the HDPE geomembranes to be overall welded
16
2
If the HDPE geomembranes are just laid piece by piece on the landfill as shown in Fig. 6(a), it is inevitable to appear to many gaps on the layer, and the sealing property could not be perfect and could lead to either air leakage or rainwater infiltration. Furthermore as waste compaction is not implemented in many landfills in China, i.e. the waste on the top layer of these landfills is loose, air is liable to suck into the gas wells under negative pressure. Therefore, if HDPE geomembranes are used as the single intermediate layer on a landfill, they should be welded together to form a uniform sealing system.
0
4.2. Management and maintenance
14 o
Extracted gas temperature / C
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Test data Linear fit
4
-2 -4
-2
0
2
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Lowest daily ambient temperature / C Fig. 9. The relationship of extracted gas temperature dependence upon ambient temperature.
The gas extraction performance of the individual wells was also improved. Fig. 8 illustrates the gas flows and the amount of each gas component in the W1 well under different extraction vacuums. Compared with the data in Fig. 5, the gas extraction ability of the well improved obviously. The data of the other wells were similar to that of W1 well.
3.3. Ambient temperature influence After amendment of the HDPE geomembrane layer, the trend of ambient temperature was descending, and the gas flow also displayed a downtrend (Fig. 7) which means the gas extraction performance was depressed. The ambient temperature would also influence the extracted gas temperature. The relationship between the extracted gas temperature and the daily lowest ambient temperature is plotted in Fig. 9. From Fig. 9, we could find that although scattered, the plot of the two temperatures clearly showed temperature dependent; i.e. with the ambient temperature descending, the temperature of the extracted gas dropped. This phenomenon could be explained by the low heat isolation capacity of the HDPE geomembrane layer as well as low heat storage capacity of the shallow landfill (about 12 m in depth). According to the literatures (Zhang et al., 2006; Fan and Zhou, 2007), the thermal conductivity of HDPE geomembrane and soil could be selected as 0.4 W m 1 K 1 and 1.6 W m 1 K 1, respectively, and the thickness of HDPE geomembrane layer and soil layer is 0.5 mm and 300 mm, respectively, therefore, the heat transfer coefficient of soil layer is calculated as 5.3 W m 2 K 1 which is much lower than that of HDPE geomembrane layer (800 W m 2 K 1). Besides, the specific heat capacity of HDPE geomembrane and soil could be selected as 1.88 J g 1 K 1 and 3.10 J g 1 K 1, the thermal storage capacity of soil layer is calculated 1290 times that of HDPE geomembrane layer.
4. Discussion After the gas extraction test under different HDPE geomembrane layer conditions, it was found that the air sealing property of HDPE geomembrane layer and ambient temperature were the main factors affecting gas extraction ability, and other aspects such as maintenance of the HDPE geomembrane layer in good shape in daily management on the landfill was also important.
Advantage and disadvantage of both compacted clay layer and HDPE geomembrane layer used as the intermediate cover on landfills are obvious. The compacted clay layer is not only easy to install and has good adaptability with waste, it but also needs less maintenance. However cracks on the compacted clay layer might occur especially in dry climate, which could weaken sealing property of the layer. The most adverse factor for using soil as the covering material is that it becomes more and more expensive and sometimes it is difficult to be sufficiently supplied. As for the HDPE geomembrane layer, it has been proved to be an alternative intermediate cover on landfills by this study, with the precondition that the HDPE geomembranes should be welded overall to guarantee the sealing property as discussed above. In addition, the following aspects about the management and maintenance of HDPE geomembrane layer should also be paid attention to. The connection condition between the HDPE geomembranes and the vertical gas well pipes should be monitored carefully. Along with the sedimentation of waste in landfills, well pipes would gradually move upwards relative to the landfill ground and may cause serious tension stress of the HDPE geomembranes on the connection points, thus the timely maintenance of the connection parts between the HDPE geomembranes and well pipes needs to be done. As the HDPE geomembrane layer is easy to be waved or wrinkled, it is preferred to compact the waste and make the landfill surface flat, which could lessen the sedimentation of the landfill and decrease the possibility of stress damage of the HDPE geomembranes. In order to prevent the HDPE geomembranes from being broken by metal or other hard waste, it is better to lay a thin liner using soil or nonwoven needle-punched geotextile on the compacted waste firstly and then lay the HDPE geomembranes on the thin liner. Considering low heat isolation capacity of the HDPE geomembrane layer, it is better to increase the depth of filling cell to maintain the temperature of the waste in cold weather. On the other hand, the problem of waste settlement as mentioned previously should be of special concern. Normally the rate of waste settlement is fast and could be at the level of 5–8% in the first half of a year, after that the settlement tends to slow down and attains to stable finally. Therefore, the maintenance of the HDPE geomembranes should be paid special attention to ensure good connection of the extraction wells and the HDPE geomembranes. Finally, the intermediate HDPE geomembrane layer is a temporary cover which should be removed when additional waste layers are added and it could be reused on new waste layers on condition that it keeps in good shape. Compared with the HDPE geomembrane layer being used as the permanent in-place cover, the temporary cover is more effective not only for cost saving but also for the need of leachate seeping.
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5. Conclusions In this study, the possibility and effectiveness of using the HDPE geomembrane layer as an intermediate cover was investigated through a gas extraction test on a landfill. Results showed that if the HDPE geomembranes are laid on the ground of the landfill piece by piece without being welded together, the geomembranes cannot become airproof. It was proven that welding the HDPE geomembranes all together to form a whole airtight layer upon a large area of the landfill could evidently increase gas extraction ability which is acceptable in application. As the heat isolation capacity of HDPE geomembrane layer is low, the gas generation ability of a shallow landfill is likely to be weakened in cold climate. In order to ensure the HDPE geomembrane layert satisfies the operational demands, some practical problems such as how to keep the connection points between the HDPE geomembranes and vertical gas well pipes in good sealing condition for long term, and the methods to prevent the HDPE geomembranes from breaking, have to be solved. Acknowledgements The authors gratefully acknowledge the financial support from the Environment Protection Ministry of Jiangsu Province (No. 2009009), Education Ministry of Jiangsu Province (No. JH07-001), Analysis & Test Fund of Nanjing University and Research Fund for Young Teachers of Center of Materials Analysis in Nanjing University. The authors thank the three anonymous reviewers for providing helpful comments on this manuscript. References Bouazzaa, G.M., Vangpaisal, T., 2006. Laboratory investigation of gas leakage rate through a GM/GCL composite liner due to a circular defect in the geomembrane. Geotextiles and Geomembranes 24, 110–115.
Bozbey, I., Guler, E., 2006. Laboratory and field testing for utilization of an excavated soil as landfill liner material. Waste Management 26, 1277–1286. Chinese Construction Ministry, 2004. Technical Code for Municipal Solid Waste Sanitary Landfill (CJJ17-2004, in Chinese). Chiriac, R., Carre, J., Perrodin, Y., Fine, L., Letoffe, J.M., 2007. Characterisation of VOCs emitted by open cells receiving municipal solid waste. Journal of Hazardous Materials 149, 249–263. Elshorbagy, W.A., Mohamed, A.M.O., 2000. Evaluation of using municipal solid waste compost in landfill closure caps in arid areas. Waste Management 20, 499–507. Fan, H., Zhou, Y., 2007. Analysis of the Borehole resistance in the vertical U-tube heat exchanger. Building Energy & Environment 26, 85–88 (in Chinese). Göktürk, E.H., 2003. Mineralogical and sorption characteristics of Ankara Clay as a landfill liner. Applied Geochemistry 18, 711–717. Herrmann, I., Svensson, M., Ecke, H., Kumpiene, J., Maurice, C., Andreas, L., Lagerkvist, A., 2009. Hydraulic conductivity of fly ash–sewage sludge mixes for use in landfill cover liners. Water Research 43, 3541–3547. Hwan, S.K., Chung, M., 2008. Effectiveness of compacted soil liner as a gas barrier layer in the landfill final cover system. Waste Management 28, 1909–1914. Kim, E.H., Cho, J.K., Yim, S., 2005. Digested sewage sludge solidification by converter slag for landfill cover. Chemosphere 59, 387–395. Müller, W., Jakob, I., Seeger, S., Gerth, R.T., 2008. Long-term shear strength of geosynthetic clay liners. Geotextiles and Geomembranes 26, 130–144. Park, J.W., Shin, H.C., 2001. Surface emission of landfill gas from solid waste landfill. Atmospheric Environment 35, 3445–3451. Perdikea, K., Mehrotra, A.K., Patrick, J., Hettiaratchi, A., 2008. Study of thin biocovers (TBC) for oxidizing uncaptured methane emissions in bioreactor landfills. Waste Management 28, 1364–1374. Samah, A.B., Alexandre, R.C., Claudia, T.P., 2008. Evolution of biodegradation of deinking by-products used as alternative cover material. Waste Management 28, 85–96. Simon, F.G., Müller, W.W., 2004. Standard and alternative landfill capping design in Germany. Environmental Science & Policy 7, 277–290. Xu, H., 2010. Sanitary landfilling disposal of municipal solid waste. Training seminar of landfill gas recovery and clean energy utilization supported by USEPA Methane to Markets Program in Wuhan, April 15–16, China. Xu, Q., Townsend, T., Reinhart, D., 2010. Attenuation of hydrogen sulfide at construction and demolition debris landfills using alternative cover materials. Waste Management 30, 660–666. Zamorano, M., Pérez, J.I., Pavés, I.A., Ridao, Á.R., 2007. Study of the energy potential of the biogas produced by an urban waste landfill in Southern Spain. Renewable and Sustainable Energy Reviews 11, 909–922. Zhang, H., Ge, X., Ye, H., 2006. Investigation of the thermal properties of wetporous soils. Acta Energiae Solaris Sinica 127, 1069–1072 (in Chinese).
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Impact of using high-density polyethylene geomembrane layer as landfill intermediate cover on landfill gas extraction Zezhi Chen a,b,d, Huijuan Gong a,c,⇑, Mengqun Zhang c, Weili Wu d, Yu Liu e, Jin Feng e a
State Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Nanjing 210093, PR China School of the Environment, Nanjing University, Nanjing 210093, PR China c Center of Materials Analysis, Nanjing University, Nanjing 210093, PR China d Jiangsu Engineering Research Center for Biomass Energy and Low Carbon Technology, Nanjing University, Nanjing 210093, PR China e Shanghai Qiyao Micropowers Technology Company, Shanghai 201203, PR China b
a r t i c l e
i n f o
Article history: Received 24 May 2010 Accepted 13 December 2010 Available online 12 January 2011
a b s t r a c t Clay is widely used as a traditional cover material for landfills. As clay becomes increasingly costly and scarce, and it also reduces the storage capacity of landfills, alternative materials with low hydraulic conductivity are employed. In developing countries such as China, landfill gas (LFG) is usually extracted for utilization during filling stage, therefore, the intermediate covering system is an important part in a landfill. In this study, a field test of LFG extraction was implemented under the condition of using high-density polyethylene (HDPE) geomembrane layer as the only intermediate cover on the landfill. Results showed that after welding the HDPE geomembranes together to form a whole airtight layer upon a larger area of landfill, the gas flow in the general pipe increased 25% comparing with the design that the HDPE geomembranes were not welded together, which means that the gas extraction ability improved. However as the heat isolation capacity of the HDPE geomembrane layer is low, the gas generation ability of a shallow landfill is likely to be weakened in cold weather. Although using HDPE geomembrane layer as intermediate cover is acceptable in practice, the management and maintenance of it needs to be investigated in order to guarantee its effective operation for a long term. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Landfilling is the primary method to treat municipal solid waste (MSW) in China. In 2008, the disposed MSW was treated by various methods at varying amounts; sanitary landfilling (54.6%), dumping (33.2%), incineration (10.2%) and compost (2.0%) (Xu, 2010). It is expected that burial of MSW in landfills will persist for at least one or two decades in China (Xu, 2010). Landfills are the major source of anthropogenic methane and carbon dioxide emissions. The landfill gas (LFG) is generated by anaerobic decomposition of organic waste. In addition to the main components of methane (50–65 vol.%) and carbon dioxide (30–40 vol.%), LFG also contains over a hundred of undesirable potentially toxic chemical compounds (Park and Shin, 2001; Chiriac et al., 2007). Due to the contribution to the greenhouse effect, the toxicological relevance, the danger of explosion, and the olfactory annoyance, collecting and combusting LFG to yield energy ⇑ Corresponding author at: State Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Nanjing 210093, PR China. Tel.: +86 25 83685262; fax: +86 25 83325180. E-mail addresses: [email protected], [email protected] (H. Gong). 0956-053X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2010.12.012
could solve both environmental pollution and energy shortage (Zamorano et al., 2007). Generally, a sanitary landfill is divided into many cells before filling waste and each of the cells covers hundreds of centiares. The waste was dumped into the cells, respectively. When one cell is filled with waste to a certain extent, another cell is then used for the sequential waste filling. The covering systems which include daily covers, intermediate covers and final covers are essential factors for the successful operation of a sanitary landfill. The daily cover is laid over the newly dumped waste in a certain cell day by day. After a period of time, the cell needs not to be filled with the waste continually for the moment, then the intermediate cover should be laid on the top of this cell, and the new waste will be dumped into another cell. After a cycle, the waste was dumped upon the former cell again. Before the landfill is fully filled, there will be several intermediate covers laying on it. When the landfill is closed, the final cover is constructed and laid over the whole landfill. The covering materials of the landfills should be low hydraulic conductive to avoid escape of LFG and infiltration of rainwater. Covering systems of landfills involve partial or complete isolation of waste from the surrounding environment. Previously,
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engineers and researchers were mainly concerned with the final covers of landfills, and many materials have been widely used, involving geosynthetic clay liners (GCLs), geomembranes (GMs), compacted soil liners (CSLs), compacted clay liners (CCLs), highdensity polyethylene (HDPE) geomembranes, nonwoven needlepunched geotextile (GT). The advantages of these materials are consistent physical properties, low leakage rates, and facile installation, etc. (Simon and Müller, 2004; Bouazzaa and Vangpaisal, 2006; Müller et al., 2008; Hwan and Chung, 2008). Other materials for substituting clay or soil have also been tried, such as deinking by-products (DBPs), fluvial clay deposit, silty soil excavated from highway or tunnel projects, soil blended with municipal solid waste compost, digested sewage sludge, mixture of incineration fly ash and sewage sludge, and sandy soil amended with hydrated lime (Elshorbagy and Mohamed, 2000; Göktürk, 2003; Kim et al., 2005; Bozbey and Guler, 2006; Samah et al., 2008; Herrmann et al., 2009; Xu et al., 2010). In recent years, in order to eliminate the uncaptured methane emission from landfills, active or passive aerated biocovers with methanotrophic ability have become a hotspot of research (Perdikea et al., 2008). In developing countries such as China, as there are more food waste components in MSW, and these components are easily degraded and can produce LFG in short time, therefore, a large amount of LFG is generated during the filling stage. On the other hand when such a landfill is closed the LFG generation would decrease sharply. At present, most of LFG projects in China are operating under the waste landfilling stage and collecting LFG as much as possible to accomplish better energy utilization as well as greenhouse gas (GHG) emission abatement is the goal for these projects. Therefore, in order to prevent the uncaptured LFG from escaping, the intermediate covers are more critical compared to final covers and daily covers for these landfills. As for the covering materials during the cell filling stage, clay is conventionally used for intermediate cover. In China, according to the Technical Code for Municipal Solid Waste Sanitary Landfills (CJJ10-2004) (Chinese Construction Ministry, 2004), for a filling sanitary landfill the depth of clay for intermediate cover should be over 30 cm. However this requirement is rarely implemented thoroughly in practice, as clay is increasingly costly and scarce, and it also reduces the storage capacity of landfills. Due to the scarce resource of clay, engineers have tried to use HDPE geomembranes instead of clay as the landfill intermediate cover. Compared with clay layer, the advantage of HDPE geomembrane layer is that it can be used repeatedly as a low hydraulic barrier and does not occupy the space of landfills, the disadvantage is that it is easily crinkled which causes adverse influence on sealing property. Furthermore using HDPE geomembrane is cost effective. Now in China, the cost of HDPE geomembrane layer per hectare is about 100 thousand RMB Yuans (around 14.9 thousand US dollars), and that of soil layer is about 120 thousands RMB Yuans (around 17.9 thousand US dollars), thus the HDPE geomembrane layer is cheaper than soil layer, and it would be much more cost saving if the HDPE geomembrane layer is reused. Accordingly the effectiveness of using the HDPE geomembrane layer as the landfill intermediate cover should be investigated before it is used in landfills. From the viewpoint of LFG extraction in the landfills, conventional clay layer is preferred as the intermediate cover since it has good compatibility with the vertical gas extraction wells. If the clay is substituted by the HDPE geomembranes, both positive and negative effects of the HDPE geomembrane layer on gas extraction performance may occur. This study was aimed at making investigation on the feasibility of using HDPE geomembranes as a substitute for intermediate cover in landfills through a gas extraction test in a landfill.
2. Material and methods 2.1. Test field and material A filling landfill near Ningbo which is located in the southeast China was selected as the test field in this study. It was constructed with total storage capacity of 647 million cubic meters, and put into use in 2006. The initial daily MSW disposal amount was 500 tons and the data increased to 800 tons in 2009. Before the test of this study, over 700,000 tons of wastes had been dumped in it. The landfill is valley shaped, the deepest height of waste layer was 20 m, and the height of the shallow layers was about 4 m. As the landfill is located in a shaly mountain, soil is difficult to obtain locally, thus HDPE geomembranes were used as the cover layer in the landfill. The HDPE geomembranes used as the covering materials are roll shaped. The thickness, width and mass per unit area of them are 0.5 mm, 4 m and 460 g m 2, respectively. The hydraulic conductivity of the HDPE geomembrane is lower than 1.0 10 7 cm s 1. The HDPE geomembranes were laid on the landfill surface. The adjacent HDPE geomembranes were overlapped without stitching or welding and many wasted tires were placed
Fig. 1. Overall view of the landfill covered by HDPE geomembrane layer.
Fig. 2. A vertical gas well constructed for the field test.
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Orifice
Mobile flare
Branch pipe HDPE geomembrane layer
Open flare Waste
Bottom layer
W1
Blower
W2 General pipe
W3
Butterfly Sample port valve
Flowmeter
W4 Fig. 3. A schematic view of the test system.
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on the overlapped area to prevent the HDPE geomembranes from being blown away by wind. An overall view of the landfill is shown in Fig. 1. Four vertical gas extraction wells were constructed for the test in the middle zone of the landfill, where the wastes were separately dumped in therecent three years. The total depth of the waste in the test field was 12 m. The four wells were arranged at the vertices of a 25 m 25 m square region, and the depth of each well is the same as that of the waste. In order to ensure air sealing, the joints of the HDPE geomembranes and the gas wells were clamped by HDPE pipe flanges, thus there were no gaps between the HDPE geomembranes and the gas wells. A pneumatic water pump was installed at the bottom of each gas well to drain the water out. The photograph of a constructed gas well for the field test is shown in Fig. 2.
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2.2. Method
Fig. 4. Gas flow and the proportion of the main components in the early test.
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-0.8 Well vacuum / kPa
Four vertical gas wells (named as W1, W2, W3, and W4, respectively) were parallel connected to form a general pipe, and the latter was joined to a mobile flare. A schematic view of the test system is shown in Fig. 3. The mobile flare is an integrated equipment including a blower, an open flare, a gas flowmeter and other necessary components. The main function of the mobile flare is to pump LFG out of the wells and then burn it to solve air pollution as well as eliminate the hidden trouble of explosion. As the mobile flare was installed on a chassis with wheels, it could be easily moved by other machines such as a bulldozer or a grab. During the test, the four wells were all opened, and the gas extraction for the whole wells as well as for individual well was investigated. The total gas flow was read by a flowmeter in the mobile flare equipment, and at the same time the gas flow of each well was calculated by the pressure drop of an orifice plate which is mounted on the corresponding branch pipe. Sample ports on the pipes were used for measuring gas components (CH4, CO2, O2, and N2), gas pressure and temperature. The total gas flow was controlled by adjusting running speed of the blower through frequency conversion and the gas flow for each well was controlled by the butterfly valve on the corresponding branch pipe. During the field testing period, the blower ran continuously allday, and in the mean time the gas flow and gas components were monitored and adjusted timely to keep equilibrium between gas generation and extraction.
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Gas extraction flow / m .h
Fig. 5. Volume percentages of the gas components and well vacuums versus gas flows of W1 well.
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Fig. 6. Appearance of the test field before and after the replacement of HDPE geomembrane layer.
3. Results At first the gas extraction test was carried out with the HDPE geomembrane layer being maintained in the original status. It was soon found that air was sucked into the gas wells and there were not only many broken holes on the HDPE geomembranes but also leakage gaps between them. Therefore, the HDPE geomembrane layer had to be replaced and the new HDPE geomembranes were welded together. After that the test resumed.
v) and oxygen (lower than 3.0%, v/v) was acceptable both for gas well operation and for combustion utilization in the engines. It was also noticed that the pipe vacuum was very low (about 1.5 kPa), which indicated serious air leakage into the gas wells. Air leakage phenomenon was obvious from the data of the individual well. Fig. 5 displays the gas flows and the volume percentages of the main components in the W1 well under different vacuum. It shows that oxygen and nitrogen concentration increased dramatically with slight increase in gas flow. The data of other wells were similar to that of W1 well.
3.1. Results in early test 3.2. Test results after the replacement of HDPE geomembrane layer As previously mentioned that in the early test period, although HDPE geomembranes were laid on the landfill and the gas wells had good airproof connection with HDPE geomembranes initially, many leakage points were found soon on the surface of the HDPE geomembranes which could impair gas extraction. The results of this gas extraction period are presented in Fig. 4. Fig. 4 displays 10-day-data recorded the gas flow and the proportion of the main gas components in the early period of the gas extraction test. The data were measured in the general pipe for the total gas flow of the four wells. During the test the gas flow was adjusted step by step, and finally it was found that if the gas flow was over 40 m3 h 1, the amount of oxygen would exceed 3.0% (v/v) which is the acceptable upper limit for oxygen concentration in gas wells. Thus the total gas extraction ability for the four wells was determined as 40 m3 h 1. Under this condition the proportion of the main gas components such as methane (over 40%, v/
Since serious air leakage was found in the test, effort to improve the sealing property of the HDPE geomembrane layer was made. The old HDPE geomembranes were replaced by new ones and they were heat welded together to guarantee the sealing property on the whole test field. Fig. 6 shows the appearance of the test field before and after the replacement of HDPE geomembrane layer. After amendment of the HDPE geomembrane layer, gas extraction test resumed for another month. Data measured in the general pipe for the total gas flow is displayed in Fig. 7. From the above results, the total gas flow increased to 50 m3 h 1 with acceptable methane and oxygen concentrations under long time running, also the vacuum (around 4.0 kPa) in the general pipe was much higher than before, that means the air sealing property of the covering layer improved greatly.
Components / % v/v
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CH4
30
CO2 N2
20 10
O2 Well vacuum / kPa
-1
0 80 70
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Components / % v/v
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60 50 40 30 9 Dec 11 Dec 13 Dec 15 Dec 17 Dec 19 Dec 21 Dec 23 Dec 25 Dec 27 Dec 29 Dec 31 Dec 2 Jan
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Fig. 7. Gas flows and volume percentages of the gas components after amendment of the HDPE geomembrane layer.
CH4
50 40
CO2
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Gas extraction flow / m .h
Fig. 8. Volume percentages of the gas components and well vacuums versus gas flows of W1 well after the amendment of the HDPE geomembrane layer.
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4.1. Necessity for the HDPE geomembranes to be overall welded
16
2
If the HDPE geomembranes are just laid piece by piece on the landfill as shown in Fig. 6(a), it is inevitable to appear to many gaps on the layer, and the sealing property could not be perfect and could lead to either air leakage or rainwater infiltration. Furthermore as waste compaction is not implemented in many landfills in China, i.e. the waste on the top layer of these landfills is loose, air is liable to suck into the gas wells under negative pressure. Therefore, if HDPE geomembranes are used as the single intermediate layer on a landfill, they should be welded together to form a uniform sealing system.
0
4.2. Management and maintenance
14 o
Extracted gas temperature / C
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Test data Linear fit
4
-2 -4
-2
0
2
4
6
8
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o
Lowest daily ambient temperature / C Fig. 9. The relationship of extracted gas temperature dependence upon ambient temperature.
The gas extraction performance of the individual wells was also improved. Fig. 8 illustrates the gas flows and the amount of each gas component in the W1 well under different extraction vacuums. Compared with the data in Fig. 5, the gas extraction ability of the well improved obviously. The data of the other wells were similar to that of W1 well.
3.3. Ambient temperature influence After amendment of the HDPE geomembrane layer, the trend of ambient temperature was descending, and the gas flow also displayed a downtrend (Fig. 7) which means the gas extraction performance was depressed. The ambient temperature would also influence the extracted gas temperature. The relationship between the extracted gas temperature and the daily lowest ambient temperature is plotted in Fig. 9. From Fig. 9, we could find that although scattered, the plot of the two temperatures clearly showed temperature dependent; i.e. with the ambient temperature descending, the temperature of the extracted gas dropped. This phenomenon could be explained by the low heat isolation capacity of the HDPE geomembrane layer as well as low heat storage capacity of the shallow landfill (about 12 m in depth). According to the literatures (Zhang et al., 2006; Fan and Zhou, 2007), the thermal conductivity of HDPE geomembrane and soil could be selected as 0.4 W m 1 K 1 and 1.6 W m 1 K 1, respectively, and the thickness of HDPE geomembrane layer and soil layer is 0.5 mm and 300 mm, respectively, therefore, the heat transfer coefficient of soil layer is calculated as 5.3 W m 2 K 1 which is much lower than that of HDPE geomembrane layer (800 W m 2 K 1). Besides, the specific heat capacity of HDPE geomembrane and soil could be selected as 1.88 J g 1 K 1 and 3.10 J g 1 K 1, the thermal storage capacity of soil layer is calculated 1290 times that of HDPE geomembrane layer.
4. Discussion After the gas extraction test under different HDPE geomembrane layer conditions, it was found that the air sealing property of HDPE geomembrane layer and ambient temperature were the main factors affecting gas extraction ability, and other aspects such as maintenance of the HDPE geomembrane layer in good shape in daily management on the landfill was also important.
Advantage and disadvantage of both compacted clay layer and HDPE geomembrane layer used as the intermediate cover on landfills are obvious. The compacted clay layer is not only easy to install and has good adaptability with waste, it but also needs less maintenance. However cracks on the compacted clay layer might occur especially in dry climate, which could weaken sealing property of the layer. The most adverse factor for using soil as the covering material is that it becomes more and more expensive and sometimes it is difficult to be sufficiently supplied. As for the HDPE geomembrane layer, it has been proved to be an alternative intermediate cover on landfills by this study, with the precondition that the HDPE geomembranes should be welded overall to guarantee the sealing property as discussed above. In addition, the following aspects about the management and maintenance of HDPE geomembrane layer should also be paid attention to. The connection condition between the HDPE geomembranes and the vertical gas well pipes should be monitored carefully. Along with the sedimentation of waste in landfills, well pipes would gradually move upwards relative to the landfill ground and may cause serious tension stress of the HDPE geomembranes on the connection points, thus the timely maintenance of the connection parts between the HDPE geomembranes and well pipes needs to be done. As the HDPE geomembrane layer is easy to be waved or wrinkled, it is preferred to compact the waste and make the landfill surface flat, which could lessen the sedimentation of the landfill and decrease the possibility of stress damage of the HDPE geomembranes. In order to prevent the HDPE geomembranes from being broken by metal or other hard waste, it is better to lay a thin liner using soil or nonwoven needle-punched geotextile on the compacted waste firstly and then lay the HDPE geomembranes on the thin liner. Considering low heat isolation capacity of the HDPE geomembrane layer, it is better to increase the depth of filling cell to maintain the temperature of the waste in cold weather. On the other hand, the problem of waste settlement as mentioned previously should be of special concern. Normally the rate of waste settlement is fast and could be at the level of 5–8% in the first half of a year, after that the settlement tends to slow down and attains to stable finally. Therefore, the maintenance of the HDPE geomembranes should be paid special attention to ensure good connection of the extraction wells and the HDPE geomembranes. Finally, the intermediate HDPE geomembrane layer is a temporary cover which should be removed when additional waste layers are added and it could be reused on new waste layers on condition that it keeps in good shape. Compared with the HDPE geomembrane layer being used as the permanent in-place cover, the temporary cover is more effective not only for cost saving but also for the need of leachate seeping.
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5. Conclusions In this study, the possibility and effectiveness of using the HDPE geomembrane layer as an intermediate cover was investigated through a gas extraction test on a landfill. Results showed that if the HDPE geomembranes are laid on the ground of the landfill piece by piece without being welded together, the geomembranes cannot become airproof. It was proven that welding the HDPE geomembranes all together to form a whole airtight layer upon a large area of the landfill could evidently increase gas extraction ability which is acceptable in application. As the heat isolation capacity of HDPE geomembrane layer is low, the gas generation ability of a shallow landfill is likely to be weakened in cold climate. In order to ensure the HDPE geomembrane layert satisfies the operational demands, some practical problems such as how to keep the connection points between the HDPE geomembranes and vertical gas well pipes in good sealing condition for long term, and the methods to prevent the HDPE geomembranes from breaking, have to be solved. Acknowledgements The authors gratefully acknowledge the financial support from the Environment Protection Ministry of Jiangsu Province (No. 2009009), Education Ministry of Jiangsu Province (No. JH07-001), Analysis & Test Fund of Nanjing University and Research Fund for Young Teachers of Center of Materials Analysis in Nanjing University. The authors thank the three anonymous reviewers for providing helpful comments on this manuscript. References Bouazzaa, G.M., Vangpaisal, T., 2006. Laboratory investigation of gas leakage rate through a GM/GCL composite liner due to a circular defect in the geomembrane. Geotextiles and Geomembranes 24, 110–115.
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