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Evaluation of using municipal solid waste compost in landfill closure caps in arid areas
Waste Management 20 (2000) 499±507
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Evaluation of using municipal solid waste compost in land®ll closure caps in arid areas W.A. Elshorbagy *, A.M.O. Mohamed Department of Civil Engineering, United Arab Emirates University, Algimi #17555, Alain, United Arab Emirates Accepted 2 March 2000
Abstract Covering systems of land®lls involve partial or complete isolation of waste materials from the surrounding environment. Available materials and management practices in arid areas may not be adequate to ful®ll the requirements of the current regulations. This study investigates the performance of a native soil available in arid areas blended with municipal solid waste compost as an in®ltration barrier layer in land®ll closure cap design. Tests to determine dierent physical properties of the produced mixture were conducted and the optimum blend of minimum hydraulic conductivity was selected. The eect of organic decomposition and thermal ¯uctuation prevailing in the arid environment upon the changes in hydraulic conductivity was evaluated experimentally. The developed mixture of 60% compost and 40% native soil was found to have a hydraulic conductivity 4.0 to 6.010ÿ9 m/s. Other tests were conducted to examine the eect of organic decomposition and thermal ¯uctuations upon the hydraulic conductivity. From the hydraulic performance viewpoint, it was concluded that the developed mixture is an alternative. Some precautions are still needed in that case to eliminate the potential emission of gases from the cover material, anticipated settlement during the active stage of biological degradation, and the increased possibilities of deterioration related to burrowing animals. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Land®ll closure cap; Municipal solid waste compost; Hydraulic conductivity; Biodegradation; Thermal cycles
1. Introduction Land®lls undergoing closure must be covered with a ®nal cover that minimizes the long-term migration of liquids through the land®ll [1]. The main objective of covering systems is to reduce or eliminate the transport of ¯uids through the waste. Covering systems must function with minimum maintenance, promote drainage, minimize erosion of the cover, accommodate settling, and have hydraulic conductivity less than or equal to that of any bottom liner system or natural soil present. The important factors that emerge from examination of the status of the site are the design elements of the cover system. Regarding the design elements, there are signi®cant dierences in the covering system design requirements for waste management facilities in arid climates and those in humid climates [2]. In humid climates, cover and/or re-vegetation is usually required for erosion protection and in®ltration control. In extreme
* Corresponding author. Tel.: +971-3-5051-581; fax: +971-3-623-154. E-mail address: [email protected] (W.A. Elshorbagy).
cases, waste piles may require multi-layer covers to limit in®ltration. In regions where substantial freezing occurs during the winter periods, the eects of freeze±thaw cycles on the performance of high clay content materials can be destructive. Covering system design components, for waste sites in humid areas are well developed [1]. However, the US EPA-recommended design may not be suitable for other types of waste management practices in arid lands. Available materials and management practices in arid areas may not be adequate in association with the current regulations. The regulations do, however, permit alternative designs if they can achieve erosion and in®ltration protection equivalent to an acceptable conventional cover system [40CFR258. 60(b)]. This indicates the signi®cance of searching different alternatives to compacted clay-based barriers in arid areas and evaluate their performance under various environmental conditions. 1.1. Hydraulic barriers alternatives in arid areas The materials used in soil-based covering systems are either natural materials, modi®ed soils, synthetic material,
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00025-8
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or waste materials. Well-graded ®ne-grained compacted soils are usually selected in case of natural soils. If available, dierent types of clay are the most likely choice because of their low hydraulic conductivity and adequate performance in eliminating the ¯uids transport through land®lls. Capillary barriers is another con®guration of natural materials which can replace the compacted cohesive soil covers in arid and semi/arid environments [3]. A capillary barrier consists of a ®ne-over-coarse soil layer sequence that acts as a barrier to downward moving moisture under unsaturated conditions due to the contrast in hydraulic conductivities. Moisture is held in the ®ne layer by capillary forces and can be removed by evapotranspiration or in the ®ne-coarse surface is sloped, by lateral transport in the ®ne soil above the interface [4]. Another alternative of barrier layers in the closure cap is the use of synthetic materials. Several types of synthetic membranes are recently produced with reasonable prices. However, many problems can still adversely aect their ecient performance. Shrink± thaw, desiccation and subsidence cracking, root and animal intrusion, and chemical and biological attacks can always pose threat to the long-term performance of the synthetic materials. The native soils present in most arid areas are usually of the coarse type that cannot be utilized as barriers without modi®cation. The grain size distribution can be altered and broadened by blending with other materials and/or additives. The addition of cement or lime in small portions has proved to be eective in modifying soil stability over time [2]. 1.2. Waste-based hydraulic barriers There has been growing interest in using waste materials as alternative hydraulic barriers for conventional materials in lining and covering land®lls. This is apparent in particular for arid areas where the clay and other ®ne soils are not readily available and usually require high prices for transport from remote locations. Another reason is attributed to the huge amounts of generated wastes and the elevating costs associated with their disposal. Over 4.1 billion metric tons of solid wastes are generated in the United States of America only, equaling about 16 metric tons per person annually [5]. A growing amount of this waste is being land®lled, and as land®lling costs increase, new disposal options are needed. As a result, many researchers and organizations have been focused on studying and characterizing dierent waste products for identifying their potential use as in®ltration barrier layers. The use of waste materials in lining and closing the land®lls has proved to be cost-eective in many cases. For example, it has been narrated that typical savings of about $20,000 to $50,000 per acre (81±202 million dollars/m2)
have been achieved by using paper sludge in place of compacted clay in land®ll covers [6]. Among the waste materials that have already been used as substitute for soil-based covers are ¯y ash, slags from iron and steel-making, non-ferrous slags, domestic refuse incinerator ash, overburden materials, dredged silts, construction rubble, wastewater treatment sludges, and paper mill sludges. Many of these materials may include trace elements of potential pollutants and/or heavy metals that can pose various environmental risks. Therefore, care should be given before using these materials to assess the possible hazard expected during in®ltration conditions. Recently, composts made from dierent wastes have been used in many sites or under evaluation for future use. The rest of the paper documents the results of an experimental study performed to evaluate the potential use of municipal solid waste compost produced by the compost factory of Dubai Emirate of the United Arab Emirates (UAE), as a hydraulic barrier alternative in closing the national land®ll disposal sites. United Arab Emirates is located in the Arab peninsula with seven emirates and a 2.4 million population. The country has one of the highest solid waste generation rates in the world; that is 750 kg/capita/year. About 70% of that waste is organic that is originated from food and paper products. Therefore, most Emirates have major composting units that collect the solid waste, screen it, and process it to produce compost used in agricultural activities. Several land®lls are present in UAE, closure of many of which is expected in near future. The native soil present at the land®ll sites is mostly sandy soil with optimum moisture content of 12% (by wet weight) corresponds to dry unit weight of 1800 kg/m3. Clay and other cohesive materials are very rare in the region. Hence, it has been decided to explore the possibility of blending the native soil with the produced municipal solid waste compost and evaluate the performance of dierent blends as hydraulic barriers in ®nal closure caps for the existing land®lls. 2. Experimental tests Samples of the native soil were obtained from Al-Jimi district in Al-Ain municipality of UAE. The selected testing procedures were carried out following procedures described by ASTM standards. Measurements of speci®c gravity and consistency limits were performed according to ASTM D854, test method for speci®c gravity of soils, and ASTM D422, method for particlesize analysis of soils. Determination of compaction parameters and permeability coecients (at maximum dry density) were carried out following ASTM D698, test methods for moisture±density relations of soils and soil-aggregate mixtures, and ASTM D2434, test method
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for permeability of granular soils, respectively. pH and conductivity measurements of 1:10 soil±water extract were conducted according to ASTM D1293, test method for pH of water, and ASTM D1125, test methods for electrical conductivity and resistivity of water, respectively. A summary of these results is shown in Table 1. The grain size distribution of the tested soil is shown in Fig. 1. According to AASHTO classi®cation system, the gradation tests show that the soil samples include 35% gravel, 43% sand, 18% silt, and 4% clay sizes. The soil can be classi®ed as silty sand. The tested soil has a coecient of uniformity Cu=14.11, which indicates that the soil has large range in grain sizes and the soil is regarded as well graded, and a coecient of curvature Cc=0.88 indicating a well graded soil. The compost used in the study was obtained from Dubai Composting factory which process the organic portion of municipal solid waste generated from the Emirate of Dubai. The processing includes screening, followed by volume reduction using 10 mm shredders, then stabilization by windrow aeration at regulated moisture contents for 3 to 4 months at the winter time, the time of the study. The moisture content of the delivered compost ranged between 38 and 44% and the dry unit weight ranged between 550 and 650 kg/m3. Samples of the produced compost were tested for their moisture contents, unit weights, and volatile suspended solids as indicators for their organic contents. The moisture contents were determined by weighing the moist samples, drying at 90 C, then weighing the dried sample. It should be noted that the drying temperature was slightly lowered to minimize the potential volatilization of present organics. Due to the heterogeneous nature of the compost and its high potential of microbial activity, many test results were characterized by
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some instability. This was even observed with dierent replicates taken from the same bag. The maximum dry unit weight of the compost was close to 900 kg/m3 corresponding to moisture content of about 38% as shown in Fig. 2. These results were obtained using standard compaction tests on both the native soil and compost. Compaction tests were performed on dierent compost± soil mixes following the ASTM procedure D698-78 to determine the optimum moisture content and the corresponding dry unit weight. The relationships between the optimum moisture content, soil±compost mix ratios, and the maximum dry unit weight are shown in Fig. 3. The results indicate that both of the optimum moisture content and the dry unit weight were linearly related to the compost percentage in the tested mixes. An ignition test was carried out by burning the sample in a mue furnace at 550 C [7±9] and the percentage of total volatile solids (TVS) were used as indicators of organic contents according to the equation: %TVS W1 ÿ W2 =W1 100
1
where W1 is the initial dry weight, and W2 is the weight of ash after ignition. The results ranged between 56 and 62% and an average value of 59% was considered to represent the initial organic content of the compost used in the tested blends. 2.1. Saturated hydraulic conductivity Constant-head tests were chosen to evaluate the hydraulic conductivity of each compost±soil mixture. The PVC columns used in the experiments had dimensions of 52 mm in diameter and 600 mm in height. The constant head was achieved using a drainage tube located
Table 1 Selected properties of tested soil Soil properties
Pore ¯uid analysis
Geotechnical Speci®c gravity
2.68
pH Conductivity (Simens)
Consistency limits Liquid limit Plastic limit
NP NP
Concentrations (ppm) Na+ K+
140 8.5
Soil gradation Gravel (wt%) Sand (wt%) Silt (et%) Clay (wt%) Soil texture
35 43 18 4 Silty sand
Mg2+ Ca2+ CaCOÿ 3 HCOÿ 3 ÿ Cl
34 48 520 140 460
Compaction Max. dry density (Mg/m3) Opt. water content (%) Permeability (m/s)
1.79 12 7.185E-06
Mineralogical composition by X-ray analysis Major: quartz, calcite, plagioclase Minor: dolomite, feldspar, kaolinite
8.1 1.18E-3
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Fig. 1. Grain size distribution of the native soil.
Fig. 2. Results of compaction tests for (a) native soil, (b) MSW compost.
near the top of the column to remove any surplus in¯ow. A 200 mm of each compost±soil mixture was compacted in ®ve layers to its maximum dry density and optimum moisture content. The specimens were bounded from the top and bottom by porous stones and ®lter papers to eliminate piping and/or clogging during experiment. Also, to prevent any disturbance and upward movement of the mixture particles, a small pressure of about 19 kn/m2 was applied on the top of the specimen by means of small spring compressed through its connection to the upper cover. This pressure
was perceived to simulate a conservative scenario of low con®ning stress and eventually high permeability. The specimens were allowed to stabilize under the continuous in¯ow of tap water for 3±4 h before monitoring the ¯ow rate with time. Following the stabilization period, the hydraulic conductivity was calculated and reported with time for different compost±soil mixtures. The results are shown in Fig. 4 for 0.30, 0.45, 0.55, and 0.65 compost mixing ratios. It can be seen that the hydraulic conductivities at dierent time increments are decreasing with increasing
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the compost percentage up to 0.55 then started to increase slightly at a mixing ratio of 0.65. Also, most of the reduction in the hydraulic conductivity with time took place within the ®rst 14 h for all mixtures. Hence, the hydraulic conductivity at 14 h was considered as a characteristic value for each mixture. The results indicate that the optimum hydraulic conductivities correspond to mixing ratios between 0.55 and 0.65. To verify
Fig. 3. Variation of optimum moisture contents and corresponding maximum dry unit weights with compost±soil mixing ratios.
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the selection of the optimum mix ratio, two additional experiments were performed. Fig. 5 presents the considered hydraulic conductivity for the original experiment (replicate 1) and the other two experiments (replicates 2 and 3). Replicates 1 and 2 indicate that the optimum mix ratio is 0.55. However, replicate 3 indicates that a further decrease in hydraulic conductivity can be still acheived with mix ratios greater then 0.65. Therefore, a compost mix ratio of 0.60 was seleected as the least variable mixture with a saturated hydraulic conductivity in the range of 1.0 to 3.0E-8 m/s. The corresponding moisture content was 26% as it appears in Fig. 5. Owing to the fact that the minimum hydraulic conductivity is achieved at a moisture content wet of optimum, the obtained hydraulic conductivities are not necessarily the minimum ones since the specimens in the previous experiments were compacted at their optimum moisture contents. Therefore, more experiments were run to ®nd the minimum hydraulic conductivity of the selected mixture (60% compost) after being compacted at water contents higher than the optimum values. Three levels of moisture content wet of optimum were tested: 30, 35, and 40%. Hydraulic conductivities corresponding to these levels and reported after 14 h of permeation were 1.4E-8, 1.1E-8, and 2.3E-8, respectively. Therefore, a moisture content of 35% corresponding to a dry unit weight of 0.94 kg/m3 produced the least hydraulic conductivity. The minimum value was found to stabilize around 4.0 to 6.0E-9 m/s after 2 weeks of permeation. 2.2. Eect of organic decomposition on hydraulic conductivity
Fig. 4. Variation of hydraulic conductivity with time for dierent compost±soil mixing ratios.
Fig. 5. Variability of the hydraulic conductivity with the compost mix ratios for three tested replicates.
The decrease of hydraulic conductivity with time can be generally attributed to soil consolidation that decreases the soil voids. Primary consolidation is signi®cant at large overburden pressures. Another factor that may promote the decrease in hydraulic conductivity with time is the decomposition of organic materials. This factor can be dominant if prevailing environmental conditions, such as temperature, moisture content, and pH, are suitable for the optimum activity of present microorganisms. To explore the eect of organic decomposition on the temporal variation of hydraulic conductivity, two experiments were tested. The ®rst experiment was left at room temperature of 22 C while the second one was placed in an incubator with a constant temperature of 38 C. In order to determine this elevated temperature of 38 C, a simple experiment was conducted during which the barrier layer was covered with a 100 mm layer of native soil and subjected to a continuous source of heat of 46 C at the top surface. The value of top surface temperature was selected in such a way it simulates an extremely high temperature in arid deserts. The steady
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state temperature at the barrier, i.e. at a depth of 100 mm from the surface, was found to be 38 C. The temporal variations of the hydraulic conductivity for the two experiments were recorded and presented as shown in Fig. 6. The ®gure shows similar trends for both cases with lower hydraulic conductivity in case of the incubated column. The hydraulic conductivity reached a value of 110ÿ8 m/s after about 15 days for the column placed at room temperature while it dropped to a value of 410ÿ9 m/s after only 11 days for the incubated column. This indicates that the elevated temperature of the incubator accommodated better environment for the microorganisms that led to faster decomposition of the organic substrates and reduction in the resulting hydraulic conductivity. The incubator's controlled temperature was close to the optimum temperature of biological growth (35 C) for mesophilic-type of bacteria that activated the metabolism process of present microorganisms [10]. As a result, the number of viable cells and biomass is increased which resulted in physical blocking of the soil voids and lowering the mixture hydraulic conductivity. The mixture inside the column acted as a batch system with decreasing amounts of substrates accompanied by emission of carbon dioxide. This ®nding was further substantiated by inspecting the measured organic contents of both columns at the end of the experiment. An average value was found by measuring the organic contents of three samples taken from the top, middle, and bottom of each column. The average organic content was 38.4% for the experiment performed at room temperature and was 31.6% for the incubated experiment. The initial organic content for both columns was 51%. Results also indicate that the hydraulic conductivity curve of the incubated specimen could be divided into two phases. The initial phase steeply decreases within the ®rst 2 days. The later phase decreases gently and tends to ¯atten out at the end of the tested period. The high rate of decomposition in the initial phase can be attributed to the activity of aerobic bacteria and other
aerobic microorganisms that concumes the residual oxygen present in the voids at the early stage of permeation. With passage of time, the permeant begins to strip the oxygen from the mixture pores and the metabolism process gradually transforms from aerobic to anaerobic. This was substantiated by the odorous gases released from the incubated column and the black ¯uids deposited in the euent tube after a day of permeation. The amount and odor of the gases intensi®ed after 2 days suggesting complete anaerobic decomposition. The emitted gas was presumably hydrogen sul®de because of its similarity to rotten eggs odor, while the black ¯uid deposited in the euent tube was possibly ferrous sul®de since the analyzed leachate showed high iron content (Table 2). It should be noted that current experiments were conducted under volume change conditions, i.e. open system. For ®eld application, gas venting systems should be installed for releasing the gas and preventing any potential formation of cracks due to gas buildup. Also, the moisture conditions required to promote biodegradation may not be possible to sustain in arid environment. However, tests were performed under these conditions as a means of simulating extreme biodegradation conditions that may occur over long periods of time. On the other hand, the presence of two phases in Fig. 6 is not related to primary consolidation, as the applied pressure (19 kN/m2) would be much smaller than the compaction induced by appaerent preconsolidation pressure. 2.3. Eect of thermal cycles on hydraulic conductivity A relevant environmental concern in arid areas is the ¯uctuation in temperature due to diurnal and seasonal eects. Experiments were conducted to test the eect of thermal cycles on the hydraulic conductivity. The compost±soil mixture selected earlier (60% compost) was subjected to thermal cycles during the continuous permeation process. The experiments were carried out
Fig. 6. Eect of temperature on the temporal variation of hydraulic conductivity.
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Table 2 Heavy metals contents in mixture leachate Element
Lead
Cadmium
Copper
Nickel
Zinc
Iron
Cobalt
Comcentration (mg/l) using buer solution of pH=5 Comcentration (mg/l) using buer solution of pH=3 Maximum regulatory concentration (mg/l)
0.340 0.708 0.40a 5.00b
0.05 0.013 0.10a 1.00b
0.354 0.687 2.0a
0.205 0.254 0.20a
1.356 4.196 2.0a
2.239 3.062 N.A.c
0.014 0.015 N.A.c
a b c
European Community. US EPA. No available regulatory concentrations.
using two columns, the ®rst had the selected mixture while the second had the selected mixture blended with 15% lime (by dry weight) to inhibit microbial activity and organic decomposition. pH of the leachate from the second column was measured and found to found to vary between 11.7 and 12.2. The two columns were maintained at 22 C (room temperature) for 3 to 4 days and then maintained at 38 C in an incubator for a similar period. This cycle was repeated up to 40 days with continuous monitoring of hydraulic conductivity. The results are shown in Fig. 7 for the two tested columns. The results indicate that the hydraulic conductivity of the lime-amended mixture was approximately constant with time and was insentive to temperatue ¯uctuations. The slight initial increase is attributed to the initial reaction of lime with organic matter in the compost and to the particle agitation in the new structured mix. The hydraulic conductivity had a little perturbation around a value of 1.0E-7 m/s for about 19 days, after which the perturbation center slightly shifted up to a value of 1.3E-7 m/s. Other than this variation, no clear trend was observed during the tested period. The results suggest that thermal cycles have little eect on hydraulic conductivity if the microbial activity is eliminated. It is worth mentioning that clay samples compacted near the
optimum or dry of optimum have been shown to undergo very little change in hydraulic conductivity when subjected to wet±dry cycles [11]. On the other hand, the hydraulic conductivity of the untreated mixture was responding to thermal cycles; it decreased at 38 C and increased at room temperature. The increase in hydraulic conductivity at room temperature is attributed to the drop of biological activity associated with de-¯occulation of the viable cells that increased the pore voids. The increase was maximum in the ®rst cycle (was reached a value of 1.2E-7 m/s) then it decayed at later cycles and completely vanished in the ®fth cycle after about 36 days. It can be also seen that the hydraulic conductivity following the ®fth cycle approached very small values (in the orders of 1.0E-11); that were close to the value obtained in the absence of thermal cycles (Fig. 6). Finally, the plots shown in Fig. 7 show that the hydraulic conductivity was sharply increasing once subjected to the low-temperature phase and remained almost constant throughout the entire phase. This can be explained by the expected sudden occurrence of de¯occulation. On the other hand, the hydraulic conductivity was gradually decreasing when subjected to the high-temperature phase that is mostly related to slow and gradual ¯occulation.
Fig. 7. Eect of thermal cycles on the temporal variation of hydraulic conductivity: I; inside the incubator (high temperature); O; outside the incubator (low temperature).
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2.4. Atterberg limits An important factor to be considered when designing the in®ltration barrier layer is the potential cracking and desiccation. This is of particular concern in arid areas due to the high temperatures and long drying periods in addition to the common dierential settlement caused by overburden pressures and biological and chemical reactions in the buried waste. USA EPA regulation [1] requires the soil of in®ltration barrier layer to have a plasticity index of at least 10% although some soils with a slightly lower plasticity index may be suitable. Boynton and Daniel [12] have shown one order of magnitude increase in the hydraulic conductivity of a compacted clay barrier after desiccation cracks developed at low con®ning pressure. Desiccation cracking requires special attention when designing land®ll cover in arid areas so that drying and/or high temperatures are avoided either during the construction or following the ®nal closure. Having a protection layer of sucient thickness above the barrier layer can eliminate the hazard of desiccation and cracking. The liquid limit, plastic limit, and plasticity index were determined for the selected optimum mixture (60% compost) by adopting the ASTM procedure D4318. These limits were: liquid limit=41%; plastic limit=notdeterminable; and plasticity index=41%. The high liquid limit of the amended soil is attributed to the high organic matter content (51%). The inability to determine plastic limit is attributed to the increased ®brous nature of the organic matter and low clay content in the mixture. Further studies are in progress to evaluate the potential desication and cracking of soil±compost mixes. 2.5. Potential leachability of heavy metal ions from the compost Heavy metal ions present in the municipal solid waste compost may be leached by the percolating ¯uids into the underlying waste. The elevated concentrations of such ions in the leached ¯uids are most likely to be insigni®cant in comparison to the usually high concentrations existing in the refuse. Yet, they were measured in this study as a means of characterizing the leachate. Also, they can be important if thickness of the closure cap was to be large. A conservative analysis was performed by testing the compost samples rather than the selected mixture. The test followed the USEPA Toxicity Charactaristics Leaching Procedure (TCLP) [13] that is a single step batch extraction test. The extracts were prepared by ®rst crushing the compost solids to pass through a 9.5-mm screen, adding a buer solution with a 20:1 liquid to solid ratio, shaking the mix for 20 h, and ®nally ¯itering. All measurements were done in Flame Atomization of GBC 906 Atomic
Absorption Spectrophotometer with double beam. Two types of buered acidic leaching solutions were used; the ®rst had a pH of 5 and the second had a pH of 3. The results of elemental analysis are listed in Table 2 for the two buer solutions along with regulatory maximum concentrations employed by USEPA and the European Community. The results show that the concentrations of all heavy metal ions were reasonably les than current regulations. Also, for the amended soil, heavy metal concentrations will be even less due to the high adsorption capacity of the soils [2]. 3. Conclusions This study has presented an overall evaluation of using a native soil available in arid areas blended with municipal solid waste compost as an in®ltration barrier layer in land®ll closure caps. Regular tests were conducted to determine the optimum moisture content, maximum dry unit weight, hydraulic conductivity, Atterberg limits, and potential leachability of heavy metals from the mix. The compost and native soil were mixed in proportions that yielded minimum hydraulic conductivity. Other tests were conducted to examine the eects of organic decomposition and thermal ¯uctuation upon the hydraulic conductivity. Speci®c ®ndings were as follows: . A mixture of 60% compost and 40% native soil (dry weights) was selected and found to have a hydraulic conductivity of 2.0 to 3.0E-8 m/s at optimum moisture content of 26% and a minimum value of 4.0 to 6.0E-9 m/s at a moisture content of 35%. . Raising the temperature from 22 to 38 C resulted in lowering the permeability by one order of magnitude. This was attributed to the activated metabolism of the present microorganisms that resulted in physical blocking of the soil voids. . The hydraulic conductivity of the selected mixture was responding to the thermal cycles in such a way it decreased at high temperature and increased at room temperature. The maximum increase occurred in the ®rst cycle. . Thermal cycles had little eect on the hydraulic conductivity when the microbial activity was eliminated by adding lime to the mixture. . Addition of compost to the natural soil increased the plastic nature of the amended soil. . Levels of heavy metal ions in the leachate from the compost samples were within acceptable standards The presented study has shown that the municipal solid waste compost investigated can be used in construction of hydraulic barrier layer in land®ll closure
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caps in arid areas. The cost bene®ts of such usage can be potentially doubled if the compost required for the mixture was manufactured at the site. This can be easily achieved via processing the received solid wastes following the common techniques of composting including screening, shredding, and windrow degradation procedures. Yet, special considerations pertinent to the compost material should be given. These include the potential emission of gases from the cover material, major settlement during the active stage of biological degradation, and the increased possibilities of deterioration related to the burrowing animals. Acknowledgements This study was funded by the scienti®c research council of United Arab Emirates University. Acknowledgements are due to Dubai Municipal Composting plant and the Central Lab Unit at UAE University. The authors would also like to extend their thanks and appreciation to Engineers Essam Khalifa, Abd-Elkadir Alkamaly, and Salim Hegazy for their diligent help oered during the experimental work. References [1] US Environmental Protection Agency. Final covers on hazardous waste land®ll and surface impoundments. EPA 530-SW-89-047, 1989.
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[2] Mohamed AMO, Antia HE. Geoenvironmental engineering. Amsterdam: Elsevier, 1998. [3] Mohamed AMO. Evaluation of a multi-layer soil cover system for controlling acid mine drainage. In: Al-Manaseer A, et al., editors. Int. Conf. On Engineering Materials, Ottawa, Canada, 1997. p. 159±172. [4] Morris CE, Stormont JC. Capillary barriers and subtitle D covers: Estimating equivalency. Journal of Environmental Engineering, ASCE 1997;123:1:3±10. [5] Amirkhanian S. Utilization of waste materials in highway industry Ð a literature review. Journal of Waste Management and Technology 1997;24(2). [6] Moo-Young HK, Zimmie TF. Closure of geotechnical properties of paper mill sludges for use in land®ll covers Journal of Geotechnical Engineering. ASCE 1998;124(10):1043. [7] Adani F, Genevini PL, Gasperi F, Zorzi G. Organic matter evolution index (OMEI) as a measure of composting eciency. Compost Science and Utilization 1997;5:2:53±62. [8] Ham RK, Norman MR, Fritschel PR. Chemical characterization of fresh kills land®ll refuse and extracts. Journal of Environmental Engineering 1993;119(6):1176±95. [9] Schwab BS, Ritchie CJ, Kain DJ, Bobrin GC, King LW, Palmisano AG. Characterization of compost from a pilot plant pcale composter utilizing simulated solid waste. Waste Management and Research 1994;12:289±303. [10] Tchobanoglous G, Theisen H, Vigil S. Integrated solid waste management: engineering principles and management issues. McGraw-Hill, Inc., 1993. [11] Johnston K, Hang, MD. Impacts of wet-dry freeze±thaw cycles on the hydraulic conductivity of Glacial Till. In: 45th Canadian Geotechnicial Conference, paper Toronto, 1992. p. 1±10 [12] Boynton SS, Daniel DE. Hydraulic conductivity tests on compacted clay. Journal of Geotech. Eng. ASCE 1985;111:4:465±78. [13] US EPA. Rules and regulations, The Federal Register. TCLP, method 1311. Federal Register 1990;55:61.
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Evaluation of using municipal solid waste compost in land®ll closure caps in arid areas W.A. Elshorbagy *, A.M.O. Mohamed Department of Civil Engineering, United Arab Emirates University, Algimi #17555, Alain, United Arab Emirates Accepted 2 March 2000
Abstract Covering systems of land®lls involve partial or complete isolation of waste materials from the surrounding environment. Available materials and management practices in arid areas may not be adequate to ful®ll the requirements of the current regulations. This study investigates the performance of a native soil available in arid areas blended with municipal solid waste compost as an in®ltration barrier layer in land®ll closure cap design. Tests to determine dierent physical properties of the produced mixture were conducted and the optimum blend of minimum hydraulic conductivity was selected. The eect of organic decomposition and thermal ¯uctuation prevailing in the arid environment upon the changes in hydraulic conductivity was evaluated experimentally. The developed mixture of 60% compost and 40% native soil was found to have a hydraulic conductivity 4.0 to 6.010ÿ9 m/s. Other tests were conducted to examine the eect of organic decomposition and thermal ¯uctuations upon the hydraulic conductivity. From the hydraulic performance viewpoint, it was concluded that the developed mixture is an alternative. Some precautions are still needed in that case to eliminate the potential emission of gases from the cover material, anticipated settlement during the active stage of biological degradation, and the increased possibilities of deterioration related to burrowing animals. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Land®ll closure cap; Municipal solid waste compost; Hydraulic conductivity; Biodegradation; Thermal cycles
1. Introduction Land®lls undergoing closure must be covered with a ®nal cover that minimizes the long-term migration of liquids through the land®ll [1]. The main objective of covering systems is to reduce or eliminate the transport of ¯uids through the waste. Covering systems must function with minimum maintenance, promote drainage, minimize erosion of the cover, accommodate settling, and have hydraulic conductivity less than or equal to that of any bottom liner system or natural soil present. The important factors that emerge from examination of the status of the site are the design elements of the cover system. Regarding the design elements, there are signi®cant dierences in the covering system design requirements for waste management facilities in arid climates and those in humid climates [2]. In humid climates, cover and/or re-vegetation is usually required for erosion protection and in®ltration control. In extreme
* Corresponding author. Tel.: +971-3-5051-581; fax: +971-3-623-154. E-mail address: [email protected] (W.A. Elshorbagy).
cases, waste piles may require multi-layer covers to limit in®ltration. In regions where substantial freezing occurs during the winter periods, the eects of freeze±thaw cycles on the performance of high clay content materials can be destructive. Covering system design components, for waste sites in humid areas are well developed [1]. However, the US EPA-recommended design may not be suitable for other types of waste management practices in arid lands. Available materials and management practices in arid areas may not be adequate in association with the current regulations. The regulations do, however, permit alternative designs if they can achieve erosion and in®ltration protection equivalent to an acceptable conventional cover system [40CFR258. 60(b)]. This indicates the signi®cance of searching different alternatives to compacted clay-based barriers in arid areas and evaluate their performance under various environmental conditions. 1.1. Hydraulic barriers alternatives in arid areas The materials used in soil-based covering systems are either natural materials, modi®ed soils, synthetic material,
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00025-8
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or waste materials. Well-graded ®ne-grained compacted soils are usually selected in case of natural soils. If available, dierent types of clay are the most likely choice because of their low hydraulic conductivity and adequate performance in eliminating the ¯uids transport through land®lls. Capillary barriers is another con®guration of natural materials which can replace the compacted cohesive soil covers in arid and semi/arid environments [3]. A capillary barrier consists of a ®ne-over-coarse soil layer sequence that acts as a barrier to downward moving moisture under unsaturated conditions due to the contrast in hydraulic conductivities. Moisture is held in the ®ne layer by capillary forces and can be removed by evapotranspiration or in the ®ne-coarse surface is sloped, by lateral transport in the ®ne soil above the interface [4]. Another alternative of barrier layers in the closure cap is the use of synthetic materials. Several types of synthetic membranes are recently produced with reasonable prices. However, many problems can still adversely aect their ecient performance. Shrink± thaw, desiccation and subsidence cracking, root and animal intrusion, and chemical and biological attacks can always pose threat to the long-term performance of the synthetic materials. The native soils present in most arid areas are usually of the coarse type that cannot be utilized as barriers without modi®cation. The grain size distribution can be altered and broadened by blending with other materials and/or additives. The addition of cement or lime in small portions has proved to be eective in modifying soil stability over time [2]. 1.2. Waste-based hydraulic barriers There has been growing interest in using waste materials as alternative hydraulic barriers for conventional materials in lining and covering land®lls. This is apparent in particular for arid areas where the clay and other ®ne soils are not readily available and usually require high prices for transport from remote locations. Another reason is attributed to the huge amounts of generated wastes and the elevating costs associated with their disposal. Over 4.1 billion metric tons of solid wastes are generated in the United States of America only, equaling about 16 metric tons per person annually [5]. A growing amount of this waste is being land®lled, and as land®lling costs increase, new disposal options are needed. As a result, many researchers and organizations have been focused on studying and characterizing dierent waste products for identifying their potential use as in®ltration barrier layers. The use of waste materials in lining and closing the land®lls has proved to be cost-eective in many cases. For example, it has been narrated that typical savings of about $20,000 to $50,000 per acre (81±202 million dollars/m2)
have been achieved by using paper sludge in place of compacted clay in land®ll covers [6]. Among the waste materials that have already been used as substitute for soil-based covers are ¯y ash, slags from iron and steel-making, non-ferrous slags, domestic refuse incinerator ash, overburden materials, dredged silts, construction rubble, wastewater treatment sludges, and paper mill sludges. Many of these materials may include trace elements of potential pollutants and/or heavy metals that can pose various environmental risks. Therefore, care should be given before using these materials to assess the possible hazard expected during in®ltration conditions. Recently, composts made from dierent wastes have been used in many sites or under evaluation for future use. The rest of the paper documents the results of an experimental study performed to evaluate the potential use of municipal solid waste compost produced by the compost factory of Dubai Emirate of the United Arab Emirates (UAE), as a hydraulic barrier alternative in closing the national land®ll disposal sites. United Arab Emirates is located in the Arab peninsula with seven emirates and a 2.4 million population. The country has one of the highest solid waste generation rates in the world; that is 750 kg/capita/year. About 70% of that waste is organic that is originated from food and paper products. Therefore, most Emirates have major composting units that collect the solid waste, screen it, and process it to produce compost used in agricultural activities. Several land®lls are present in UAE, closure of many of which is expected in near future. The native soil present at the land®ll sites is mostly sandy soil with optimum moisture content of 12% (by wet weight) corresponds to dry unit weight of 1800 kg/m3. Clay and other cohesive materials are very rare in the region. Hence, it has been decided to explore the possibility of blending the native soil with the produced municipal solid waste compost and evaluate the performance of dierent blends as hydraulic barriers in ®nal closure caps for the existing land®lls. 2. Experimental tests Samples of the native soil were obtained from Al-Jimi district in Al-Ain municipality of UAE. The selected testing procedures were carried out following procedures described by ASTM standards. Measurements of speci®c gravity and consistency limits were performed according to ASTM D854, test method for speci®c gravity of soils, and ASTM D422, method for particlesize analysis of soils. Determination of compaction parameters and permeability coecients (at maximum dry density) were carried out following ASTM D698, test methods for moisture±density relations of soils and soil-aggregate mixtures, and ASTM D2434, test method
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for permeability of granular soils, respectively. pH and conductivity measurements of 1:10 soil±water extract were conducted according to ASTM D1293, test method for pH of water, and ASTM D1125, test methods for electrical conductivity and resistivity of water, respectively. A summary of these results is shown in Table 1. The grain size distribution of the tested soil is shown in Fig. 1. According to AASHTO classi®cation system, the gradation tests show that the soil samples include 35% gravel, 43% sand, 18% silt, and 4% clay sizes. The soil can be classi®ed as silty sand. The tested soil has a coecient of uniformity Cu=14.11, which indicates that the soil has large range in grain sizes and the soil is regarded as well graded, and a coecient of curvature Cc=0.88 indicating a well graded soil. The compost used in the study was obtained from Dubai Composting factory which process the organic portion of municipal solid waste generated from the Emirate of Dubai. The processing includes screening, followed by volume reduction using 10 mm shredders, then stabilization by windrow aeration at regulated moisture contents for 3 to 4 months at the winter time, the time of the study. The moisture content of the delivered compost ranged between 38 and 44% and the dry unit weight ranged between 550 and 650 kg/m3. Samples of the produced compost were tested for their moisture contents, unit weights, and volatile suspended solids as indicators for their organic contents. The moisture contents were determined by weighing the moist samples, drying at 90 C, then weighing the dried sample. It should be noted that the drying temperature was slightly lowered to minimize the potential volatilization of present organics. Due to the heterogeneous nature of the compost and its high potential of microbial activity, many test results were characterized by
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some instability. This was even observed with dierent replicates taken from the same bag. The maximum dry unit weight of the compost was close to 900 kg/m3 corresponding to moisture content of about 38% as shown in Fig. 2. These results were obtained using standard compaction tests on both the native soil and compost. Compaction tests were performed on dierent compost± soil mixes following the ASTM procedure D698-78 to determine the optimum moisture content and the corresponding dry unit weight. The relationships between the optimum moisture content, soil±compost mix ratios, and the maximum dry unit weight are shown in Fig. 3. The results indicate that both of the optimum moisture content and the dry unit weight were linearly related to the compost percentage in the tested mixes. An ignition test was carried out by burning the sample in a mue furnace at 550 C [7±9] and the percentage of total volatile solids (TVS) were used as indicators of organic contents according to the equation: %TVS W1 ÿ W2 =W1 100
1
where W1 is the initial dry weight, and W2 is the weight of ash after ignition. The results ranged between 56 and 62% and an average value of 59% was considered to represent the initial organic content of the compost used in the tested blends. 2.1. Saturated hydraulic conductivity Constant-head tests were chosen to evaluate the hydraulic conductivity of each compost±soil mixture. The PVC columns used in the experiments had dimensions of 52 mm in diameter and 600 mm in height. The constant head was achieved using a drainage tube located
Table 1 Selected properties of tested soil Soil properties
Pore ¯uid analysis
Geotechnical Speci®c gravity
2.68
pH Conductivity (Simens)
Consistency limits Liquid limit Plastic limit
NP NP
Concentrations (ppm) Na+ K+
140 8.5
Soil gradation Gravel (wt%) Sand (wt%) Silt (et%) Clay (wt%) Soil texture
35 43 18 4 Silty sand
Mg2+ Ca2+ CaCOÿ 3 HCOÿ 3 ÿ Cl
34 48 520 140 460
Compaction Max. dry density (Mg/m3) Opt. water content (%) Permeability (m/s)
1.79 12 7.185E-06
Mineralogical composition by X-ray analysis Major: quartz, calcite, plagioclase Minor: dolomite, feldspar, kaolinite
8.1 1.18E-3
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Fig. 1. Grain size distribution of the native soil.
Fig. 2. Results of compaction tests for (a) native soil, (b) MSW compost.
near the top of the column to remove any surplus in¯ow. A 200 mm of each compost±soil mixture was compacted in ®ve layers to its maximum dry density and optimum moisture content. The specimens were bounded from the top and bottom by porous stones and ®lter papers to eliminate piping and/or clogging during experiment. Also, to prevent any disturbance and upward movement of the mixture particles, a small pressure of about 19 kn/m2 was applied on the top of the specimen by means of small spring compressed through its connection to the upper cover. This pressure
was perceived to simulate a conservative scenario of low con®ning stress and eventually high permeability. The specimens were allowed to stabilize under the continuous in¯ow of tap water for 3±4 h before monitoring the ¯ow rate with time. Following the stabilization period, the hydraulic conductivity was calculated and reported with time for different compost±soil mixtures. The results are shown in Fig. 4 for 0.30, 0.45, 0.55, and 0.65 compost mixing ratios. It can be seen that the hydraulic conductivities at dierent time increments are decreasing with increasing
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the compost percentage up to 0.55 then started to increase slightly at a mixing ratio of 0.65. Also, most of the reduction in the hydraulic conductivity with time took place within the ®rst 14 h for all mixtures. Hence, the hydraulic conductivity at 14 h was considered as a characteristic value for each mixture. The results indicate that the optimum hydraulic conductivities correspond to mixing ratios between 0.55 and 0.65. To verify
Fig. 3. Variation of optimum moisture contents and corresponding maximum dry unit weights with compost±soil mixing ratios.
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the selection of the optimum mix ratio, two additional experiments were performed. Fig. 5 presents the considered hydraulic conductivity for the original experiment (replicate 1) and the other two experiments (replicates 2 and 3). Replicates 1 and 2 indicate that the optimum mix ratio is 0.55. However, replicate 3 indicates that a further decrease in hydraulic conductivity can be still acheived with mix ratios greater then 0.65. Therefore, a compost mix ratio of 0.60 was seleected as the least variable mixture with a saturated hydraulic conductivity in the range of 1.0 to 3.0E-8 m/s. The corresponding moisture content was 26% as it appears in Fig. 5. Owing to the fact that the minimum hydraulic conductivity is achieved at a moisture content wet of optimum, the obtained hydraulic conductivities are not necessarily the minimum ones since the specimens in the previous experiments were compacted at their optimum moisture contents. Therefore, more experiments were run to ®nd the minimum hydraulic conductivity of the selected mixture (60% compost) after being compacted at water contents higher than the optimum values. Three levels of moisture content wet of optimum were tested: 30, 35, and 40%. Hydraulic conductivities corresponding to these levels and reported after 14 h of permeation were 1.4E-8, 1.1E-8, and 2.3E-8, respectively. Therefore, a moisture content of 35% corresponding to a dry unit weight of 0.94 kg/m3 produced the least hydraulic conductivity. The minimum value was found to stabilize around 4.0 to 6.0E-9 m/s after 2 weeks of permeation. 2.2. Eect of organic decomposition on hydraulic conductivity
Fig. 4. Variation of hydraulic conductivity with time for dierent compost±soil mixing ratios.
Fig. 5. Variability of the hydraulic conductivity with the compost mix ratios for three tested replicates.
The decrease of hydraulic conductivity with time can be generally attributed to soil consolidation that decreases the soil voids. Primary consolidation is signi®cant at large overburden pressures. Another factor that may promote the decrease in hydraulic conductivity with time is the decomposition of organic materials. This factor can be dominant if prevailing environmental conditions, such as temperature, moisture content, and pH, are suitable for the optimum activity of present microorganisms. To explore the eect of organic decomposition on the temporal variation of hydraulic conductivity, two experiments were tested. The ®rst experiment was left at room temperature of 22 C while the second one was placed in an incubator with a constant temperature of 38 C. In order to determine this elevated temperature of 38 C, a simple experiment was conducted during which the barrier layer was covered with a 100 mm layer of native soil and subjected to a continuous source of heat of 46 C at the top surface. The value of top surface temperature was selected in such a way it simulates an extremely high temperature in arid deserts. The steady
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state temperature at the barrier, i.e. at a depth of 100 mm from the surface, was found to be 38 C. The temporal variations of the hydraulic conductivity for the two experiments were recorded and presented as shown in Fig. 6. The ®gure shows similar trends for both cases with lower hydraulic conductivity in case of the incubated column. The hydraulic conductivity reached a value of 110ÿ8 m/s after about 15 days for the column placed at room temperature while it dropped to a value of 410ÿ9 m/s after only 11 days for the incubated column. This indicates that the elevated temperature of the incubator accommodated better environment for the microorganisms that led to faster decomposition of the organic substrates and reduction in the resulting hydraulic conductivity. The incubator's controlled temperature was close to the optimum temperature of biological growth (35 C) for mesophilic-type of bacteria that activated the metabolism process of present microorganisms [10]. As a result, the number of viable cells and biomass is increased which resulted in physical blocking of the soil voids and lowering the mixture hydraulic conductivity. The mixture inside the column acted as a batch system with decreasing amounts of substrates accompanied by emission of carbon dioxide. This ®nding was further substantiated by inspecting the measured organic contents of both columns at the end of the experiment. An average value was found by measuring the organic contents of three samples taken from the top, middle, and bottom of each column. The average organic content was 38.4% for the experiment performed at room temperature and was 31.6% for the incubated experiment. The initial organic content for both columns was 51%. Results also indicate that the hydraulic conductivity curve of the incubated specimen could be divided into two phases. The initial phase steeply decreases within the ®rst 2 days. The later phase decreases gently and tends to ¯atten out at the end of the tested period. The high rate of decomposition in the initial phase can be attributed to the activity of aerobic bacteria and other
aerobic microorganisms that concumes the residual oxygen present in the voids at the early stage of permeation. With passage of time, the permeant begins to strip the oxygen from the mixture pores and the metabolism process gradually transforms from aerobic to anaerobic. This was substantiated by the odorous gases released from the incubated column and the black ¯uids deposited in the euent tube after a day of permeation. The amount and odor of the gases intensi®ed after 2 days suggesting complete anaerobic decomposition. The emitted gas was presumably hydrogen sul®de because of its similarity to rotten eggs odor, while the black ¯uid deposited in the euent tube was possibly ferrous sul®de since the analyzed leachate showed high iron content (Table 2). It should be noted that current experiments were conducted under volume change conditions, i.e. open system. For ®eld application, gas venting systems should be installed for releasing the gas and preventing any potential formation of cracks due to gas buildup. Also, the moisture conditions required to promote biodegradation may not be possible to sustain in arid environment. However, tests were performed under these conditions as a means of simulating extreme biodegradation conditions that may occur over long periods of time. On the other hand, the presence of two phases in Fig. 6 is not related to primary consolidation, as the applied pressure (19 kN/m2) would be much smaller than the compaction induced by appaerent preconsolidation pressure. 2.3. Eect of thermal cycles on hydraulic conductivity A relevant environmental concern in arid areas is the ¯uctuation in temperature due to diurnal and seasonal eects. Experiments were conducted to test the eect of thermal cycles on the hydraulic conductivity. The compost±soil mixture selected earlier (60% compost) was subjected to thermal cycles during the continuous permeation process. The experiments were carried out
Fig. 6. Eect of temperature on the temporal variation of hydraulic conductivity.
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Table 2 Heavy metals contents in mixture leachate Element
Lead
Cadmium
Copper
Nickel
Zinc
Iron
Cobalt
Comcentration (mg/l) using buer solution of pH=5 Comcentration (mg/l) using buer solution of pH=3 Maximum regulatory concentration (mg/l)
0.340 0.708 0.40a 5.00b
0.05 0.013 0.10a 1.00b
0.354 0.687 2.0a
0.205 0.254 0.20a
1.356 4.196 2.0a
2.239 3.062 N.A.c
0.014 0.015 N.A.c
a b c
European Community. US EPA. No available regulatory concentrations.
using two columns, the ®rst had the selected mixture while the second had the selected mixture blended with 15% lime (by dry weight) to inhibit microbial activity and organic decomposition. pH of the leachate from the second column was measured and found to found to vary between 11.7 and 12.2. The two columns were maintained at 22 C (room temperature) for 3 to 4 days and then maintained at 38 C in an incubator for a similar period. This cycle was repeated up to 40 days with continuous monitoring of hydraulic conductivity. The results are shown in Fig. 7 for the two tested columns. The results indicate that the hydraulic conductivity of the lime-amended mixture was approximately constant with time and was insentive to temperatue ¯uctuations. The slight initial increase is attributed to the initial reaction of lime with organic matter in the compost and to the particle agitation in the new structured mix. The hydraulic conductivity had a little perturbation around a value of 1.0E-7 m/s for about 19 days, after which the perturbation center slightly shifted up to a value of 1.3E-7 m/s. Other than this variation, no clear trend was observed during the tested period. The results suggest that thermal cycles have little eect on hydraulic conductivity if the microbial activity is eliminated. It is worth mentioning that clay samples compacted near the
optimum or dry of optimum have been shown to undergo very little change in hydraulic conductivity when subjected to wet±dry cycles [11]. On the other hand, the hydraulic conductivity of the untreated mixture was responding to thermal cycles; it decreased at 38 C and increased at room temperature. The increase in hydraulic conductivity at room temperature is attributed to the drop of biological activity associated with de-¯occulation of the viable cells that increased the pore voids. The increase was maximum in the ®rst cycle (was reached a value of 1.2E-7 m/s) then it decayed at later cycles and completely vanished in the ®fth cycle after about 36 days. It can be also seen that the hydraulic conductivity following the ®fth cycle approached very small values (in the orders of 1.0E-11); that were close to the value obtained in the absence of thermal cycles (Fig. 6). Finally, the plots shown in Fig. 7 show that the hydraulic conductivity was sharply increasing once subjected to the low-temperature phase and remained almost constant throughout the entire phase. This can be explained by the expected sudden occurrence of de¯occulation. On the other hand, the hydraulic conductivity was gradually decreasing when subjected to the high-temperature phase that is mostly related to slow and gradual ¯occulation.
Fig. 7. Eect of thermal cycles on the temporal variation of hydraulic conductivity: I; inside the incubator (high temperature); O; outside the incubator (low temperature).
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2.4. Atterberg limits An important factor to be considered when designing the in®ltration barrier layer is the potential cracking and desiccation. This is of particular concern in arid areas due to the high temperatures and long drying periods in addition to the common dierential settlement caused by overburden pressures and biological and chemical reactions in the buried waste. USA EPA regulation [1] requires the soil of in®ltration barrier layer to have a plasticity index of at least 10% although some soils with a slightly lower plasticity index may be suitable. Boynton and Daniel [12] have shown one order of magnitude increase in the hydraulic conductivity of a compacted clay barrier after desiccation cracks developed at low con®ning pressure. Desiccation cracking requires special attention when designing land®ll cover in arid areas so that drying and/or high temperatures are avoided either during the construction or following the ®nal closure. Having a protection layer of sucient thickness above the barrier layer can eliminate the hazard of desiccation and cracking. The liquid limit, plastic limit, and plasticity index were determined for the selected optimum mixture (60% compost) by adopting the ASTM procedure D4318. These limits were: liquid limit=41%; plastic limit=notdeterminable; and plasticity index=41%. The high liquid limit of the amended soil is attributed to the high organic matter content (51%). The inability to determine plastic limit is attributed to the increased ®brous nature of the organic matter and low clay content in the mixture. Further studies are in progress to evaluate the potential desication and cracking of soil±compost mixes. 2.5. Potential leachability of heavy metal ions from the compost Heavy metal ions present in the municipal solid waste compost may be leached by the percolating ¯uids into the underlying waste. The elevated concentrations of such ions in the leached ¯uids are most likely to be insigni®cant in comparison to the usually high concentrations existing in the refuse. Yet, they were measured in this study as a means of characterizing the leachate. Also, they can be important if thickness of the closure cap was to be large. A conservative analysis was performed by testing the compost samples rather than the selected mixture. The test followed the USEPA Toxicity Charactaristics Leaching Procedure (TCLP) [13] that is a single step batch extraction test. The extracts were prepared by ®rst crushing the compost solids to pass through a 9.5-mm screen, adding a buer solution with a 20:1 liquid to solid ratio, shaking the mix for 20 h, and ®nally ¯itering. All measurements were done in Flame Atomization of GBC 906 Atomic
Absorption Spectrophotometer with double beam. Two types of buered acidic leaching solutions were used; the ®rst had a pH of 5 and the second had a pH of 3. The results of elemental analysis are listed in Table 2 for the two buer solutions along with regulatory maximum concentrations employed by USEPA and the European Community. The results show that the concentrations of all heavy metal ions were reasonably les than current regulations. Also, for the amended soil, heavy metal concentrations will be even less due to the high adsorption capacity of the soils [2]. 3. Conclusions This study has presented an overall evaluation of using a native soil available in arid areas blended with municipal solid waste compost as an in®ltration barrier layer in land®ll closure caps. Regular tests were conducted to determine the optimum moisture content, maximum dry unit weight, hydraulic conductivity, Atterberg limits, and potential leachability of heavy metals from the mix. The compost and native soil were mixed in proportions that yielded minimum hydraulic conductivity. Other tests were conducted to examine the eects of organic decomposition and thermal ¯uctuation upon the hydraulic conductivity. Speci®c ®ndings were as follows: . A mixture of 60% compost and 40% native soil (dry weights) was selected and found to have a hydraulic conductivity of 2.0 to 3.0E-8 m/s at optimum moisture content of 26% and a minimum value of 4.0 to 6.0E-9 m/s at a moisture content of 35%. . Raising the temperature from 22 to 38 C resulted in lowering the permeability by one order of magnitude. This was attributed to the activated metabolism of the present microorganisms that resulted in physical blocking of the soil voids. . The hydraulic conductivity of the selected mixture was responding to the thermal cycles in such a way it decreased at high temperature and increased at room temperature. The maximum increase occurred in the ®rst cycle. . Thermal cycles had little eect on the hydraulic conductivity when the microbial activity was eliminated by adding lime to the mixture. . Addition of compost to the natural soil increased the plastic nature of the amended soil. . Levels of heavy metal ions in the leachate from the compost samples were within acceptable standards The presented study has shown that the municipal solid waste compost investigated can be used in construction of hydraulic barrier layer in land®ll closure
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caps in arid areas. The cost bene®ts of such usage can be potentially doubled if the compost required for the mixture was manufactured at the site. This can be easily achieved via processing the received solid wastes following the common techniques of composting including screening, shredding, and windrow degradation procedures. Yet, special considerations pertinent to the compost material should be given. These include the potential emission of gases from the cover material, major settlement during the active stage of biological degradation, and the increased possibilities of deterioration related to the burrowing animals. Acknowledgements This study was funded by the scienti®c research council of United Arab Emirates University. Acknowledgements are due to Dubai Municipal Composting plant and the Central Lab Unit at UAE University. The authors would also like to extend their thanks and appreciation to Engineers Essam Khalifa, Abd-Elkadir Alkamaly, and Salim Hegazy for their diligent help oered during the experimental work. References [1] US Environmental Protection Agency. Final covers on hazardous waste land®ll and surface impoundments. EPA 530-SW-89-047, 1989.
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[2] Mohamed AMO, Antia HE. Geoenvironmental engineering. Amsterdam: Elsevier, 1998. [3] Mohamed AMO. Evaluation of a multi-layer soil cover system for controlling acid mine drainage. In: Al-Manaseer A, et al., editors. Int. Conf. On Engineering Materials, Ottawa, Canada, 1997. p. 159±172. [4] Morris CE, Stormont JC. Capillary barriers and subtitle D covers: Estimating equivalency. Journal of Environmental Engineering, ASCE 1997;123:1:3±10. [5] Amirkhanian S. Utilization of waste materials in highway industry Ð a literature review. Journal of Waste Management and Technology 1997;24(2). [6] Moo-Young HK, Zimmie TF. Closure of geotechnical properties of paper mill sludges for use in land®ll covers Journal of Geotechnical Engineering. ASCE 1998;124(10):1043. [7] Adani F, Genevini PL, Gasperi F, Zorzi G. Organic matter evolution index (OMEI) as a measure of composting eciency. Compost Science and Utilization 1997;5:2:53±62. [8] Ham RK, Norman MR, Fritschel PR. Chemical characterization of fresh kills land®ll refuse and extracts. Journal of Environmental Engineering 1993;119(6):1176±95. [9] Schwab BS, Ritchie CJ, Kain DJ, Bobrin GC, King LW, Palmisano AG. Characterization of compost from a pilot plant pcale composter utilizing simulated solid waste. Waste Management and Research 1994;12:289±303. [10] Tchobanoglous G, Theisen H, Vigil S. Integrated solid waste management: engineering principles and management issues. McGraw-Hill, Inc., 1993. [11] Johnston K, Hang, MD. Impacts of wet-dry freeze±thaw cycles on the hydraulic conductivity of Glacial Till. In: 45th Canadian Geotechnicial Conference, paper Toronto, 1992. p. 1±10 [12] Boynton SS, Daniel DE. Hydraulic conductivity tests on compacted clay. Journal of Geotech. Eng. ASCE 1985;111:4:465±78. [13] US EPA. Rules and regulations, The Federal Register. TCLP, method 1311. Federal Register 1990;55:61.