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Experimental study of the intrinsic permeability of municipal solid waste
Waste Management 102 (2020) 304–311
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Experimental study of the intrinsic permeability of municipal solid waste Xiao Bing Xu a, William Powrie b, Wen Jie Zhang c,⇑, David Stuart Holmes d, Hui Xu e, Richard Beaven b a
Zhejiang University of Technology, 310023 Hangzhou, China University of Southampton, SO17 1BJ Southampton, UK c Shanghai University, 200444 Shanghai, China d Ecologia Environmental Solutions Ltd, ME9 8BZ Kent, UK e Zhejiang Sci-Tech University, 310018 Hangzhou, China b
a r t i c l e
i n f o
Article history: Received 21 August 2019 Revised 16 October 2019 Accepted 21 October 2019
Keywords: Permeability Waste Liquid Gas Particle size Porosity
a b s t r a c t Changing patterns of municipal solid waste (MSW) management, for example sorting for recycling and mechanical–biological treatment (MBT), will change the nature of the residual material going to landfill and in particular its intrinsic permeability. This is an important parameter, not least because of its influence on gas and leachate flows and the ramifications for gas and leachate management. This paper reports the results of laboratory permeability tests on specimens of MSW recovered from boreholes drilled in a Chinese landfill, under both liquid and gas flow. The test results are used to assess the intrinsic permeability of the waste, and are compared with corresponding data from raw and MBT municipal solid wastes from developed countries in the context of differences in waste composition, porosity and particle size. For the Chinese waste, the intrinsic permeability decreased with depth, while at a given depth the permeability determined with gas flow was consistently larger than that determined with liquid flow. Intrinsic permeabilities determined in liquid flow showed no clear trend of variation with effective particle diameter d10, but reduced with drainable porosity (the drainable porosity, ne, being a more appropriate and useful measure than the total porosity, n). Conversely, intrinsic permeabilities determined in gas flow showed a clear decrease with decreasing d10, but no consistent variation with porosity. These differences are potentially significant in assessing the impacts and interactions between gas and liquid flows; some reasons for them are suggested. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Municipal solid waste (MSW) pretreatment techniques generally combine the mechanical, thermal and biological processes, with the objectives of recovering materials from waste, producing energy and/or minimizing the amount of waste to be landfilled (Komilis et al., 1999). The introduction of regulations to limit the amount of biodegradable waste disposed in landfills (CEC, 1999) has led to an increasing number of MSW pretreatment facilities, such as mechanical–biological treatment (MBT) plant, in EU countries. In China, the proportion of kitchen waste (or food waste) in landfilled MSW has historically been high (generally above ⇑ Corresponding author. E-mail addresses: [email protected] (X.B. Xu), [email protected] (W. Powrie), [email protected] (W.J. Zhang), [email protected] (D.S. Holmes), [email protected] (H. Xu), r.p.beaven@soton. ac.uk (R. Beaven). https://doi.org/10.1016/j.wasman.2019.10.039 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
50% by wet weight) and the proportion of recyclable wastes (e.g. paper, cardboard, metal, glass) low, owing to informal recycling by human scavengers. To improve efficiency in resource use and the disposal of MSW, China has issued a plan requiring 46 cities to carry out mandatory garbage sorting by the end of 2020, guiding residents to sort hazardous waste, kitchen waste and recyclable waste (NDRC and MOHURD, 2017). This will lead to a change from mixed MSW disposal to sorted disposal. Pretreatment facilities will include composting plant for kitchen waste and MBT plant for other waste. As a result, the waste to be landfilled in China will undergo changes in composition and particle size. Leachate and gas control remain major environmental concerns for MSW landfills, particularly as sustainable landfilling technologies such as flushing and aeration become more prevalent. All of these require a good understanding of liquid and gas flow in landfills, and the governing parameter of waste permeability. The intrinsic permeability, ki, reflects the ability of a landfilled waste to
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allow fluids to pass through it. It depends on the properties of the waste, and not the fluid. It may in principle be determined from conductivity tests on waste samples that are 100% saturated with a single-phase fluid (liquid or gas). The hydraulic conductivity of liquid saturated MSW has been investigated in a number of seminal laboratory, pilot and fullscale studies (e.g. Oweis et al., 1990; Chen and Chynoweth, 1995; Powrie and Beaven, 1999; Jang et al., 2002; Jain et al., 2006; Staub et al., 2009; Hossain et al., 2009; Reddy et al., 2009; Stoltz et al., 2010; Wu et al., 2012; Xu et al., 2014; Ke et al., 2017; Miguel et al., 2018; Breitmeyer et al., 2019). Early studies generally indicated a large decrease of saturated hydraulic conductivity ks with increasing effective stress r’v, saturated density qsat, dry unit weight cd, deposit depth or age, and suitable empirical relationships proposed – for example, ks = 2.1 r’v2.71 and ks = 1.15 105 qsat30.14 for raw (crude) MSW from the UK in the large-scale tests reported by Powrie and Beaven (1999). An upper bound relationship ks = 4.64 exp(7.53cd/cw), where cw is the unit weight of water, was suggested by Reddy et al. (2009). Later research focused on the relationship between ks and porosity n or void ratio e. Laboratory tests by Hossain et al. (2009) and Stoltz et al. (2010) indicated that a decrease of n due to compression resulted in a decrease of permeability. In the study by Stoltz et al. (2010), the intrinsic permeability, ki, of shredded fresh MSW from France with different n, was described by an empirical power function ki = C1 nC2, where C1 and C2 are constants. Breitmeyer et al., 2019 suggested ki = 1.5 104 e8.36/(1 + e) based on laboratory and field tests on fresh and decomposed MSW. Hossain et al. (2009) reported that, for specimens of MSW with similar density, ks decreased with degradation. This was attributed to the decreased size of pores (flow channels) associated with a reduction in the measured particle size on degradation. Laboratory tests on shredded Chinese borehole MSW by Xu et al. (2014) showed a decrease in ki with compression and degradation. Ke et al. (2017) developed an empirical equation relating ks to porosity and the degree of degradation based on laboratory tests on synthetic Chinese MSW, and found (like Powrie and Beaven, 1999) that compression had a much greater effect in decreasing ks than degradation. Studies of the permeability of gas saturated (i.e., dry) MSW are comparatively rare. Laboratory tests on sorted bulky waste, incombustible waste and incineration ash from Japan in gas flow were reported by Kallel et al. (2004). The results showed a decrease in ki with decreasing n, and an increase in ki with increasing effective particle diameter d10 (with a best-fit line given by ki = 103 d210) for wastes with 0.387 < n < 0.505. Xu et al. (2014) studied ki of shredded Chinese borehole MSW (deposit age 6.2 years) based on the Kozeny-Carman model (Eq. (3)), and found that the intrinsic permeability coefficient k0 decreased with decreasing porosity caused by compression (k0 = 2 109 n12). The intrinsic permeability depends primarily on the geometric properties of the particles and pores, which are usually represented by the particle size and porosity respectively. The particle size and / or shape are influenced by the waste composition, although currently there is no quantitative or linked understanding of the combined effects of waste composition, porosity, particle size and shape on ki. In this paper, laboratory liquid and gas permeability tests of borehole MSW samples recovered from a landfill in China were conducted. The test results are compared with ki data for MSW and MBT waste in developed countries, focusing on the influences from waste composition, porosity and particle size. The influences from particle shape are also discussed. The results of the study may be used to assess the impact on ki of variations in MSW composition (through subsequent impacts on porosity, particle size and / or shape) associated with changes in waste management and disposal strategies.
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2. Materials and methods 2.1. Materials Samples of MSW were retrieved from different depths from boreholes in the Qizishan landfill, China. Detailed information on the landfill and the sampling process is given by Xu et al. (2014) and Zhang and Lin (2019). Sample diameters were about 96 mm for wastes taken from depths<20 m, and 82 mm for wastes taken from depths > 20 m. In all cases, samples were approximately 200 mm in length. On each sample, a liquid flow permeability test was carried out first. The sample was then dried for 48 h in an oven at a temperature of 60 °C (Zekkos, 2005), to avoid ignition of combustible matter. After carrying out a gas permeability test on the dried material, each sample was sorted manually to determine its composition (Table E1 in E-component) and particle size distribution (Fig.E1 in E-component) according to ASTM D422. The average grain specific gravity of each sample (Table E2 in E-component) was measured using the water displacement method in accordance with the technical code CJJ176-2012 (MOHURD, 2012). Waste porosities (Table E2) were determined from the relevant measured grain specific gravity and dry density (in turn calculated as the dry mass divided by the as sampled volume). 2.2. Test apparatus and procedure The permeability tests were carried out in rigid-walled cylinders of inner diameter 100 mm, as shown in Fig. 1(a) for liquid flow and Fig. 1(b) for gas flow. In both cases, the imposed flow direction was vertical. Waste retrieved from the borehole was placed in a cylinder to form a ~ 180 mm high column, to the target dry density as indicated in Table E2. Porous plates were placed at the bottom and top of the sample. The sample was then saturated with tap water and allowed to drain freely under gravity for 24 h, to reach the field capacity. During this process, the drainable porosity (i.e., the difference between the saturated volumetric water content and the volumetric field capacity) was determined. Then tap water was added to the fully drained column, maintaining a constant head of water of 20 mm above the top of the sample. The effluent volume was recorded at intervals of 5 s until a steady-state outflow was reached. Biogas generation during the liquid permeability test was assumed to be negligible, given the short time required for the test. In the gas permeability tests, the gas pressure gradient was measured using two pressure tappings at a spacing of 140 mm along the direction of flow. Each pressure tapping was connected to a water column manometer with a measurement accuracy of ± 1 mm. The inlet flow rate and pressure were measured using a flowmeter and pressure gauge installed at the top of the cylinder, respectively. 2.3. Analytical methods Prior to the interpretation of experimental data, the models used to analyze ki are described. ki can be determined using Darcy’s law with respect to liquid or gas, as appropriate:
ki ¼
k s lL qL g
ð1Þ
ki ¼
k d lG qG g
ð2Þ
where ki (m2) is the intrinsic permeability; ks and kd (ms1) are the conductivities of the waste in liquid saturated (s) and dry (d) states,
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Flowmeter
20 mm
Inlet port Water
20 mm
Overflow port
Porous plate
180 mm
Sample
Pressure gauge
Cavity
Porous plate
180 mm
Inlet port
Sample
100 mm
100 mm
Outlet
Outlet
Pressure tapping
140 mm
306
Fig. 1. Apparatus for permeability tests: (a) liquid, (b) gas.
respectively; qL and qG (kgm3) are the liquid and gas densities, respectively; lL and lG (kgm1s1) are the liquid and gas viscosities, respectively. At the controlled, standard test temperature of 20 °C, the values of qL, lL and lG are 103 kgm3, 103 kgm1s1 and 1.8 105 kgm1s1, respectively. Based on the Kozeny-Carman model (Kozeny, 1927; Carman, 1939), ki can be expressed as (Bear, 1972):
ki ¼ k0
n3 ð1 nÞ
2
¼
1 cs
2S 2 0
n3 ð1 nÞ2
ð3Þ
where k0 (m2) is termed the intrinsic permeability coefficient; c is an empirical parameter in the Kozeny-Carman model; S0 (m1) is the specific surface area; and s is the tortuosity of the porous medium, which is defined as the average effective length of a streamline (flowline), Le, divided by the corresponding direct length L. For spherical particles in a cubic packing, there is S0 = 6/d, where d (m) is the diameter of the spheres. For soils, the effective particle diameter d10 is widely used to estimate ki, as the smaller particles govern the size of the pore channels controlling fluid flow (Terzaghi et al., 1996). Therefore, S0 in Eq. (3) is considered to be inversely proportional to d10. Tortuosity is usually related to the porosity n of the porous medium (Ghanbarian et al., 2013). In numerical simulations of a granular matrix with fibrous inclusions, Yazdchi et al., (2011) found a linear dependence of s on n. Xu et al., (2014) obtained an intrinsic permeability coefficient k0 = 2 109 n12 in gas flow tests on a shredded Chinese MSW recovered from a borehole 6.2 years after deposition in the landfill. Tortuosity may also be affected by the particle size of the porous media. Therefore, s in Eq. (3) is in itself expected to depend on n and d10. This is reflected in Eq. (4):
ki ¼ C 1 nC2 d10
C3
n3 ð1 nÞ2
where C1,C2 and C3 are fitting parameters.
ð4Þ
For waste, water may be trapped in unconnected pores, e.g. closed-ended containers such as bottles or cans (Beaven et al., 2011). Thus the drainable porosity, ne, might be a more appropriate measure of the interconnected pore space available for liquid flow. Mavko and Nur, (1997) suggest incorporating ‘‘percolation” into the Kozeny-Carman relationship by replacing n by (n nc), where nc is the threshold porosity before which connected flow paths do not exist. Assuming ne = n nc, Eq. (4) can be written in terms of effective porosity ne as Eq. (5). A similar approach would be expected to apply for gas flow.
ki ¼ C 1 ne C 2 d10
C3
ne 3 ð1 ne Þ2
ð5Þ
3. Results and discussions 3.1. Intrinsic permeability determined in liquid flow In this section, the results of liquid permeability tests on Chinese MSW recovered from boreholes in the Qizishan landfill are presented and discussed with reference to data from rigid-walled permeameter tests on fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) and German MBT waste (Pimolthai and Wagner, 2014). The waste compositions are shown in Table E1. The Chinese borehole MSW shows a reduction in the proportion of organic waste (and a corresponding increase in the proportion of inorganic waste) with increasing depth. This is consistent with a greater degree of degradation in the older, deeper layers and the relatively rapid degradation of kitchen waste, which forms the main component of organic waste in fresh Chinese MSW. The kitchen waste quickly becomes indistinguishable, resulting in its classification as ‘‘organic, other”. The Chinese borehole MSW had larger inorganic content and smaller overall organic content than the fresh MSW from the tipping face of a UK landfill tested by Powrie and
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of the intrinsic permeability ki, calculated from the measured or inferred Darcy conductivity ks or kd and the properties of the permeant fluid according to Eqs. (1) and (2). Relationships between the intrinsic permeability determined in liquid flow tests and the dry density (which can be taken as a measure of the degree of compaction of a given waste) is shown in Fig. 2. For both types of MSW, ki decreased with increasing qd – much more significantly in the case of the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) than for the Chinese borehole MSW. The single data point for the German MBT waste (Pimolthai and Wagner, 2014) seems to be close to the trend line for the fresh UK MSW, but the lack of corroborating data and the very different waste compositions preclude any suggestion that the ki-qd relationships are the same. Fig. 3(a) shows that the intrinsic permeability ki determined in liquid flow tests decreases with decreasing porosity n, both for the Chinese borehole MSW and the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000). A single relationship between ki and n for the three different wastes may be expressed as ki = 8 108n18 (R2 = 0.808: the dotted curve in Fig. 3(a)). Fig. 3(b) shows a clear trend of decreasing ki with decreasing ne for both the Chinese borehole MSW and the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000). The dotted curve in Fig. 3(b) represents a fitted trend line, ki = 4 1010ne2.214 (R2 = 0.803). The liquid intrinsic permeability plotted against the effective diameter d10 determined by sieving is shown in Fig. 4. It indicates no clear relationship between ki and d10. As stated by Powrie and Beaven (1999), ki is much more affected by the porosity (Fig. 3). For the Chinese borehole MSW in deep layer and the fresh MSW in UK with similar porosity (n ~ 0.54 to 0.61, ne ~ 0.14 to 0.15), ki showed a slight decrease with decreasing d10 (see the dotted trend line in Fig. 4). However, it should be noted that the relatively large value of d10 for the fresh UK MSW (Fig. 4) might cover up the effect of particle shape. As mentioned above, the fresh UK MSW might contain more 2D particles. To investigate the effects of both porosity and particle size on intrinsic permeability, ki determined in liquid flow tests is plotted against n (ne) and d10 (see Fig. 5). Based on Eq.(4) and Eq.(5), the best-fit curved surface (using Orthogonal Distance Regression of Origin Pro 9.1, OriginLab Corporation, 2013) is plotted in Fig. 5(a) and Fig. 5(b), respectively. It shows that Eq. (4), relating ki to n and d10, does not fit the test data well (ki = 1.17 1011n1.848d0.234 10 n3/(1 n)2, R2 = 0.698; see Fig. 5(a)). Fig. 5(b) shows that a better fit is obtained based on Eq.(5), relating ki to ne and d10 (ki = 5.29 1011ne1.641d0.135 ne3/(1 ne)2, R2 = 0.815). 10
1.E-10
Intrinsic permeability ki (m2 )
Beaven, (1999) and Beaven (2000), of which the main organic constituent was paper and cardboard. The German MBT waste was taken from the composting heaps of a final treatment process prior to landfilling (Pimolthai and Wagner, 2014) and comprised almost entirely organic waste (98%) – principally paper, textiles and wood. Waste composition affects the permeability through its influence on particle size and / or shape. Waste components were categorized according to the size and / or shape by Kolsch (1995) as either 0D (grains), 1D (fibres, sticks or strings), 2D (flat particles with two long dimensions and one short, e.g., plastics, textiles and paper), or 3D (bulky waste, e.g., brick and box). The effect of particle size and/or shape on permeability is discussed below. Key constitutive parameters including dry density, average particle specific gravity, porosity, drainable porosity and field capacity of the three wastes are shown in Table E2. The Chinese borehole MSW showed a decrease in total porosity n and drainable porosity ne with increasing depth, and an increase of field capacity hf with increasing depth. The decrease in drainable porosity ne, which reflects the relatively large pores available for liquid flow under gravity, was more significant than the decrease in total porosity n. The particle specific gravity Gs and the bulk dry density cd also increased with increasing depth. These trends are all consistent with the findings of Powrie and Beaven (1999) and Beaven (2000) for UK wastes subjected to an increase in vertical stress, and result from the compression of the waste (skeleton and potentially individual particles) with increasing vertical stress/depth of burial in a landfill. The German MBT waste (Pimolthai and Wagner, 2014) had the smallest porosity and the largest dry density cd. This was probably a result of the generally smaller particle size (i.e., the absence of particles >10 mm; Fig.E1). Particle size distributions for each waste are shown in Fig.E1. In the case of the Chinese MSW recovered for this study from the Qizishan landfill, the maximum particle size was limited by the diameter of the borehole (82–96 mm). The proportion of indeterminate fines increased with increasing depth, with d10 of shallow, middle and deep MSW of 0.25 mm, 0.12 mm and 0.075 mm respectively. The uniformity coefficients Cu (d60/d10) were 173, 121 and 21, and the curvature coefficients Cc (d230/(d60d10)) were 0.54, 0.17 and 0.73, for shallow, middle and deep MSW respectively, indicating a wide range of particle size with a lack of particles in the middle (1 ~ 10 mm). The German MBT waste (Pimolthai and Wagner, 2014) had more fines but a relatively larger value of d10 (about 0.5 mm) than the Chinese MSW from the Qizishan landfill. The shape of the particle size distribution curve for the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) was similar to that of the fresh Chinese MSW reported by Sun et al. (2008), but the actual sizes were generally a factor of at least 2 greater (e.g. d10 ~ 20 mm, compared with ~10 mm). Particle size determination generally follows the sieving method for soils, with the waste sieved through a number of screens to grade a sample by size. However, for 1D and 2D particles, the notion of a representative diameter becomes problematic as they have at least one dimension extremely small. The same applies to 3D waste components of irregular shape. Furthermore, 2D particles tend to orientate with their shortest dimension vertical during waste deposition and compression, leading to increased flow tortuosity and anisotropy (Qin et al., 2019). The unsuitability of a single representative particle dimension and the effect of 2D particles on fluid flow may be less significant for MBT waste than that for crude unprocessed waste, as a result of shredding or other mechanical processing. Thus the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) might contain more 2D particles classified as large diameter components than either the Chinese borehole MSW or the German MBT waste. To facilitate comparison between tests carried out with liquid and gas flow, the conductivity data obtained are presented in terms
Shallow
Middle
1.E-11
Deep
1.E-12 1.E-13
1.E-14 1.E-15
Borehole MSW-China(This study) Fresh MSW-UK(Powrie and Beaven 1999) MBT-Germany (Pimolthai and Wagner 2014)
1.E-16
1.E-17 0.2
0.4
0.6 Dry density ρd (g/cm3 )
0.8
1
Fig. 2. Relationships between intrinsic permeability determined in liquid flow tests and dry density.
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Intrinsic permeability ki (m2 )
(a) 1.E-10 ki = 8×10 -8n18 R² = 0.808
1.E-11
Deep
1.E-12
Middle
Shallow
1.E-13
1.E-14 1.E-15
Borehole MSW-China (This study) Fresh MSW-UK(Powrie and Beaven 1999) MBT-Germany (Pimolthai and Wagner 2014)
1.E-16 1.E-17
0.3
0.4
0.5 0.6 Porosity n
0.7
0.8
Intrinsic permeability ki (m2)
(b) 1.E-10 ki = 4×10 -10n e2.214 R² = 0.803
1.E-11
Shallow Middle
Deep
1.E-12 1.E-13
1.E-14
Borehole MSW-China (This study)
Fresh MSW-UK(Powrie and Beaven 1999) 1.E-15 0
0.1
0.2 Drainable porosity ne
0.3
0.4
Fig. 3. Relationship between intrinsic permeability determined in liquid flowtests and porosity: (a) total porosity n; (b) drainable porosity ne.
Intrinsic permeability ki (m2 )
1.E-10 Shallow Middle Deep
1.E-11 1.E-12
1.E-13
Fig. 5. Intrinsic permeability determined in liquid flow tests: (a) related to n and d10; (b) related to ne and d10.
Borehole MSW-China(This study) Fresh MSW-UK(Powrie and Beaven 1999) MBT-Germany (Pimolthai and Wagner 2014) Similar porosity (n of 0.54-0.61, ne of 0.14~0.15 )
1.E-14
1.E-15 1.E-16
1.E-17 0
5
10 15 Effective diameter d10 (mm)
20
Fig. 4. Relationship between intrinsic permeability determined in liquid flow tests and effective diameter d10.
3.2. Intrinsic permeability determined in gas flow Intrinsic permeabilities determined in gas flow tests in rigidwalled permeameter on the Chinese MSW recovered from the Qizishan landfill are compared with data for Japanese MBT waste obtained by Kallel et al. (2004) and for UK MBT obtained by Holmes (2013). Waste compositions and densities etc are compared in Table E3 and Table E4 in E-component. For the Chinese borehole MSW, these are similar to those in the liquid permeability tests (Table E1 and Table E2 in E-component), although the samples had been dried and re-packed for the gas flow tests. In the study reported by Kallel et al. (2004), source-separated incombustible (IW1) and bulky wastes (BW1) were shredded and then sieved after ferrous metal recovery, and then tested to deter-
mine the intrinsic permeability in gas flow of the under-sieve fraction destined for landfill. As shown in Table E3, IW1 comprised mainly plastics (35%) and glass/metal (60%). BW1 contained more wood waste and less plastics, glass and metal. The MBT waste (maximum particle size of 9 mm and 10 mm, respectively) used in the gas permeability tests of Holmes (2013) was from a processing facility in the UK. About 62.13% of the waste was unidentifiable and classified as other inorganic waste (Table E3). The remaining fraction of organic waste was very small (<4%). Compared with the MBT waste in Japan, the MBT waste in UK had similar specific gravity but larger porosity and smaller dry density (Table E4). Particle size distributions of the wastes used in gas permeability tests are shown in Fig.E2 in E-component. The Japanese (Kallel et al., 2004) and UK MBT wastes (Holmes, 2013) comprised more fines than the Chinese borehole MSW, with larger values of d10. The UK MBT waste was finer than that from Japan. The values of d10 were 0.3 mm and 0.25 mm for the sub-10 mm and sub-9 mm UK MBT wastes, and 0.7 mm and 0.5 mm for the Japanese IW1 and BW1, respectively. The Chinese borehole MSW might contain more 2D particles classified as large diameter components than both of the MBT wastes. The relationship between the intrinsic permeability determined in gas flow tests and waste dry density is shown in Fig. 6. For the
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Intrinsic permeability ki (m2 )
1.E-09
Borehole MSW-China (This study) MBT IW1-Japan(Kallel et al. 2004) MBT BW1-Japan (Kallel et al. 2004) MBT sub-10mm-UK (Holmes 2013) MBT sub-9mm-UK (Holmes 2013)
1.E-10 Shallow
Middle Deep
1.E-11 0.2
0.4
0.6 Dry density ρd (g/cm3 )
0.8
1
Fig. 6. Relationship between intrinsic permeability determined in gas flow tests and dry density.
Chinese borehole MSW and the UK MBT waste (Holmes, 2013), ki decreased with increasing qd with a similar fitted trend. The values of ki for the Japanese MBT wastes (Kallel et al., 2004) did not conform to the trend line for the other wastes, and showed no clear relationship with qd. The intrinsic permeabilities determined in gas flow tests are plotted against total porosity n in Fig. 7(a). For the Chinese borehole MSW and the UK MBT wastes (Holmes, 2013), ki followed a similar decreasing trend with decreasing n. Values of ki determined for the Japanese MBT waste (Kallel et al., 2004) did not follow the
Intrinsic permeability ki (m2 )
(a) 1.E-09
Borehole MSW-China (This study) MBT IW1-Japan (Kallel et al. 2004) MBT BW1-Japan (Kallel et al. 2004) MBT sub-10mm-UK(Holmes 2013) MBT sub-9mm-UK(Holmes 2013)
1.E-10
Shallow Middle
Deep
trend line for the other wastes, and showed no clear relationship with n. Data of ki from gas flow tests on Chinese borehole MSW, based on the assumption ne = n - nc, are plotted against ne in Fig. 7(b), and show a decrease with decreasing ne. The relationship between intrinsic permeability determined in gas flow tests and effective diameter d10 is shown in Fig. 8. There is a trend of decreasing ki with decreasing d10. For the middle layer Chinese borehole MSW and the sub-9 mm UK MBT waste (Holmes, 2013), which have similar porosity (n of 0.67–0.72), the decrease in ki with decreasing d10 is slight. Kallel et al. (2004) also reported a decrease of ki determined in gas flow tests with decreasing d10 for different wastes with n of 0.30 ~ 0.50. Feng et al. (2017) reviewed the data of Kallel et al. (2004) and Xu et al. (2014) and suggested ki = bd210, where the model parameter b is dependent on n. They also suggested improving the model of ki, as d10 is not sufficient to reflect the influences from particle size distribution. Although the Japanese MBT waste had the smallest porosity, it had the largest value of ki (Fig. 7(a)) – perhaps as a result of its relatively large value of d10 (Fig.E2 in E-component). The intrinsic permeability determined in gas flow tests is apparently more sensitive to d10 than the intrinsic permeability determined in liquid flow. This needs further study. To investigate the effects of both porosity and particle size on intrinsic permeability, ki is plotted against n and d10 in Fig. 9. The best-fit curved surface (determined using Orthogonal Distance Regression of Origin Pro 9.1, OriginLab Corporation 2013) based on Eq.(4) (ki = 5 1012n7.646d0.900 n3/(1-n)2, R2 = 0.826) is also 10 shown. However, the indicated decrease in ki with increasing n is considered to be an unrealistic artefact of the influence of the data for the Japanese MBT waste (Kallel et al., 2004), which are inconsistent with the results from the other waste types (Fig. 7(a)). The increase in ki with n (ne) determined in liquid flow is apparently more linear than that determined in gas flow. ki shows a clear increase with d10 when determined in gas flow, but no obvious dependence when determined in liquid flow. These apparent differences might be a result of one or all of (i) the use of d10 to quantify the effect of particle size; in reality, the particle size distribution, particle shape and orientation are likely to have a significant influence; (ii) differences in test conditions (e.g., treatment of specimen, and processing of porosity, particle size distribution and permeability tests) in the various studies; (iii) different flow behaviours in waste between liquid and gas.
1.E-11 0.4
0.5
0.6 Porosity n
0.7
0.8
(b) 1.E-09
1.E-09
Borehole MSW-China (This study) MBT IW1-Japan (Kallel et al. 2004) MBT BW1-Japan (Kallel et al. 2004) MBT sub-10mm-UK (Holmes 2013) MBT sub-9mm-UK (Holmes 2013) Porosity of 0.67~0.72
Intrinsic permeability ki (m2)
Borehole MSW-China (This study) Intrinsic permeability ki (m2 )
In an attempt to reduce the degree of variability in test conditions and other parameters, the intrinsic permeabilities
1.E-10
1.E-10
Shallow Middle Deep
1.E-11
Shallow Middle Deep
1.E-11
0.1
0.2 0.3 Drainable porosity ne
0.4
Fig. 7. Relationship between intrinsic permeability determined in gas flow tests and porosity: (a) total porosity n, (b) drainable porosity ne.
0
0.1
0.2
0.3 0.4 0.5 0.6 Effective diameter d10 (mm)
0.7
0.8
Fig. 8. Relationship between intrinsic permeability determined in gas flow tests and effective diameter d10.
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less degradation, and hence causes a decrease of porosity. Therefore, the Chinese landfilled waste may experience a decrease of ki (smaller than 4 1012 m2 which is the value of ki for current Chinese waste in deep layers) in the future due to the decrease of porosity and particle size. However, it still needs further study. 4. Conclusions Laboratory liquid and gas permeability tests have been carried out on samples of MSW recovered from a landfill in China, and the results compared with data reported in the literature to assess the effects of waste porosity and particle size and the nature of the permeant (liquid or gas) on the inferred intrinsic permeability, ki. The main conclusions are as follows:
Fig. 9. Intrinsic permeability determined in gas flow tests: related to n and d10.
determined in liquid and gas flow will now be compared for specimens of waste from the same depth of the Qizishan landfill. The intrinsic permeabilities are plotted against porosity in Fig. 10. For specimens from the same depth, the value of ki obtained from gas flow tests is consistently greater than that obtained from liquid flow tests. This is in agreement with the findings of Stoltz et al. (2010) and Xu et al. (2014). Possible reasons include: (i) a decreased pore space available for liquid flow due to a lack of liquid saturation (i.e., gas trapped in waste) (Stoltz et al. 2010; Xu et al. 2014) (ii) a decreased pore space available for liquid flow due to the sorption of water in unsaturated pores (iii) enhancement of gas flow rates due to ‘‘gas slippage”(i.e., the Klinkenberg effect, which manifests as an additional flux due to collisions between gas molecules and the pore walls: Klinkenberg, 1941) (iv) drying and re-packing of the waste for the gas flow tests affecting the geometric properties of both the solid particles and the pores. With the development of sorted disposal (i.e., composting plant for kitchen waste, MBT and incineration plants for other waste) in China, the waste to be landfilled may contain much more fine (0D) inorganic particles and less 1D, 2D and 3D particles in the future. It tends to make the waste easier for a good compaction and have
1. For the Chinese MSW, both liquid and gas intrinsic permeability decreased with increasing depth, due mainly to the smaller porosity and more fine particles in deeper layer. For the Chinese borehole MSW drilled from the same depth, the intrinsic permeability determined in gas flow was larger than that determined in liquid flow. 2. The intrinsic permeability determined in liquid flow tests shows no clear relationship with effective particle diameter d10. It is however considerably affected by the porosity – especially the drainable porosity, ne, which gives a better indication of the relative volume of pores available for liquid flow. The intrinsic permeability determined in gas flow tests shows a clear decrease with decreasing d10, but no consistent variation with porosity. 3. The different behaviours in ki in relation to n(ne) and d10 between liquid and gas might have been due to: (i) the use of d10 (rather than a more complex reflection of particle size distribution and shape) to quantify the effect of particle size, (ii) the different test conditions involved, and (iii) the different behaviours between liquid and gas flow in waste. 4. The intrinsic permeability of Chinese landfilled MSW is considered to decrease with the development of sorted disposal. However, it still needs further study.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement
Intrinsic permeability ki (m2)
1.E-09
The work was supported by the National Natural Science Foundation of China (Grant Nos. 51708508 and 41772300), the Zhejiang Provincial Natural Science Foundation of China (Grant Nos. LY17E080021) and the fund from China Scholarship Council.
Borehole MSW-China-Gas-n (This study) Borehole MSW-China-Liquid -n (This study) Borehole MSW-China-Gas-ne (This study) Borehole MSW-China-Lqiud -ne (This study)
1.E-10
Appendix A. Supplementary material Shallow
1.E-11
Shallow
Middle Deep
Middle Deep
Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.10.039. References
1.E-12 0.1
0.2
0.3 0.4 0.5 0.6 Porosity n or Drainable porosity ne
0.7
0.8
Fig. 10. Comparison between intrinsic permeabilities determined in gas and liquid flow tests.
Bear, J., 1972. Dynamics of fluids in porous media. American Elsevier Pub.Co.. Beaven, R.P., 2000. The hydrogeological and geotechnical properties of household waste in relation to sustainable landfilling. University of London, UK. PhD dissertation. Beaven, R.P., Powrie, W., Zardava, K., 2011. Hydraulic properties of MSW. In: Zekkos, D. (Ed.), Chapter 1 in Geotechnical characterization, field measurement and
X.B. Xu et al. / Waste Management 102 (2020) 304–311 laboratory testing of municipal solid waste. American Society of Civil Engineers Special Technical Publication ASCE, Reston, VA. Breitmeyer, R.J., Benson, C.H., Edil, T.B., 2019. Effects of compression and decomposition on saturated hydraulic conductivity of municipal solid waste in bioreactor landfills. J. Geotech. Geoenviron. Eng. 145 (4), 4019011. Chen, T.H., Chynoweth, D.P., 1995. Hydraulic conductivity of compacted municipal solid waste. Bioresour. Technol. 51 (2–3), 205–212. Carman, P.C., 1939. Permeability of saturated sands, soils and clays. J. Agri. Sci. 29 (2), 262–273. CEC (Council of the European Union), 1999. Directive 1999/31/EC on the landfill of waste. Official Journal of the European Communities, L182, 1-19. Feng, S., Zheng, Q., Chen, H.X., 2017. Unsaturated flow parameters of municipal solid waste. Waste Manage. 63, 107–121. Holmes, D.S., 2013. Air flow in waste materials and the quantification of the transfer and transport of the water vapour component. University of Southampton, UK. PhD dissertation. Hossain, M.S., Penmethsa, K.K., Hoyos, L., 2009. Permeability of municipal solid waste in bioreactor landfill with degradation. Geotech. Geolog. Eng. 27 (1), 43–51. Jain, P., Powell, J., Townsend, T.G., Reinhart, D.R., 2006. Estimating the hydraulic conductivity of landfilled municipal solid waste using the borehole permeameter test. J. Environ. Eng. 132 (6), 645–652. Jang, Y.S., Kim, Y.W., Lee, S.I., 2002. Hydraulic properties and leachate level analysis of Kimpo Metropolitan Landfill. Korea. Waste Manage. 22 (3), 261–267. Kallel, A., Tanaka, N., Matsuto, T., 2004. Gas permeability and tortuosity for packed layers of processed municipal solid wastes and incinerator residue. Waste Manage. Res. 22, 186–194. Ke, H., Hu, J., Xu, X.B., Wang, W.F., Chen, Y.M., Zhan, L.T., 2017. Evolution of saturated hydraulic conductivity with compression and degradation for municipal solid waste. Waste Manage. 65, 63–74. Klinkenberg, L.J., 1941. The permeability of porous media to liquids and gases. In: Drilling and production practice. American Petroleum Institute, pp. 200–213. Kolsch, F., 1995. Material values for some mechanical properties of domestic waste. Fifth International Waste Management and Landfill Symposium Cagliari, Italy. Komilis, D.P., Ham, R.K., Stegmann, R., 1999. The effect of municipal solid waste pretreatment on landfill behavior: a literature review. Waste Manage. Res. 17, 10–19. Kozeny, J., 1927. Ueberkapillare Leitung des Wassersim Boden. Wien, Akad. Wiss. 136 (2a), 271. Ghanbarian, B., Hunt, A.G., Ewing, R.P., Sahimi, M., 2013. Tortuosity in porous media: a critical review. Soil Sci. Soc. Am. J. 77 (5), 1461–1477. Mavko, G., Nur, A., 1997. The effect of a percolation threshold in the Kozeny-Carman relation. Geophysics 62, 1480–1482. Miguel, M.G., Mortatti, B.C., Filho, J.L.P., Pereira, S.Y., 2018. Saturated hydraulic conductivity of municipal solid waste considering the influence of biodegradation. J. Environ. Eng. 144 (9), 04018080.
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MOHURD (Ministry of Housing and Urban-Rural Development of the People’s Republic of China), Technical code for geotechnical engineering of municipal solid waste sanitary landfill - CJJ176-2012. China Architecture & Building Press, Beijing, 2012. NDRC (National Development and Reform Commission), MOHURD (Ministry of Housing and Urban-Rural Development). The Implementation Plan of Garbage Sorting. 2017. OriginLab Corporation. Origin Pro 9.1. 2013 Oweis, I.S., Smith, D.A., Ellwood, R.B., Greene, D.S., 1990. Hydraulic characteristics of municipal refuse. J. Geotech. Eng. 116 (4), 539–553. Pimolthai, P., Wagner, J.-F., 2014. Soil mechanical properties of MBT waste from Luxembourg, Germany and Thailand. Songklanakarin J. Sci. Technol. 36 (6), 701–709. Powrie, W., Beaven, R.P., 1999. Hydraulic properties of household waste and implications for landfills. Proc. Inst. Civ. Eng. Geotech. Eng. 137 (4), 235–247. Qin, R., Ke, H., Hu, J., Lan, J.W., 2019. Analysis of particle composition and structure of MSW based on permeability. China Environ. Sci. 39 (1), 203–209 (in Chinese). Reddy, K.R., Hettiarachchi, H., Parakalla, N., Gangathulasi, J., Bogner, J., Lagier, T., 2009. Hydraulic conductivity of MSW in landfills. J. Environ. Eng. 135 (8), 677– 683. Staub, M., Galietti, B., Oxarango, L., Khire, M.V., Gourc, J.P., 2009. Porosity and hydraulic conductivity of MSW using laboratory-scale tests. Proceedings of 3rd International Workshop ‘‘Hydro-Physico-Mechanics of Landfills”, Germany. Stoltz, G., Gour, J.P., Oxarango, L., 2010. Liquid and gas permeability of unsaturated municipal solid waste under compression. J. Contam. Hydrol. 118 (1–2), 27–42. Sun, X., Xu, D., Li, X., Li, B., 2008. Analysis of composition and caloric value of municipal solid waste in Shanghai. J. Tongji Univ. (Natural Sci.) 36 (3), 356–360. Terzaghi, K., Peck, R.B., Mesri, G., 1996. Soil mechanics in engineering practice. JohnWiley & Sons, USA. Wu, H., Chen, T., Wang, H., Lu, W., 2012. Field air permeability and hydraulic conductivity of landfilled municipal solid waste in China. J. Environ. Manage. 98, 15–22. Xu, X.B., Zhan, T.L.T., Chen, Y.M., Beaven, R.P., 2014. Intrinsic and relative permeabilities of shredded municipal solid wastes from the Qizishan landfill. China. Canadian Geotech. J. 51 (11), 1243–1252. Yazdchi, K., Srivastava, S., Luding, S., 2011. On the validity of the Carman-Kozeny equation in random fibrous media. Proceedings of the International Conference on Particle-based Methods – Fundamentals and Applications. ECCOMAS. Zekkos, D.P., 2005. Evaluation of static and dynamic properties of municipal solid waste. University of California, Berkeley, USA. PhD dissertation. Zhang, W.-J., Lin, M.-F., 2019. Evaluating the dual porosity of landfilled municipal solid waste. Environ. Sci. Pollut. Res. 26 (12), 12080–12088.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Experimental study of the intrinsic permeability of municipal solid waste Xiao Bing Xu a, William Powrie b, Wen Jie Zhang c,⇑, David Stuart Holmes d, Hui Xu e, Richard Beaven b a
Zhejiang University of Technology, 310023 Hangzhou, China University of Southampton, SO17 1BJ Southampton, UK c Shanghai University, 200444 Shanghai, China d Ecologia Environmental Solutions Ltd, ME9 8BZ Kent, UK e Zhejiang Sci-Tech University, 310018 Hangzhou, China b
a r t i c l e
i n f o
Article history: Received 21 August 2019 Revised 16 October 2019 Accepted 21 October 2019
Keywords: Permeability Waste Liquid Gas Particle size Porosity
a b s t r a c t Changing patterns of municipal solid waste (MSW) management, for example sorting for recycling and mechanical–biological treatment (MBT), will change the nature of the residual material going to landfill and in particular its intrinsic permeability. This is an important parameter, not least because of its influence on gas and leachate flows and the ramifications for gas and leachate management. This paper reports the results of laboratory permeability tests on specimens of MSW recovered from boreholes drilled in a Chinese landfill, under both liquid and gas flow. The test results are used to assess the intrinsic permeability of the waste, and are compared with corresponding data from raw and MBT municipal solid wastes from developed countries in the context of differences in waste composition, porosity and particle size. For the Chinese waste, the intrinsic permeability decreased with depth, while at a given depth the permeability determined with gas flow was consistently larger than that determined with liquid flow. Intrinsic permeabilities determined in liquid flow showed no clear trend of variation with effective particle diameter d10, but reduced with drainable porosity (the drainable porosity, ne, being a more appropriate and useful measure than the total porosity, n). Conversely, intrinsic permeabilities determined in gas flow showed a clear decrease with decreasing d10, but no consistent variation with porosity. These differences are potentially significant in assessing the impacts and interactions between gas and liquid flows; some reasons for them are suggested. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Municipal solid waste (MSW) pretreatment techniques generally combine the mechanical, thermal and biological processes, with the objectives of recovering materials from waste, producing energy and/or minimizing the amount of waste to be landfilled (Komilis et al., 1999). The introduction of regulations to limit the amount of biodegradable waste disposed in landfills (CEC, 1999) has led to an increasing number of MSW pretreatment facilities, such as mechanical–biological treatment (MBT) plant, in EU countries. In China, the proportion of kitchen waste (or food waste) in landfilled MSW has historically been high (generally above ⇑ Corresponding author. E-mail addresses: [email protected] (X.B. Xu), [email protected] (W. Powrie), [email protected] (W.J. Zhang), [email protected] (D.S. Holmes), [email protected] (H. Xu), r.p.beaven@soton. ac.uk (R. Beaven). https://doi.org/10.1016/j.wasman.2019.10.039 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
50% by wet weight) and the proportion of recyclable wastes (e.g. paper, cardboard, metal, glass) low, owing to informal recycling by human scavengers. To improve efficiency in resource use and the disposal of MSW, China has issued a plan requiring 46 cities to carry out mandatory garbage sorting by the end of 2020, guiding residents to sort hazardous waste, kitchen waste and recyclable waste (NDRC and MOHURD, 2017). This will lead to a change from mixed MSW disposal to sorted disposal. Pretreatment facilities will include composting plant for kitchen waste and MBT plant for other waste. As a result, the waste to be landfilled in China will undergo changes in composition and particle size. Leachate and gas control remain major environmental concerns for MSW landfills, particularly as sustainable landfilling technologies such as flushing and aeration become more prevalent. All of these require a good understanding of liquid and gas flow in landfills, and the governing parameter of waste permeability. The intrinsic permeability, ki, reflects the ability of a landfilled waste to
X.B. Xu et al. / Waste Management 102 (2020) 304–311
allow fluids to pass through it. It depends on the properties of the waste, and not the fluid. It may in principle be determined from conductivity tests on waste samples that are 100% saturated with a single-phase fluid (liquid or gas). The hydraulic conductivity of liquid saturated MSW has been investigated in a number of seminal laboratory, pilot and fullscale studies (e.g. Oweis et al., 1990; Chen and Chynoweth, 1995; Powrie and Beaven, 1999; Jang et al., 2002; Jain et al., 2006; Staub et al., 2009; Hossain et al., 2009; Reddy et al., 2009; Stoltz et al., 2010; Wu et al., 2012; Xu et al., 2014; Ke et al., 2017; Miguel et al., 2018; Breitmeyer et al., 2019). Early studies generally indicated a large decrease of saturated hydraulic conductivity ks with increasing effective stress r’v, saturated density qsat, dry unit weight cd, deposit depth or age, and suitable empirical relationships proposed – for example, ks = 2.1 r’v2.71 and ks = 1.15 105 qsat30.14 for raw (crude) MSW from the UK in the large-scale tests reported by Powrie and Beaven (1999). An upper bound relationship ks = 4.64 exp(7.53cd/cw), where cw is the unit weight of water, was suggested by Reddy et al. (2009). Later research focused on the relationship between ks and porosity n or void ratio e. Laboratory tests by Hossain et al. (2009) and Stoltz et al. (2010) indicated that a decrease of n due to compression resulted in a decrease of permeability. In the study by Stoltz et al. (2010), the intrinsic permeability, ki, of shredded fresh MSW from France with different n, was described by an empirical power function ki = C1 nC2, where C1 and C2 are constants. Breitmeyer et al., 2019 suggested ki = 1.5 104 e8.36/(1 + e) based on laboratory and field tests on fresh and decomposed MSW. Hossain et al. (2009) reported that, for specimens of MSW with similar density, ks decreased with degradation. This was attributed to the decreased size of pores (flow channels) associated with a reduction in the measured particle size on degradation. Laboratory tests on shredded Chinese borehole MSW by Xu et al. (2014) showed a decrease in ki with compression and degradation. Ke et al. (2017) developed an empirical equation relating ks to porosity and the degree of degradation based on laboratory tests on synthetic Chinese MSW, and found (like Powrie and Beaven, 1999) that compression had a much greater effect in decreasing ks than degradation. Studies of the permeability of gas saturated (i.e., dry) MSW are comparatively rare. Laboratory tests on sorted bulky waste, incombustible waste and incineration ash from Japan in gas flow were reported by Kallel et al. (2004). The results showed a decrease in ki with decreasing n, and an increase in ki with increasing effective particle diameter d10 (with a best-fit line given by ki = 103 d210) for wastes with 0.387 < n < 0.505. Xu et al. (2014) studied ki of shredded Chinese borehole MSW (deposit age 6.2 years) based on the Kozeny-Carman model (Eq. (3)), and found that the intrinsic permeability coefficient k0 decreased with decreasing porosity caused by compression (k0 = 2 109 n12). The intrinsic permeability depends primarily on the geometric properties of the particles and pores, which are usually represented by the particle size and porosity respectively. The particle size and / or shape are influenced by the waste composition, although currently there is no quantitative or linked understanding of the combined effects of waste composition, porosity, particle size and shape on ki. In this paper, laboratory liquid and gas permeability tests of borehole MSW samples recovered from a landfill in China were conducted. The test results are compared with ki data for MSW and MBT waste in developed countries, focusing on the influences from waste composition, porosity and particle size. The influences from particle shape are also discussed. The results of the study may be used to assess the impact on ki of variations in MSW composition (through subsequent impacts on porosity, particle size and / or shape) associated with changes in waste management and disposal strategies.
305
2. Materials and methods 2.1. Materials Samples of MSW were retrieved from different depths from boreholes in the Qizishan landfill, China. Detailed information on the landfill and the sampling process is given by Xu et al. (2014) and Zhang and Lin (2019). Sample diameters were about 96 mm for wastes taken from depths<20 m, and 82 mm for wastes taken from depths > 20 m. In all cases, samples were approximately 200 mm in length. On each sample, a liquid flow permeability test was carried out first. The sample was then dried for 48 h in an oven at a temperature of 60 °C (Zekkos, 2005), to avoid ignition of combustible matter. After carrying out a gas permeability test on the dried material, each sample was sorted manually to determine its composition (Table E1 in E-component) and particle size distribution (Fig.E1 in E-component) according to ASTM D422. The average grain specific gravity of each sample (Table E2 in E-component) was measured using the water displacement method in accordance with the technical code CJJ176-2012 (MOHURD, 2012). Waste porosities (Table E2) were determined from the relevant measured grain specific gravity and dry density (in turn calculated as the dry mass divided by the as sampled volume). 2.2. Test apparatus and procedure The permeability tests were carried out in rigid-walled cylinders of inner diameter 100 mm, as shown in Fig. 1(a) for liquid flow and Fig. 1(b) for gas flow. In both cases, the imposed flow direction was vertical. Waste retrieved from the borehole was placed in a cylinder to form a ~ 180 mm high column, to the target dry density as indicated in Table E2. Porous plates were placed at the bottom and top of the sample. The sample was then saturated with tap water and allowed to drain freely under gravity for 24 h, to reach the field capacity. During this process, the drainable porosity (i.e., the difference between the saturated volumetric water content and the volumetric field capacity) was determined. Then tap water was added to the fully drained column, maintaining a constant head of water of 20 mm above the top of the sample. The effluent volume was recorded at intervals of 5 s until a steady-state outflow was reached. Biogas generation during the liquid permeability test was assumed to be negligible, given the short time required for the test. In the gas permeability tests, the gas pressure gradient was measured using two pressure tappings at a spacing of 140 mm along the direction of flow. Each pressure tapping was connected to a water column manometer with a measurement accuracy of ± 1 mm. The inlet flow rate and pressure were measured using a flowmeter and pressure gauge installed at the top of the cylinder, respectively. 2.3. Analytical methods Prior to the interpretation of experimental data, the models used to analyze ki are described. ki can be determined using Darcy’s law with respect to liquid or gas, as appropriate:
ki ¼
k s lL qL g
ð1Þ
ki ¼
k d lG qG g
ð2Þ
where ki (m2) is the intrinsic permeability; ks and kd (ms1) are the conductivities of the waste in liquid saturated (s) and dry (d) states,
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Flowmeter
20 mm
Inlet port Water
20 mm
Overflow port
Porous plate
180 mm
Sample
Pressure gauge
Cavity
Porous plate
180 mm
Inlet port
Sample
100 mm
100 mm
Outlet
Outlet
Pressure tapping
140 mm
306
Fig. 1. Apparatus for permeability tests: (a) liquid, (b) gas.
respectively; qL and qG (kgm3) are the liquid and gas densities, respectively; lL and lG (kgm1s1) are the liquid and gas viscosities, respectively. At the controlled, standard test temperature of 20 °C, the values of qL, lL and lG are 103 kgm3, 103 kgm1s1 and 1.8 105 kgm1s1, respectively. Based on the Kozeny-Carman model (Kozeny, 1927; Carman, 1939), ki can be expressed as (Bear, 1972):
ki ¼ k0
n3 ð1 nÞ
2
¼
1 cs
2S 2 0
n3 ð1 nÞ2
ð3Þ
where k0 (m2) is termed the intrinsic permeability coefficient; c is an empirical parameter in the Kozeny-Carman model; S0 (m1) is the specific surface area; and s is the tortuosity of the porous medium, which is defined as the average effective length of a streamline (flowline), Le, divided by the corresponding direct length L. For spherical particles in a cubic packing, there is S0 = 6/d, where d (m) is the diameter of the spheres. For soils, the effective particle diameter d10 is widely used to estimate ki, as the smaller particles govern the size of the pore channels controlling fluid flow (Terzaghi et al., 1996). Therefore, S0 in Eq. (3) is considered to be inversely proportional to d10. Tortuosity is usually related to the porosity n of the porous medium (Ghanbarian et al., 2013). In numerical simulations of a granular matrix with fibrous inclusions, Yazdchi et al., (2011) found a linear dependence of s on n. Xu et al., (2014) obtained an intrinsic permeability coefficient k0 = 2 109 n12 in gas flow tests on a shredded Chinese MSW recovered from a borehole 6.2 years after deposition in the landfill. Tortuosity may also be affected by the particle size of the porous media. Therefore, s in Eq. (3) is in itself expected to depend on n and d10. This is reflected in Eq. (4):
ki ¼ C 1 nC2 d10
C3
n3 ð1 nÞ2
where C1,C2 and C3 are fitting parameters.
ð4Þ
For waste, water may be trapped in unconnected pores, e.g. closed-ended containers such as bottles or cans (Beaven et al., 2011). Thus the drainable porosity, ne, might be a more appropriate measure of the interconnected pore space available for liquid flow. Mavko and Nur, (1997) suggest incorporating ‘‘percolation” into the Kozeny-Carman relationship by replacing n by (n nc), where nc is the threshold porosity before which connected flow paths do not exist. Assuming ne = n nc, Eq. (4) can be written in terms of effective porosity ne as Eq. (5). A similar approach would be expected to apply for gas flow.
ki ¼ C 1 ne C 2 d10
C3
ne 3 ð1 ne Þ2
ð5Þ
3. Results and discussions 3.1. Intrinsic permeability determined in liquid flow In this section, the results of liquid permeability tests on Chinese MSW recovered from boreholes in the Qizishan landfill are presented and discussed with reference to data from rigid-walled permeameter tests on fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) and German MBT waste (Pimolthai and Wagner, 2014). The waste compositions are shown in Table E1. The Chinese borehole MSW shows a reduction in the proportion of organic waste (and a corresponding increase in the proportion of inorganic waste) with increasing depth. This is consistent with a greater degree of degradation in the older, deeper layers and the relatively rapid degradation of kitchen waste, which forms the main component of organic waste in fresh Chinese MSW. The kitchen waste quickly becomes indistinguishable, resulting in its classification as ‘‘organic, other”. The Chinese borehole MSW had larger inorganic content and smaller overall organic content than the fresh MSW from the tipping face of a UK landfill tested by Powrie and
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of the intrinsic permeability ki, calculated from the measured or inferred Darcy conductivity ks or kd and the properties of the permeant fluid according to Eqs. (1) and (2). Relationships between the intrinsic permeability determined in liquid flow tests and the dry density (which can be taken as a measure of the degree of compaction of a given waste) is shown in Fig. 2. For both types of MSW, ki decreased with increasing qd – much more significantly in the case of the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) than for the Chinese borehole MSW. The single data point for the German MBT waste (Pimolthai and Wagner, 2014) seems to be close to the trend line for the fresh UK MSW, but the lack of corroborating data and the very different waste compositions preclude any suggestion that the ki-qd relationships are the same. Fig. 3(a) shows that the intrinsic permeability ki determined in liquid flow tests decreases with decreasing porosity n, both for the Chinese borehole MSW and the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000). A single relationship between ki and n for the three different wastes may be expressed as ki = 8 108n18 (R2 = 0.808: the dotted curve in Fig. 3(a)). Fig. 3(b) shows a clear trend of decreasing ki with decreasing ne for both the Chinese borehole MSW and the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000). The dotted curve in Fig. 3(b) represents a fitted trend line, ki = 4 1010ne2.214 (R2 = 0.803). The liquid intrinsic permeability plotted against the effective diameter d10 determined by sieving is shown in Fig. 4. It indicates no clear relationship between ki and d10. As stated by Powrie and Beaven (1999), ki is much more affected by the porosity (Fig. 3). For the Chinese borehole MSW in deep layer and the fresh MSW in UK with similar porosity (n ~ 0.54 to 0.61, ne ~ 0.14 to 0.15), ki showed a slight decrease with decreasing d10 (see the dotted trend line in Fig. 4). However, it should be noted that the relatively large value of d10 for the fresh UK MSW (Fig. 4) might cover up the effect of particle shape. As mentioned above, the fresh UK MSW might contain more 2D particles. To investigate the effects of both porosity and particle size on intrinsic permeability, ki determined in liquid flow tests is plotted against n (ne) and d10 (see Fig. 5). Based on Eq.(4) and Eq.(5), the best-fit curved surface (using Orthogonal Distance Regression of Origin Pro 9.1, OriginLab Corporation, 2013) is plotted in Fig. 5(a) and Fig. 5(b), respectively. It shows that Eq. (4), relating ki to n and d10, does not fit the test data well (ki = 1.17 1011n1.848d0.234 10 n3/(1 n)2, R2 = 0.698; see Fig. 5(a)). Fig. 5(b) shows that a better fit is obtained based on Eq.(5), relating ki to ne and d10 (ki = 5.29 1011ne1.641d0.135 ne3/(1 ne)2, R2 = 0.815). 10
1.E-10
Intrinsic permeability ki (m2 )
Beaven, (1999) and Beaven (2000), of which the main organic constituent was paper and cardboard. The German MBT waste was taken from the composting heaps of a final treatment process prior to landfilling (Pimolthai and Wagner, 2014) and comprised almost entirely organic waste (98%) – principally paper, textiles and wood. Waste composition affects the permeability through its influence on particle size and / or shape. Waste components were categorized according to the size and / or shape by Kolsch (1995) as either 0D (grains), 1D (fibres, sticks or strings), 2D (flat particles with two long dimensions and one short, e.g., plastics, textiles and paper), or 3D (bulky waste, e.g., brick and box). The effect of particle size and/or shape on permeability is discussed below. Key constitutive parameters including dry density, average particle specific gravity, porosity, drainable porosity and field capacity of the three wastes are shown in Table E2. The Chinese borehole MSW showed a decrease in total porosity n and drainable porosity ne with increasing depth, and an increase of field capacity hf with increasing depth. The decrease in drainable porosity ne, which reflects the relatively large pores available for liquid flow under gravity, was more significant than the decrease in total porosity n. The particle specific gravity Gs and the bulk dry density cd also increased with increasing depth. These trends are all consistent with the findings of Powrie and Beaven (1999) and Beaven (2000) for UK wastes subjected to an increase in vertical stress, and result from the compression of the waste (skeleton and potentially individual particles) with increasing vertical stress/depth of burial in a landfill. The German MBT waste (Pimolthai and Wagner, 2014) had the smallest porosity and the largest dry density cd. This was probably a result of the generally smaller particle size (i.e., the absence of particles >10 mm; Fig.E1). Particle size distributions for each waste are shown in Fig.E1. In the case of the Chinese MSW recovered for this study from the Qizishan landfill, the maximum particle size was limited by the diameter of the borehole (82–96 mm). The proportion of indeterminate fines increased with increasing depth, with d10 of shallow, middle and deep MSW of 0.25 mm, 0.12 mm and 0.075 mm respectively. The uniformity coefficients Cu (d60/d10) were 173, 121 and 21, and the curvature coefficients Cc (d230/(d60d10)) were 0.54, 0.17 and 0.73, for shallow, middle and deep MSW respectively, indicating a wide range of particle size with a lack of particles in the middle (1 ~ 10 mm). The German MBT waste (Pimolthai and Wagner, 2014) had more fines but a relatively larger value of d10 (about 0.5 mm) than the Chinese MSW from the Qizishan landfill. The shape of the particle size distribution curve for the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) was similar to that of the fresh Chinese MSW reported by Sun et al. (2008), but the actual sizes were generally a factor of at least 2 greater (e.g. d10 ~ 20 mm, compared with ~10 mm). Particle size determination generally follows the sieving method for soils, with the waste sieved through a number of screens to grade a sample by size. However, for 1D and 2D particles, the notion of a representative diameter becomes problematic as they have at least one dimension extremely small. The same applies to 3D waste components of irregular shape. Furthermore, 2D particles tend to orientate with their shortest dimension vertical during waste deposition and compression, leading to increased flow tortuosity and anisotropy (Qin et al., 2019). The unsuitability of a single representative particle dimension and the effect of 2D particles on fluid flow may be less significant for MBT waste than that for crude unprocessed waste, as a result of shredding or other mechanical processing. Thus the fresh UK MSW (Powrie and Beaven, 1999; Beaven, 2000) might contain more 2D particles classified as large diameter components than either the Chinese borehole MSW or the German MBT waste. To facilitate comparison between tests carried out with liquid and gas flow, the conductivity data obtained are presented in terms
Shallow
Middle
1.E-11
Deep
1.E-12 1.E-13
1.E-14 1.E-15
Borehole MSW-China(This study) Fresh MSW-UK(Powrie and Beaven 1999) MBT-Germany (Pimolthai and Wagner 2014)
1.E-16
1.E-17 0.2
0.4
0.6 Dry density ρd (g/cm3 )
0.8
1
Fig. 2. Relationships between intrinsic permeability determined in liquid flow tests and dry density.
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Intrinsic permeability ki (m2 )
(a) 1.E-10 ki = 8×10 -8n18 R² = 0.808
1.E-11
Deep
1.E-12
Middle
Shallow
1.E-13
1.E-14 1.E-15
Borehole MSW-China (This study) Fresh MSW-UK(Powrie and Beaven 1999) MBT-Germany (Pimolthai and Wagner 2014)
1.E-16 1.E-17
0.3
0.4
0.5 0.6 Porosity n
0.7
0.8
Intrinsic permeability ki (m2)
(b) 1.E-10 ki = 4×10 -10n e2.214 R² = 0.803
1.E-11
Shallow Middle
Deep
1.E-12 1.E-13
1.E-14
Borehole MSW-China (This study)
Fresh MSW-UK(Powrie and Beaven 1999) 1.E-15 0
0.1
0.2 Drainable porosity ne
0.3
0.4
Fig. 3. Relationship between intrinsic permeability determined in liquid flowtests and porosity: (a) total porosity n; (b) drainable porosity ne.
Intrinsic permeability ki (m2 )
1.E-10 Shallow Middle Deep
1.E-11 1.E-12
1.E-13
Fig. 5. Intrinsic permeability determined in liquid flow tests: (a) related to n and d10; (b) related to ne and d10.
Borehole MSW-China(This study) Fresh MSW-UK(Powrie and Beaven 1999) MBT-Germany (Pimolthai and Wagner 2014) Similar porosity (n of 0.54-0.61, ne of 0.14~0.15 )
1.E-14
1.E-15 1.E-16
1.E-17 0
5
10 15 Effective diameter d10 (mm)
20
Fig. 4. Relationship between intrinsic permeability determined in liquid flow tests and effective diameter d10.
3.2. Intrinsic permeability determined in gas flow Intrinsic permeabilities determined in gas flow tests in rigidwalled permeameter on the Chinese MSW recovered from the Qizishan landfill are compared with data for Japanese MBT waste obtained by Kallel et al. (2004) and for UK MBT obtained by Holmes (2013). Waste compositions and densities etc are compared in Table E3 and Table E4 in E-component. For the Chinese borehole MSW, these are similar to those in the liquid permeability tests (Table E1 and Table E2 in E-component), although the samples had been dried and re-packed for the gas flow tests. In the study reported by Kallel et al. (2004), source-separated incombustible (IW1) and bulky wastes (BW1) were shredded and then sieved after ferrous metal recovery, and then tested to deter-
mine the intrinsic permeability in gas flow of the under-sieve fraction destined for landfill. As shown in Table E3, IW1 comprised mainly plastics (35%) and glass/metal (60%). BW1 contained more wood waste and less plastics, glass and metal. The MBT waste (maximum particle size of 9 mm and 10 mm, respectively) used in the gas permeability tests of Holmes (2013) was from a processing facility in the UK. About 62.13% of the waste was unidentifiable and classified as other inorganic waste (Table E3). The remaining fraction of organic waste was very small (<4%). Compared with the MBT waste in Japan, the MBT waste in UK had similar specific gravity but larger porosity and smaller dry density (Table E4). Particle size distributions of the wastes used in gas permeability tests are shown in Fig.E2 in E-component. The Japanese (Kallel et al., 2004) and UK MBT wastes (Holmes, 2013) comprised more fines than the Chinese borehole MSW, with larger values of d10. The UK MBT waste was finer than that from Japan. The values of d10 were 0.3 mm and 0.25 mm for the sub-10 mm and sub-9 mm UK MBT wastes, and 0.7 mm and 0.5 mm for the Japanese IW1 and BW1, respectively. The Chinese borehole MSW might contain more 2D particles classified as large diameter components than both of the MBT wastes. The relationship between the intrinsic permeability determined in gas flow tests and waste dry density is shown in Fig. 6. For the
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Intrinsic permeability ki (m2 )
1.E-09
Borehole MSW-China (This study) MBT IW1-Japan(Kallel et al. 2004) MBT BW1-Japan (Kallel et al. 2004) MBT sub-10mm-UK (Holmes 2013) MBT sub-9mm-UK (Holmes 2013)
1.E-10 Shallow
Middle Deep
1.E-11 0.2
0.4
0.6 Dry density ρd (g/cm3 )
0.8
1
Fig. 6. Relationship between intrinsic permeability determined in gas flow tests and dry density.
Chinese borehole MSW and the UK MBT waste (Holmes, 2013), ki decreased with increasing qd with a similar fitted trend. The values of ki for the Japanese MBT wastes (Kallel et al., 2004) did not conform to the trend line for the other wastes, and showed no clear relationship with qd. The intrinsic permeabilities determined in gas flow tests are plotted against total porosity n in Fig. 7(a). For the Chinese borehole MSW and the UK MBT wastes (Holmes, 2013), ki followed a similar decreasing trend with decreasing n. Values of ki determined for the Japanese MBT waste (Kallel et al., 2004) did not follow the
Intrinsic permeability ki (m2 )
(a) 1.E-09
Borehole MSW-China (This study) MBT IW1-Japan (Kallel et al. 2004) MBT BW1-Japan (Kallel et al. 2004) MBT sub-10mm-UK(Holmes 2013) MBT sub-9mm-UK(Holmes 2013)
1.E-10
Shallow Middle
Deep
trend line for the other wastes, and showed no clear relationship with n. Data of ki from gas flow tests on Chinese borehole MSW, based on the assumption ne = n - nc, are plotted against ne in Fig. 7(b), and show a decrease with decreasing ne. The relationship between intrinsic permeability determined in gas flow tests and effective diameter d10 is shown in Fig. 8. There is a trend of decreasing ki with decreasing d10. For the middle layer Chinese borehole MSW and the sub-9 mm UK MBT waste (Holmes, 2013), which have similar porosity (n of 0.67–0.72), the decrease in ki with decreasing d10 is slight. Kallel et al. (2004) also reported a decrease of ki determined in gas flow tests with decreasing d10 for different wastes with n of 0.30 ~ 0.50. Feng et al. (2017) reviewed the data of Kallel et al. (2004) and Xu et al. (2014) and suggested ki = bd210, where the model parameter b is dependent on n. They also suggested improving the model of ki, as d10 is not sufficient to reflect the influences from particle size distribution. Although the Japanese MBT waste had the smallest porosity, it had the largest value of ki (Fig. 7(a)) – perhaps as a result of its relatively large value of d10 (Fig.E2 in E-component). The intrinsic permeability determined in gas flow tests is apparently more sensitive to d10 than the intrinsic permeability determined in liquid flow. This needs further study. To investigate the effects of both porosity and particle size on intrinsic permeability, ki is plotted against n and d10 in Fig. 9. The best-fit curved surface (determined using Orthogonal Distance Regression of Origin Pro 9.1, OriginLab Corporation 2013) based on Eq.(4) (ki = 5 1012n7.646d0.900 n3/(1-n)2, R2 = 0.826) is also 10 shown. However, the indicated decrease in ki with increasing n is considered to be an unrealistic artefact of the influence of the data for the Japanese MBT waste (Kallel et al., 2004), which are inconsistent with the results from the other waste types (Fig. 7(a)). The increase in ki with n (ne) determined in liquid flow is apparently more linear than that determined in gas flow. ki shows a clear increase with d10 when determined in gas flow, but no obvious dependence when determined in liquid flow. These apparent differences might be a result of one or all of (i) the use of d10 to quantify the effect of particle size; in reality, the particle size distribution, particle shape and orientation are likely to have a significant influence; (ii) differences in test conditions (e.g., treatment of specimen, and processing of porosity, particle size distribution and permeability tests) in the various studies; (iii) different flow behaviours in waste between liquid and gas.
1.E-11 0.4
0.5
0.6 Porosity n
0.7
0.8
(b) 1.E-09
1.E-09
Borehole MSW-China (This study) MBT IW1-Japan (Kallel et al. 2004) MBT BW1-Japan (Kallel et al. 2004) MBT sub-10mm-UK (Holmes 2013) MBT sub-9mm-UK (Holmes 2013) Porosity of 0.67~0.72
Intrinsic permeability ki (m2)
Borehole MSW-China (This study) Intrinsic permeability ki (m2 )
In an attempt to reduce the degree of variability in test conditions and other parameters, the intrinsic permeabilities
1.E-10
1.E-10
Shallow Middle Deep
1.E-11
Shallow Middle Deep
1.E-11
0.1
0.2 0.3 Drainable porosity ne
0.4
Fig. 7. Relationship between intrinsic permeability determined in gas flow tests and porosity: (a) total porosity n, (b) drainable porosity ne.
0
0.1
0.2
0.3 0.4 0.5 0.6 Effective diameter d10 (mm)
0.7
0.8
Fig. 8. Relationship between intrinsic permeability determined in gas flow tests and effective diameter d10.
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less degradation, and hence causes a decrease of porosity. Therefore, the Chinese landfilled waste may experience a decrease of ki (smaller than 4 1012 m2 which is the value of ki for current Chinese waste in deep layers) in the future due to the decrease of porosity and particle size. However, it still needs further study. 4. Conclusions Laboratory liquid and gas permeability tests have been carried out on samples of MSW recovered from a landfill in China, and the results compared with data reported in the literature to assess the effects of waste porosity and particle size and the nature of the permeant (liquid or gas) on the inferred intrinsic permeability, ki. The main conclusions are as follows:
Fig. 9. Intrinsic permeability determined in gas flow tests: related to n and d10.
determined in liquid and gas flow will now be compared for specimens of waste from the same depth of the Qizishan landfill. The intrinsic permeabilities are plotted against porosity in Fig. 10. For specimens from the same depth, the value of ki obtained from gas flow tests is consistently greater than that obtained from liquid flow tests. This is in agreement with the findings of Stoltz et al. (2010) and Xu et al. (2014). Possible reasons include: (i) a decreased pore space available for liquid flow due to a lack of liquid saturation (i.e., gas trapped in waste) (Stoltz et al. 2010; Xu et al. 2014) (ii) a decreased pore space available for liquid flow due to the sorption of water in unsaturated pores (iii) enhancement of gas flow rates due to ‘‘gas slippage”(i.e., the Klinkenberg effect, which manifests as an additional flux due to collisions between gas molecules and the pore walls: Klinkenberg, 1941) (iv) drying and re-packing of the waste for the gas flow tests affecting the geometric properties of both the solid particles and the pores. With the development of sorted disposal (i.e., composting plant for kitchen waste, MBT and incineration plants for other waste) in China, the waste to be landfilled may contain much more fine (0D) inorganic particles and less 1D, 2D and 3D particles in the future. It tends to make the waste easier for a good compaction and have
1. For the Chinese MSW, both liquid and gas intrinsic permeability decreased with increasing depth, due mainly to the smaller porosity and more fine particles in deeper layer. For the Chinese borehole MSW drilled from the same depth, the intrinsic permeability determined in gas flow was larger than that determined in liquid flow. 2. The intrinsic permeability determined in liquid flow tests shows no clear relationship with effective particle diameter d10. It is however considerably affected by the porosity – especially the drainable porosity, ne, which gives a better indication of the relative volume of pores available for liquid flow. The intrinsic permeability determined in gas flow tests shows a clear decrease with decreasing d10, but no consistent variation with porosity. 3. The different behaviours in ki in relation to n(ne) and d10 between liquid and gas might have been due to: (i) the use of d10 (rather than a more complex reflection of particle size distribution and shape) to quantify the effect of particle size, (ii) the different test conditions involved, and (iii) the different behaviours between liquid and gas flow in waste. 4. The intrinsic permeability of Chinese landfilled MSW is considered to decrease with the development of sorted disposal. However, it still needs further study.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement
Intrinsic permeability ki (m2)
1.E-09
The work was supported by the National Natural Science Foundation of China (Grant Nos. 51708508 and 41772300), the Zhejiang Provincial Natural Science Foundation of China (Grant Nos. LY17E080021) and the fund from China Scholarship Council.
Borehole MSW-China-Gas-n (This study) Borehole MSW-China-Liquid -n (This study) Borehole MSW-China-Gas-ne (This study) Borehole MSW-China-Lqiud -ne (This study)
1.E-10
Appendix A. Supplementary material Shallow
1.E-11
Shallow
Middle Deep
Middle Deep
Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.10.039. References
1.E-12 0.1
0.2
0.3 0.4 0.5 0.6 Porosity n or Drainable porosity ne
0.7
0.8
Fig. 10. Comparison between intrinsic permeabilities determined in gas and liquid flow tests.
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