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WMR0010.1177/0734242X16675684Waste Management & ResearchFeng et al.

Original Article

Three-dimensional modelling of leachate recirculation using vertical wells in bioreactor landfills

Waste Management & Research 2016, Vol. 34(12) 1307–1315 © The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X16675684 wmr.sagepub.com

Shi-Jin Feng1,2, Zheng-Wei Chen1 and Ben-Yi Cao1

Abstract Bioreactor landfills use leachate recirculation to enhance the biodegradation of municipal solid waste and accelerate landfill stabilisation, which can provide significant environmental and economic benefits. Vertical wells are operated as a major method for leachate recirculation systems. The objectives of this article are to analyse the leachate migration in bioreactor landfills using vertical wells and to offer theoretical basis for the design of leachate recirculation systems. A three-dimensional numerical model was built using FLAC-3D, and this model can consider the saturated and unsaturated flow of leachate within anisotropic waste to reflect the actual conditions. First, main influence factors of leachate migration were analysed, including the vertical well height, hydraulic conductivity, and anisotropic coefficient, in a single-well recirculation system. Then, the effects of different configurations of a group-well system were studied and the optimal well spacing was obtained. Some key design parameters (e.g. the recirculation flow rate, volume of impact zone, radius of impact zone and time to reach steady state) were also evaluated. The results show that the hydraulic conductivity has a great impact on the optimal height of vertical wells and uniform configuration is the best option in terms of both volume of impact zone and time to reach steady state. Keywords Bioreactor landfills, leachate recirculation system, vertical wells, three-dimensional model, group-well configurations

Introduction Although there are an increasing number of treatments of municipal solid waste (MSW), landfilling remains the major method all around the world. A traditional landfill consists of a bottom liner system, leachate collection and removal systems (LCRSs), and a final cover system that prevents moisture infiltration (Reddy et al., 2015; USEPA, 2009). The low water content in traditional landfills results in a slow degradation rate of MSW. The moisture content is required to be around 50% to create an optimal environment for biodegradation (ITRC, 2006). Thus, bioreactor landfills, which can increase the moisture content through the recirculation of leachate back into landfills, are becoming increasingly popular for the environmental and economic benefits. Surface spraying, surface ponds, horizontal trenches (HTs), vertical wells (VWs), and drainage blankets (DBs) are wellrecognised leachate recirculation methods (Reddy et al., 2015). VWs are the most commonly used system for adding liquids to bioreactor landfills (Reinhart and Townsend, 1997). In a leachate recirculation system using VWs, spraying pipes surrounded by highly permeable filter material are installed inside a landfill, and leachate is injected back into the landfill through these pipes. Townsend (1995) developed an equation to describe leachate recirculation through LCS in porous media under saturated conditions. Khanbilvardi et al. (1995) developed a two-dimensional (2D) moisture flow model to investigate the leachate migration in landfills and to study the effect of leachate head. McCreanor and

Reinhart (1999) employed the SUTRA (United States Geological Survey, USA) to simulate the leachate migration recirculated with VWs in homogenous and isotropic waste. Haydar and Khire (2005) studied the leachate migration in the bioreactor landfills with HTs using HYDRUS-2D (University of California, Riverside, USA). Leachate migration in the bioreactor landfills with VWs was investigated by Khire and Mukherjee (2007). They studied the hydraulic conductivity of the waste, VW backfill, and spacing of the leachate collection pipes using HYDRUS-2D. A 2D numerical model was conducted by Jain et al. (2010a; 2010b) using SEEP/W (GEO-SLOPE International Ltd., Calgary, Canada) to investigate the moisture distribution in homogeneous waste. They presented an axisymmetric landfill model, and simulated leachate recirculation considering well dimensions and waste properties. Reddy et al. (2015) built a leachate recirculation model using FLAC-2D (ITASCA Consulting

1Department

of Geotechnical Engineering, Tongji University, Shanghai, China 2Key Laboratory Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai, China Corresponding author: Shi-Jin Feng, Department of Geotechnical Engineering, Tongji University, Si Ping Road 1239, Shanghai 200092, China. Email: [email protected]

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Group Inc., Minneapolis, USA) to simulate the leachate migration with DBs, investigating the effect of MSW properties and the leachate injection rate. Feng et al. (2014, 2015a, 2015b) proposed a theoretical model that considered the effects of biodegradation on the settlement of MSW, and analysed the leachate migration in the bioreactor landfills using surface irrigation, HTs, and VWs. Researchers generally used 2D software such as SUTRA, HYDRUS-2D, FLAC-2D, and SEEP/W to simulate VW recirculation systems. Among these studies, some researchers simplified the actual conditions into a plane model, where the VWs were actually simulated as trenches, inevitably causing deviations; some built axisymmetric models, which can only be applied to a single-well recirculation system. The main objective of this article is to develop a method for designing bioreactor landfills using VWs as leachate recirculation systems. This article employed FLAC-3D (ITASCA Consulting Group Inc., Minneapolis, USA) to propose three-dimensional (3D) saturated–unsaturated models and simulate leachate recirculation for both single-well and group-well recirculation systems. First, a single-well recirculation system was simulated, and MSW properties and VW height were evaluated. Then a groupwell recirculation system was analysed considering the groupwell configurations and well spacing. A 3D model can reflect the actual conditions and simulate group-well systems reasonably, so the analytical results can be regarded as a reference for the design of leachate recirculation systems in bioreactor landfills.

k k ( s ) qi = − il [ p − ρ w x j g j ],l (1) µ

where qi is the flow rate, p is the pore pressure, kil is the saturated permeability, k ( s ) is the relative permeability, µ is the coefficient of dynamic viscosity, ρ w is the leachate density, xj is the coordinate, and g j is the gravity. The fluid mass balance can be expressed as: −qi ,i + qv =

∂θ (2) ∂t

where qv is the fluid source intensity, t is the time, and θ is the water content. The balance of momentum can be expressed as:

The relative hydraulic conductivity k is related to saturation:

k ( s ) = s 2 (3 − 2 s ) (4)

Equations (1)–(4) are solved numerically with the FLAC-3D program using the finite difference method.

The flow of leachate through MSW was modelled by FLAC-3D. The flow in both saturated and unsaturated zones was modelled. Darcy’s law is used to describe the flow of fluid:

LCRS: leachate collection and removal system; MSW: municipal solid waste.

Modelling development Numerical flow model

Figure 1. Conceptual model of a bioreactor landfill with VW: (a) the cross section and (b) 3D.

σ ij , j + ρ gi = ρ

dvi (3) dt

where σ ij , j is the gradient of normal stress, ρ = ρ d + ns ρ w is the density, ρ d is the density of the dry MSW, ρ w is the density of the leachate, vi is the velocity, n is the porosity, and s is the saturation.

Conceptual model To simulate actual conditions, a 3D model (40 m long × 40 m wide × 20 m high) was established for the saturated–unsaturated flow. Watson (1993) used VWs with the diameter of 1.2 m at the Delaware landfill, and the recirculation rate ranged from 108 to 1080 m3 day-1. As can be seen from Figure 1, a VW with the diameter of 0.3 m and injection screen height of 2 m is settled at the centre of the model domain. There is an injection pipe inside the injection screen, backfilled with gravel. H is the landfill height and h is the VW height. Zones of the saturation of above 0.8 are defined as the impact zone. Steady state is defined as the state when volume of impact zone does not vary with time. The dim is defined as the radius of impact zone in the horizontal plane at the bottom of the injection screen, the Vim is the volume of impact zone, and the Ts is the time to reach steady state. Among the primitive mesh shapes in FLAC-3D, brick and radial cylinder meshes were used to build the landfill model. A grid size of 0.25 m was used for all model simulations, and the number of mesh elements was 256,000. The top and side boundaries were set to be impermeable. The bottom grid points in the landfill model were fixed to zero pore water pressure, representing the LCRSs.

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Feng et al. Table 1. Values of properties and variables used in numerical simulations. Properties

Value

Density (kg m-3) Fluid density (kg m-3) Initial saturation Residual saturation Bulk modulus (Pa) Shear modulus (Pa) Fluid modulus (Pa) Porosity MSW Gravel Saturated hydraulic conductivity (cm s-1) MSW Gravel

1100 1000 0.4 0.2 1.5×105 1.0×105 1.0×106 0.45 0.40

Scenarios Variables

A single-well

A group-well

Vertical well height: h (m)

4, 8, 12, 16, 20, 24, 28, 32, 36, 40

12, 16

Landfill height: H (m)

20, 30, 40

20, 30

Hydraulic conductivity: kv (cm s-1)

1×10-3, 5×10-4, 3×10-4, 1×10-4, 5×10-5

1×10-4, 5×10-4

Anisotropic coefficient: A Well spacing: D (m) Recirculation rate: Q (m3 day-1)

1, 2, ,3, 5, 10

1, 5

—

2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 10, 26, 56, 86, 100

1×10–3 1×10–2

10, 26, 56, 86, 100

MSW: municipal solid waste.

The hydraulic properties of MSW were selected based on the typical values published by Benson and Wang (1998) and Stoltz et al. (2011). Benson and Wang (1998) performed laboratory testing on field-collected MSW, and a specific weight of 7 kN m-3, a residual moisture content of 11%, and a porosity of 53% was obtained. Stoltz et al. (2011) carried out similar testing and they obtained a specific weight of 6.08 kN m-3, a residual moisture content of 20%, and a porosity of 58.6%. Kadambala et al. (2011) reported that the MSW saturated hydraulic conductivity ranges from 5.4 × 10-6 to 6.1 × 10-5 cm s-1 with an MSW density of 700 kg m-3. McCreanor (1998) investigated leachate routing in a bioreactor landfill using the mathematical model SUTRA, and the hydraulic conductivity of 10-3, 10-4, and 10-5 cm s-1 were selected. Landva et al. (1998) determined hydraulic conductivity in the order of 2 × 10-6 to 2 × 10-3 cm s-1 in the vertical direction and 4 × 10-5 to 1 × 10-3 cm s-1 in the horizontal direction. Thus, the value of hydraulic conductivity in both vertical and horizontal directions was chosen from 10-3 to 10-5 cm s-1. The MSW material properties in this study are summarised in Table 1. Dimensionless VW location h/H for different values of k and landfill height H was investigated to determine the optimal location of recirculation screen. A series of leachate recirculation flow rates (Q) were simulated to investigate the effects of MSW properties, including hydraulic conductivity and anisotropic coefficient. The relationship between horizontal hydraulic conductivity and vertical hydraulic conductivity is expressed as follows:

kh = A × kv (5)

where kh is the horizontal hydraulic conductivity, kv is the vertical hydraulic conductivity, and A is the anisotropic coefficient.

Modelling scenarios for recirculation systems are shown in Table 1. Group-well recirculation models were built to investigate the optimal well spacing and to compare different configurations for group wells.

Model validation To ensure the accuracy of this model, validations were conducted on previously published in situ and modelling studies.

Validation with the results of Grellier et al. (2005) Electrical resistivity tomography (ERT) was applied to moisture measurements in a French bioreactor landfill by Grellier et al. (2005). The landfill site studied was 10 m in depth and 50 m in width with a HT for leachate injection. Before recirculation, resistivity sensors were installed at a shallow depth below the surface by the investigators. During the recirculation process, the moisture resistivity could be captured by these sensors. The electrical resistivity has mathematical relationship with the moisture content, so the moisture evolution can be studied. In this in situ measurement, leachate was injected back into the landfill using a HT that was 1 m wide and 1 m deep. A numerical model that was 50 m wide and 10 m high was built in this study using FLAC-3D. The HT that was 0.5 m wide and 0.5 m high was set near the ground surface, and it was backfilled with gravel. A grid size of 0.25 m was used in this model. All boundaries were set to be impermeable. The validation adopts the following parameters: The residual moisture content of MSW and gravel is 20% and 2%, respectively; the porosity of

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Figure 2. Comparison between saturation contours obtained (a) from the field experimental data by Grellier et al. (2005) and (b) in this study.

MSW and gravel is 63% and 47%, respectively; the saturated hydraulic conductivity of MSW and gravel is 1.0 × 10-3 cm s-1 and 1.0 × 10-2 cm s-1, respectively. Both the saturation contours obtained by Grellier et al. (2005) and by this study are shown in Figure 2. It can be seen from Figure 2(a) that the leachate tends to migrate in both horizontal and vertical directions because of the anisotropy and gravity of the MSW. Owing to the inhomogeneity of MSW, the saturation contours have complex geometries. Saturation contours in Figure 2(b) show that leachate migration simulated by FLAC-3D has a reasonable agreement with that of Figure 2(a) with regard to the impact zone and the migration rate.

Validation with the results of Reddy et al. (2014) Reddy et al. (2014) used FLAC-2D to model the two-phase flow in landfills with DBs, considering the migration of nonwetting fluid. Laboratory, field, and modelling studies previously published were used to validate the model built using FLAC-2D. The moisture distribution was analysed for a wide range of MSW properties, DB dimensions, and leachate injection rates until the steady state condition was reached. The conceptual model was a bioreactor landfill of 100 m in width and 30 m in height with DB. A DB that was 0.3 m thick was assumed to be placed in the middle of the model, 25 m above the LCRS. The width of the DB (WB) was varied from 10 to 80 m, and there was a single leachate injection pipe located at the centre of the blanket. The residual saturation, initial saturation, and the initial porosity of MSW was assumed to be 25%, 40%, and 45%, respectively. The recirculation was simulated considering both the anisotropy and inhomogeneity of MSW, and k ranged from 10-2 to 10-6 cm s-1. The maximum wetted width (WWmax) and the wetted area for different DB configurations were compared by varying the leachate injection rates and the saturated hydraulic conductivity. The WB was normalized with the depth from DB to the LCRS (DB) for the WB/DB value of 2.4, 1.6, 0.8, and 0.4. In this article, we conducted similar modelling using FLAC-3D and compared our results to the results presented by Reddy et al. (2014). Validation was carried out considering the MSW as isotropic, and the same hydraulic properties and size of landfill model were set in FLAC-3D. Different values of WB/DB (i.e. 0.4 and 0.8) and k (i.e. 10-3 cm s-1 and 10-4 cm s-1) were simulated, and the values of WWmax/WB were compared with those of Reddy et al.

Figure 3. Normalised wetted width under different DHT/WHT with different leachate recirculation rates and saturated hydraulic conductivities of MSW.

(2014). Figure 3 shows the variation of normalised wetted width with leachate injection rate under different WB/DB and k. Both the modelling results presented by Reddy et al. (2014) and in this article are shown in Figure 3. It can be found that the normalised wetted widths obtained from both models are relatively close.

Results and discussions In this study, over 200 simulations using FLAC-3D were carried out to analyse the effects of the MSW properties and the VW height (h) on the leachate migration in bioreactor landfills.

Single-well recirculation system Influence of the VW height. Figure 4 shows the variation of volume of impact zone with dimensionless VW height for different values of hydraulic conductivity and landfill heights. Three groups of curves in Figure 4 represent different k of MSW (i.e. 10-3 cm s-1, 3 × 10-4 cm s-1 and 10-4 cm s-1), and each group contains three curves representing different H (i.e. 20 m, 30 m, and 40 m). For a single curve (e.g. under the condition of k = 3 × 10-4 cm s-1 and H = 30 m), the volume of impact zone increases with the dimensionless VW height, and a peak volume appears when h/H

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Table 2. The suggested values of h/H to settle the injection screen in practical engineering. k (cm s-1)

1×10-3

The optimal 1 h/H:

Figure 4. Variation of volume of impact zone with dimensionless VW height h/H.

is around 0.8, which is defined as the optimal h/H. This means that if a VW is set at the upper middle part of a landfill, the volume of impact zone will reach the highest point, which can maximise the acceleration effects of leachate on biodegradation progresses. Similar results can be found in the other two curves in the same group, which indicates that the relationship between optimal dimensionless VW height and H is not obvious. This is also true for the other two groups of curves, the only difference is that the value of the optimal dimensionless VW height varies with k. For example, the optimal values of dimensionless VW height range from 0.5 to 0.6 for k = 10-3 cm s-1. This is because when the value of h/H is lower than the optimal value, leachate tends to flow downward directly out of landfills, however, when the value of h/H is higher than the optimal value, the recirculated leachate spreads more widely with fewer zones reaching the saturation value of 0.8. On the other hand, the impact zone increases with the dimensionless VW height when k = 10-4 cm s-1, and the optimal value of h/H is 1. This means that for a higher value of h/H, the leachate spreads more widely and leachate can accumulate in the pore space because of the low conductivity. The group of k = 3 × 10-4 cm s-1 has an optimal value of h/H of around 0.8. The optimal value of h/H rises when the value of k decreases, and this rule can be applied to conditions with different H. Table 2 provides reference values of the optimal h/H corresponding to different k, and it can be used as guidelines on the design of bioreactor landfills. Simulations for different recirculation rates were carried out, and the result indicates that the recirculation rate makes no difference to the optimal h/H. Influence of the hydraulic conductivity. Hydraulic conductivity is the most important hydraulic parameter of MSW, so it is necessary to study the impact of hydraulic conductivity of MSW on moisture distribution in landfills. Four values of k (i.e. 10-3 cm s-1, 5 × 10-4 cm s-1, 10-4 cm s-1, and 5 × 10-5 cm s-1) were investigated, and it can be seen from Figure 5 that the hydraulic conductivity of waste plays an important role in leachate migration with

0.8~1

0.6~0.8

0.5~0.6

respect to Vim and Ts. Figure 5(a) shows that Vim increases linearly with Q for any k, and the value of Vim is roughly inversely proportional to k. For example, when Q is set at 100 m3 day-1, Vim is 125, 265, 1600, and 3215 m3 for k = 10-3, 5 × 10-4, 10-4, and 5 × 105 cm s-1, respectively. This is because the MSW skeleton with a lower k may obstruct the downward flow of leachate, and it is difficult for leachate to flow out of the bottom of landfill, so Vim can be greater. The difference of Vim for different k increases with Q, and it is observed that k has a greater impact on Vim than Q. The impact of k on Ts is shown in Figure 5(b). Ts hardly changes with Q; however, it should be noted that it takes a longer time for MSW with a lower k to reach the steady state. For instance, the maximum value of Ts is about 6 days for k = 10-3 cm s-1, however, it takes 134 days to reach steady state for a k of 5 × 10-5 cm s-1. So landfill operators can take some actions to reduce k, such as compacting the MSW, thereby increasing the Vim. However, the decrease in k will increase Ts, so the balance should be reached in practical engineering. Influence of anisotropic condition of MSW. Figure 6(a) shows the variation of Vim with T for various A (i.e. 1, 2, 3, 5, and 10). For the A of 10, the maximum Vim is obtained when T is approximately 130 days, however, the steady state is not reached within 300 days for the A of 1. A larger A can dramatically reduce Ts and increase Vim and dim. It should be noted that the value of Vim slightly decreases after the peak, because the leachate flows in both directions before it arrives at the bottom boundary, after which, however, the leachate tends to flow vertically, resulting in horizontal shrinkage of impact zone and an decrease in Vim. The impact of A on p is shown in Figure 6(b). A peak value of p of about 220 and 30 kPa appears at the injection screen when A is set as 1 and 10, respectively. A larger A represents larger horizontal hydraulic conductivity, making it easier for leachate to flow horizontally and obtaining a lower p.

Group-well recirculation system For a group-well recirculation system, four different configurations were simulated, including uniform and staggered configurations. Further, the height of some VWs was adjusted so that adjacent wells have different heights, which is called an HL pattern. The corresponding configurations are defined as ‘uniformHL’ and ‘staggered-HL’. A comparison between these four configurations was made. A landfill model with the height of 20 m was built, and k was set as 10-3 cm s-1, and under this condition, the optimal h/H ranges from 0.5 to 0.6 corresponding to the h from 10 to 12 m. Among these four configurations, the well spacing was set to be equal to ensure that four configurations

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Figure 5. The influence of hydraulic conductivity on (a) volume of impact zone and (b) time to reach steady state under different leachate recirculation rates.

Figure 6. The influence of anisotropy of MSW on (a) variation of volume of impact zone with time and (b) pore water pressure distribution with depth.

were simulated for the same number of wells per area. Figure 7(a) shows the uniform configuration, and all nine wells were set at the height of 10 m, which is the optimal height. Figure 7(b) shows the staggered configuration, and similarly, wells are located at a height of 10 m. In Figure 7(c), it can been found that four wells are located at the height of 12 m, 2 m higher than the other five wells. Wells are arranged uniformly in the horizontal plane, but in the vertical plane they are distinct with regard to high and low wells. It is called the ‘uniform-HL’ configuration, and Figure 7(d) shows the ‘staggered-HL’ configuration.

Figure 8(a) presents the variation of Vim with D, and it shows that the Vim reaches the peak when D = 12 m, so Dopt is 12 m under this condition. Then, a single-well recirculation system was simulated under the same MSW properties, landfill size, and VW height as the previous simulation, and dim was found out to be 5.9 m, almost half of the Dopt. Simulations were carried out under different Q for both uniform and staggered configuration systems, and the comparison between Dopt and 2dim is presented in Figure 8(b). It can be seen from these five groups of comparisons that the maximum difference is about 9.5% when Q is 10 m3 day-1 for the staggered-configured system, which provides the rationale behind the idea that one can evaluate group-well spacing D by only simulating single-well conditions. To test the feasibility of this method, a series of validations were carried out under different k, A, H, Q. The result shows that the maximum difference is around 7%, which means that in practical engineering, 2dim can be used to design the well spacing for a group-well recirculation system. Figure 9 provides reference values of the optimal well spacing for different k and Q, and it can be used as guidelines on the design of bioreactor landfills.

Influence of well spacing. In a single-well recirculation system, the dim is defined as the radius of impact zone in the horizontal plane at the bottom of the VW. For a group-well recirculation system, uniform or staggered configurations, the well spacing (D) is an important design parameter: An excessive D may prevent MSW between two adjacent wells from being moistened, resulting in an inadequate Vim; on the other hand, a too small D can decrease Vim for a single well and increase operation costs. Therefore, both of these conditions have negative effects on recirculation efficiency. In this article, the optimal well spacing (Dopt) was investigated for group-well recirculation systems. The leachate migration in the model with nine uniformly configured VWs was simulated for different D.

Influence of configuration. Four configurations for group wells were compared in this article. Saturation contours in

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Figure 7. Conceptual models of bioreactor landfills with different group-well configurations: (a) Uniform; (b) staggered; (c) uniform-HL; and (d) staggered-HL. LCRS: leachate collection and removal system; MSW: municipal solid waste; VWs: vertical well.

Figure 9. The reference value of well spacing of group-well recirculation systems in bioreactor landfills.

Figure 8. (a) Variation of volume of impact zone with well spacing; (b) comparison between Dopt and 2dim for different leachate recirculation rates.

the horizontal plane at the height of 8 m are presented in Figure 10(a), for the uniform-HL configuration. It shows that the value of saturation in five zones first increases corresponding to

the five lower VWs, followed by the increase in saturation around the other four wells with a height of 12 m. Finally, the shape of saturation contour appears a square, similar to that of the horizontal configuration of VWs. Figure 10(b) compares the configurations with regard to Vim and Ts. Volumes of impact zone for uniform and uniform-HL configurations are slightly greater than those for staggered and staggered-HL configurations, while Ts is smaller. This indicates that the uniform configuration for VWs in the horizontal plane is a better choice. It should be noted that for both uniform and staggered configurations, the HL pattern will increase the Vim for approximately 6.5%, however, it will take a little more time to reach steady state. Similar comparisons were made under different k and A, and the results are close for all conditions. Overall, the uniform configuration with HL pattern is the optimal choice.

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Waste Management & Research 34(12) Declaration of conflicting interests The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Much of the work described in this article was supported by the National Natural Science Foundation of China under Grant Nos. 41072201, 41172245, and 41222021, the National Basic Research Program of China (973 Program) under Grant No. 2012CB719803, and the Program for New Century Excellent Talents in University under Grant No. NCET-13-0421.

References Figure 10. (a) Saturation contours during the recirculation process under uniform-HL configuration; (b) comparison between Vim and Ts under different group-well configurations.

Conclusion A 3D numerical model was established using FLAC-3D software to study the leachate recirculation in bioreactor landfills with VWs. Main influence factors including the VW height, hydraulic conductivity, and anisotropic coefficient were investigated by evaluating the volume of impact zone, radius of impact zone, maximum pore water pressure, and time to reach steady state. It should be noted that hydraulic conductivity has a great impact on the optimal height of VW, and the optimal values of h/H rises when the value of k decreases. The reference value of h/H are given in this article, which could offer an insight into the rule of leachate recirculation in bioreactor landfills using VWs and provide landfill operators with preliminary guidelines for landfill design. The relationship between the optimal spacing (Dopt) for group-well and radius of impact zone for (dim) single-well was studied in this article. The results show that under the same MSW properties, VW height, and landfill height there is no significant difference between Dopt and 2dim, which indicates that 2dim can be used as a reference value for the design of VW spacing. Different configurations of group-well recirculation systems, e.g. uniform, staggered, uniform-HL, and staggeredHL configurations were compared in this article. Landfill models with these VW configurations were built and recirculation was simulated until steady state. Comparisons were carried out under different conditions, and the results show that the uniform configuration is the best option in terms of both volume of impact zone and time to reach steady state. The HL pattern, which means adjacent VWs have different height, can reach a maximum Vim.

Acknowledgements The writers would like to greatly acknowledge all these financial supports and express the sincerest gratitude.

Benson CH and Wang X (1998) Soil water characteristic curves for solid waste. Environmental Geotechnics Report 98: 13. Feng SJ, Zhang X and Cao BY (2014) Leachate recirculation in bioreactor landfills considering the effect of MSW settlement on hydraulic properties. Environmental Earth Sciences 72: 2315–2323. Feng SJ, Cao BY, Zhang X, et al. (2015a) Leachate recirculation in bioreactor landfills considering the stratification of MSW hydraulic conductivity. Environmental Earth Sciences 73: 3349–3359. Feng SJ, Cao BY, Zhang X, et al. (2015b) Modeling of leachate recirculation using vertical wells in bioreactor landfills. Environmental Science and Pollution Research 22: 9067–9079. Grellier S, Bouyé JM, Guérin R, et al. (2005) Electrical resistivity tomography (ERT) applied to moisture measurements in bioreactor: Principles, in situ measurements and results. In International workshop hydrophysico-mechanics (HPM) landfill, 21–22 March 2005, Grenoble, France. Haydar MM and Khire MV (2005) Leachate recirculation using horizontal trenches in bioreactor landfills. Journal of Geotechnical and Geoenvironmental Engineering 131: 837–847. ITASCA Consulting Group Inc. (2005) Fast Lagrangian analysis of continua in 3 Dimensions (FLAC-3D) version 5.0: Fluid-mechanical interaction. ITRC (2006) Characterization, design, construction, and monitoring of bioreactor landfills. Washington: Interstate Technology and Regulatory Council Alternative Landfill Technologies Team. Jain P, Townsend TG and Tolaymat TM (2010a) Steady-state design of horizontal systems for liquids addition at bioreactor landfills. Waste Management 30: 2560–2569. Jain P, Townsend TG and Tolaymat TM (2010b) Steady-state design of vertical wells for liquids addition at bioreactor landfills. Waste Management 30: 2022–2029. Kadambala R, Townsend TG, Jain P, et al. (2011) Temporal and spatial pore water pressure distribution surrounding a vertical landfill leachate recirculation well. International Journal of Environmental Research and Public Health 8: 1692–1706. Khanbilvardi RM, Ahmed S and Gleason PJ (1995) Flow investigation for landfill leachate (FILL). Journal of Environmental Engineering 121: 45–57. Khire MV and Mukherjee M (2007) Leachate injection using vertical wells in bioreactor landfills. Waste Management 27: 1233–1247. Landva AO, Pelkey SG and Valsangkar AJ (1998) Coefficient of permeability of municipal refuse. In: Proceedings of the 3rd international congress on environmental geotechnics, 7–11 September, Lisbon, portugal. pp.63–68. McCreanor PT (1998) Landfill leachate recirculation systems: Mathematical modeling and validation. Doctoral dissertation, University of Central Florida. McCreanor PT and Reinhart DR (1999) Hydrodynamic modeling of leachate recirculating landfills. Waste Management & Research 17: 465–469. Reddy KR, Giri RK and Kulkarni HS (2014) Design of drainage blankets for leachate recirculation in bioreactor landfills using two-phase flow modeling. Computers and Geotechnics 62: 77–89.

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Feng et al. Reddy KR, Giri RK and Kulkarni HS (2015) Two-phase modeling of leachate recirculation using drainage blankets in bioreactor landfills. Environmental Modeling & Assessment 20(5): 475–490. Reinhart DR and Townsend TG (1997) Landfill bioreactor design & operation. CRC press. Stoltz G, Tinet AJ, Staub MJ, et al. (2011) Moisture retention properties of municipal solid waste in relation to compression. Journal of Geotechnical and Geoenvironmental Engineering 138: 535–543. Townsend TG (1995) Leachate recycle at solid waste landfills using horizontal injection. Doctoral dissertation, University of Florida. USEPA (2009) Municipal solid waste generation, recycling, and disposal in the United States: facts and figures for 2009. Washington: Solid Waste and Emergency Response, pp.1–12. Watson R (1993) State of Delaware’s case history review: A full scale active waste management approach. In: Modern doublelined landfill management seminar, 2–4 March, Saratoga Springs, New York.

Appendix Notation A dim D DB Dopt gj h

anisotropic coefficient (dimensionless) radius of impact zone (m) well spacing (m) depth from DB to the LCRS (m) optimal well spacing (m) acceleration of gravity (N kg-1) vertical well height (m)

H k kil k (s) n p qi qv Q s t T Ts Vim vi WB WWmax xj θ µ ρ ρ d ρ w σ ij,j

landfill height (m) hydraulic conductivity (cm s-1) saturated permeability (m2) relative permeability (dimensionless) porosity of MSW (dimensionless) pore pressure (kPa) flow rate (m s-1) fluid source intensity (s-1) recirculation rate (m3 day-1) saturation (dimensionless) time (s) recirculation time (s) time to reach steady state (s) volume of impact zone (m3) velocity (m s-1) width of the DB (m) maximum wetted width (m) coordinate (m) water content (dimensionless) coefficient of dynamic viscosity (kg m-1 s-1) density of MSW (kg m-3) density of the dry MSW (kg m-3) leachate density (kg m-3) gradient of normal stress (N m-3)