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International Conference on Solid Waste 2011
Moving Towards Sustainable Resource Management
Conference Proceedings
[
2nd
– 6th
May 2011
Hong Kong SAR, P.R. China
Edited by
Prof. Jonathan W.C. Wong
Department of Biology, Sino-Forest Applied Research Centre for Pearl River Delta Environment
Hong Kong Baptist University, Hong Kong SAR, P.R. China
Prof. Klause Fricke
Leichtweiß-Institute Waste and Resource Management
Technical University of Braunschweig, Germany
Prof. Rao Y. Surampalli
Department of Civil Engineering
University of Nebraska–Lincoln at Omaha Campus, Omaha, USA
Dr. Ammaiyappan Selvam
Sino-Forest Applied Research Centre for Pearl River Delta Environment
Hong Kong Baptist University, Hong Kong SAR, P.R. China
692 Proceedings of the International Conference on Solid Waste 2011- Moving Towards Sustainable Resource Management,
Hong Kong SAR, P.R. China, 2 – 6 May 2011
EFFECTIVENESS OF DRAINAGE BLANKET FOR LEACHATE RECIRCULATION IN
HETEROGENEOUS AND ANISOTROPIC MUNICIPAL SOLID WASTE
K.R. Reddy * , H.S. Kulkarni
University of Illinois at Chicago, Department of Civil and Materials Engineering, West Taylor Street,
Chicago, Illinois 60607, U.S.A
* Corresponding author. Tel: (312) 996-4775 Fax: (312) 996-2426, E-mail: kreddy@uic.edu
ABSTRACT The main objective of this paper is to examine the effect of heterogeneous and anisotropic
municipal solid waste (MSW) on moisture distribution in a bioreactor landfill with drainage blanket (DB) as
leachate recirculation system (LRS). Two-phase flow modeling was performed by representing relative
permeabilities of leachate and landfill gas with van Genuchten function. Predicted saturation level, wetted
area, pore water pressure, and outflow rate of leachate were found to be significantly different for
homogeneous isotropic, heterogeneous isotropic, and heterogeneous anisotropic MSW conditions. It is
recommended that the heterogeneous and anisotropic MSW conditions should be used in the design of DB
for effective leachate distribution.
Keywords: Bioreactor landfill, Moisture distribution, Drainage blanket, Two-phase flow
Introduction
Bioreactor landfills, which involve leachate recirculation, are being increasingly considered to accelerate
biodegradation of MSW in landfills [1-2]. Drainage blankets (DBs) are recently introduced as leachate
recirculation systems (LRS) that are constructed during the waste filling operations, and they consist of a
permeable layer spread over a large area with leachate injected into it using injection pipe(s). Since the
MSW exists in unsaturated condition, distribution of injected leachate depends on the relative
permeabilities of leachate and landfill gas. Moreover, MSW is heterogeneous and anisotropic, thus leachate
distribution can be quite complex. The main purpose of this study is to determine the effects of
heterogeneous and anisotropic unsaturated MSW on moisture distribution using DB as LRS. Three different
MSW conditions; homogeneous isotropic (uniform hydraulic properties throughout the depth in the landfill),
heterogeneous isotropic (varying the hydraulic properties with depth, but having isotropic distribution of
hydraulic properties in each layer) and heterogeneous anisotropic (varying the hydraulic properties in
horizontal and vertical direction in each layer), were modeled using a two-phase numerical model. The
model results (saturation levels, pore water pressure distribution, wetted MSW area, and outflow rate
computed in leachate collection and removal system (LCRS)) are compared for the three different MSW
hydraulic conditions.
Mathematical Model
The two-phase flow in unsaturated MSW based on Darcy‘s law can be described by the following two
governing equations:
)( kkww
j
w
r
w
ij
w
i xgPx
kq
(1)
)( kkgg
j
g
r
g
ww
ij
g
i xgPx
kq
(2)
The relative permeabilities are modeled using van Genuchten function as:
2/111aa
e
b
e
w
r SS (3)
aa
e
c
e
g
r SS2/111
(4)
Where: q = flow of fluid; kij = saturated mobility coefficient, which is defined as ratio of intrinsic
permeability to dynamic viscosity; κr = relative permeability for the fluid (function of saturation); μ=
Proceedings of the International Conference on Solid Waste 2011- Moving Towards Sustainable Resource Management,
Hong Kong SAR, P.R. China, 2 – 6 May 2011 693
dynamic viscosity; P = pore pressure; = fluid density; g= gravity; a, b and c are constant parameters for
van Genuchten function; Se = effective saturation and Sr = residual wetting fluid saturation.
For the purpose of this study, a bioreactor landfill model of 100m wide and 20m height is considered. LCRS
is located at bottom of the landfill. A DB 60 m wide, 0.3 m thickness is placed at 5 m above LCRS and is
located in center of the landfill cell (Fig. 1). A typical leachate injection rate of Qi = 26 m3/day is applied.
Figure 1. Landfill model with drainage blanket for leachate recirculation in MSW
Figure 2. Sauration isochrones in (a) homogeneous isotropic MSW; (b) heterogeneous isotropic MSW; (c)
heterogeneous anisotropic MSW and (d) MSW wetted area
Hydraulic properties of MSW include saturated hydraulic conductivity and the soil water characteristics
curve (SWCC) parameters. Three different hydraulic waste conditions are assumed: (1) homogeneous and
isotropic with saturated hydraulic conductivity (ksat) of 1x10-4 cm/s (2) heterogeneous and isotropic with ksat
varying with depth (assumed that MSW is filled in ten layers (Fig. 1), each layer‘s saturated hydraulic
conductivity calculated based on the applied normal pressure per Reddy et al. [1]), and (3) heterogeneous
and anisotropic with the vertical saturated hydraulic conductivity (kv) varying with depth as in the case of
heterogeneous and isotropic case, but horizontal hydraulic conductivity is assumed ten times the kv in each
694 Proceedings of the International Conference on Solid Waste 2011- Moving Towards Sustainable Resource Management,
Hong Kong SAR, P.R. China, 2 – 6 May 2011
layer. The unsaturated hydraulic properties of MSW are adapted from Haydar and Khire [3]. All simulations
are performed to assess leachate distribution until steady-state condition is reached or 4 weeks, whichever is
less. Prior to these simulations, the model was validated based on the previous mathematical modeling
results of Haydar and Khire [3] using a numerical model and assuming homogeneous and isotropic MSW.
Results
In case of homogeneous and isotropic MSW, leachate recirculation reached steady-state condition in 17
days. However, even though the leachate recirculation was continued for four weeks, steady state condition
was not reached in heterogeneous and isotropic case or heterogeneous and anisotropic case. Leachate
recirculation in these two cases was simulated for four weeks. Interestingly, because of heterogeneity of
MSW, the injected leachate in the bottom layers increased the saturated area of MSW (Fig. 2d). The
maximum saturation in all three MSW conditions was 100%. However, the evolution of the saturation
contours was different in these three MSW conditions. Because of the lower permeability of MSW in the
deep layers, the saturated area has increased due to lateral spreading of leachate (Fig. 2b and 2d). In case of
heterogeneous and anisotropic MSW (kh = 10kv), it can be seen that the injected leachate has migrated in
lateral direction more than in vertical downward direction. Therefore, the lateral wetted area has increased
substantially in this case (Fig. 2c and 2d). The maximum pore water pressure developed in the landfill
during the leachate recirculation is plotted in Fig. 3a. Evidently, the pore water pressure is as high as 205
kPa in case of homogeneous and isotropic case, and this value is observed only near the injection pipe in the
DB, while at other locations it was significantly lower. The maximum pore water pressure in case of
heterogeneous and isotropic case increased to 405 kPa which is 97% increase compared to homogeneous
and isotropic case. This large increase in pore water pressure is due to the low permeability MSW in deep
layers that has reduced pore sizes. On the contrary, in case of heterogeneous and anisotropic case, because
of the anisotropy, the pore water pressure developed has reduced to 305 kPa (around 13% decrease
compared to heterogeneous and isotropic case). Because of anisotropic property of MSW, the pore water
pressure developed in the system has dissipated in the horizontal direction and thus the value of pore water
pressure has reduced compared to heterogeneous and isotropic case. However, because of the
heterogeneous MSW, increase of about 70% in the pore water pressure is observed compared to
homogeneous and isotropic case.
Figure 3. (a) Maximum pore water pressure developed in landfill, and (b) Outflow rate in LCRS for
different MSW conditions
Outflow from the LCRS plotted in Fig. 3b shows that the steady-state flow has reached in 17 days in
homogeneous and isotropic case. Steady state is defined as the condition when the inflow is equal to outflow.
In homogeneous and isotropic case, the injected leachate has migrated downward and reached LCRS. The
outflow at steady state condition in homogeneous and isotropic case is 24.6 m3/day/m. On the contrary, in
case of heterogeneous and isotropic case, even though the leachate injection is continued for four weeks, the
steady-state flow did not occur. Because of the low permeability MSW, less leachate is allowed to migrate
towards the LCRS thus after four weeks of recirculation; the outflow rate computed at LCRS is 23.6
Proceedings of the International Conference on Solid Waste 2011- Moving Towards Sustainable Resource Management,
Hong Kong SAR, P.R. China, 2 – 6 May 2011 695
m3/day/m. In the case of heterogeneous and anisotropic case, because of the anisotropic MSW, the injected
leachate has migrated laterally thus has reduced the outflow at LCRS, about 17.7 m3/day/m.
Conclusions
The effect of heterogeneous and anisotropic unsaturated MSW on moisture distribution using DB as LRS in
bioreactor landfill is quantified. Steady-state flow condition is observed only in case of homogeneous and
isotropic case; however, for heterogeneous and isotropic and heterogeneous and anisotropic cases, the
steady-state flow condition was not attained even after continuous leachate recirculation for four weeks.
Further, the results of saturation level, wetted area of MSW, pore water pressure developed, and outflow
rate in LCRS demonstrate the significance of accounting for heterogeneous and anisotropic hydraulic
characteristics of MSW in the design of DBs for effective leachate distribution.
References
[1] Reddy, K.R., Hettiarachchi, H., Parakalla, N., Gangathulasi, J., Bogner, J. and Lagier, T. 2009.
Hydraulic conductivity of MSW in landfills, Journal of Environmental Engineering, 135 : 1-7.
[2] Kulkarni, H.S. and Reddy, K.R. 2010. Modeling of moisture distribution under continuous and
intermittent leachate recirculation in bioreactor landfills. In Proc. 6th International Congress on
Environmental Geotechnics, 2010, Delhi, India, p. 1718.
[3] Haydar, M.M., and Khire, M.V. 2007. ―Leachate recirculation using permeable blankets in
engineered landfills. J. Geotechnical and Geoenvironmental Engrg. 131: 837-847.
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