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.