58    THE EFFECT OF CAPILLARITY ON MOISTURE PROFILES
IN LANDFILLS
James J. Noble, Adjunct Associate Professor
Girish M. Nair, Chemical Research Engineer
Center for Environmental Management
Tufts University
Medford, Massachusetts 02155
INTRODUCTION
Moisture content is probably the single most important variable affecting the rate of biodegrada-
tion of municipal solid waste (MSW) in a sanitary landfill.1'2 The water in a landfill has many critical
functions. It is a reactant in the first step of waste decomposition (hydrolysis of lignocellulosic
materials), and a transport medium by which the nutrients reach the bacteria and the enzymes reach
the substrate. Water also helps in modifying the conformational structure of enzymes, in dissolving
metabolites, in exposing more of the substrate surface area to microbial attack, and in controlling cell
turgidity.2,3 Prediction of landfill biodegradation rates, therefore, requires estimates of the moisture
content and its distribution in the waste layer.
Waste-layer moisture content, which may vary with depth, can be predicted by two approaches.
One is to simulate moisture transport as unsaturated flow in a porous medium. In this approach, two
fluxes govern moisture flow: the Darcy flux which always acts in the direction of gravity, and the
diffusivity flux (due to capillarity) which acts in the direction of decreasing moisture content. The
diffusivity flux can under certain conditions oppose the Darcy flux, leading to a unique upward flow.
The other approach to predicting moisture content is based on water-balance principles. It neglects the
capillarity effect, and bases moisture flow on the Darcy flux alone.
Capillarity can be shown to play an important role in determining the lateral spread rate in
horizontal infiltration problems. Numerical solutions of the resulting non-linear diffusion equation
have been presented elsewhere.4 The purpose of the present study is to evaluate the importance of
capillarity on vertical moisture profiles in landfills and to determine the conditions under which
modelling of moisture transport should take capillarity into account.
MATHEMATICAL MODELS
Unsaturated-Flow Models
The unsaturated-flow approach to modelling water transport in landfills is based on the Richards5
equation, which describes one-phase flow through a porous medium and does not account for any
hydrodynamic resistance due to the entrapped gases in the waste layer. Two-phase flow studies by
Curtis and Watson6 and Phuc and Morel-Seytoux7 determined that the effect of soil air (gases) on
water infiltration rates is more significant when there is an air-impervious bottom layer and to lateral
passage enabling air to escape.
The Richards equation can be written as5
68/6t +  dq/Sz = 0, (1)
where       q  =  K(0) - D(8)(o$/6z), (2)
and 6    = volumetric moisture content,
t    =  time (days),
z    =  depth from surface (feet),
q    =  vertical moisture flux (Darcy velocity) (feet/day),
K(0)  =  hydraulic conductivity (feet/day),
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
545