21    THE EFFECTS OF MASS TRANSFER ON LANDFILL
STABILIZATION RATES
James J. Noble, Associate Professor of Chemical Engineering
Thelma Nunez-McNally, Chemical Research Engineer
Berrin Tansel, Environmental Research Engineer
Center for Environmental Management
Tufts University
Medford, Massachusetts 02155
INTRODUCTION
Rates of municipal solid waste (MSW) decomposition and attendant sanitary landfill stabilization
are often seen to be quite slow. Indeed it is not uncommon to qualitatively gauge the extent of
biodegradation in landfills by the legibility of buried newspapers. These may be recovered essentially
intact after 20-30 years. Recent archaeological excavations of sanitary landfills by Rathje1 have also
shown that when 10 year old rubbish is exhumed, it may appear to be mummified or "entombed".
The long time scales for landfill stabilization stand in marked contrast to the fairly short time scales
for decomposition demonstrated by the myriad of published papers on biomass conversion and the
extracellular enzymatic hydrolysis of cellulosic materials in the laboratory. Three important reasons
that landfills decompose so slowly are: 1) lack of sufficient moisture for optimal enzyme activity; 2)
imbalance between the production of organic acids by acid-forming bacteria and the conversion of
these acids into methane by methanogenic bacteria; and 3) mass transfer limitations in the landfill
environment.
The main purpose of this chapter is to speculate on the role of mass transfer limitations in landfills
and to discuss possible consequences. It is argued that under certain circumstances mass transfer is the
rate controlling step in landfill stabilization and that detailed knowledge of metabolic pathways for
the microorganisms may not be always necessary for predictive purposes. This work is part of a larger
study on landfill dynamics at Tufts University, Center for Environmental Management, which seeks
to develop a first-round systems model for landfill behavior.
At the outset it is important to clarify our view that the rate limiting step for biodegradation in a
landfill may change as a function of landfill age. Thus one may argue that since MSW includes a small
readily soluble fraction, the initial rate limiting step might be methane formation because of insufficient time to develop viable methanogen populations. To fix ideas better, the readily biodegradable
material might comprise 5% of the MSW and convert completely to methane in 2 to 3 years. This
behavior may well be that observed in lysimeter studies of several years duration.2 Our focus here,
however, is the next phase of degradation when the more recalcitrant and insoluble lignocellulosic
material would become the most biodegradable remaining substrate. In this latter period, cellulose
hydrolysis then becomes rate limiting with the resulting 15-50 year degradation time scales. This view
has been discussed in a recent experimental bench study of landfill gas production enhancement.3
In what follows we review the role of moisture content in landfill biodegradation and describe the
microporous structure of lignocellulosic materials. The latter greatly affects moisture distribution and
mass transfer in the landfill. Lastly some estimates of the time scales for biodegradation based on
mass transfer control are calculated. This analysis follows obvious parallels with heterogeneous
catalysis in chemical engineering systems.
THE ROLE OF LANDFILL MOISTURE
By way of review, the primary environmental variables affecting landfill stabilization rates are:4
1. Moisture content
2. pH
3. Nutrient content
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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