Environmental Engineering Reference
In-Depth Information
3.
M
ATHEMATICAL
M
ODELS FOR
L
ANDFILL
G
AS
P
RODUCTION
P
REDICTION
Mathematical models for LFG production prediction are tools used to estimate, along the
time, gas or methane production from a given amount of waste starting from some input data
supplied by the model user.
In technical literature several models have been proposed (EMCON, 1980) (Boyle, 1976)
(Ham, 1979) (Halvadakis, 1983) (Findikakis, 1988) (Christensen et al., 1996) (Swarbrick and
Lethlean, 1995) (Young, 1995) (Bonori et al., 2001) in order to both estimate the potential
amount of LFG that can be produced from a given amount of waste and forecasting the
temporal evolution of LFG production.
The models proposed in literature can be classified into: empirical models, stoichiometric
models, biochemical models and ecological models.
Empirical models are based on black box approach, correlating input, related to typology
of landfilled waste, and output related to LFG production, in reference to collected data from
existing landfills and experimental data. These kind of models are very site and data specific.
Stoichiometric models are based on a global stoichiometric reaction, in which the
reactant is waste, represented by an empirical chemical formula, and the products include
methane and carbon dioxide. Stoichiometric models differ in the selected stoichiometric
reaction, in waste components considered and whether they include or not cellular growth.
Obtainable results are generally overestimated, since actual biodegradation efficiency is not
taken into account.
Biochemical models are based on biodegradable substrate removal equations, using more
or less complex kinetics, considering more or less substrates and including or not the
parameters influencing the degradation (temperature, moisture, compaction, etc).
Ecological models describes the dynamic coexistence of the different bacterial
populations which compete in the waste degradation. Actually these are the only models
which are able to highlight nutrients imbalances and substrate characteristics, but their
application is quite complex due to the non homogeneous characteristics of landfills and
moreover it is too onerous in the frame of LFG collection and recovery.
However, quite often the proposed models are a mixing of different elements, mainly
using a stoichiometric sub-model which uses waste composition as input, supplying the
amount of biodegradable compounds as output, and a biochemical-kinetic sub-model which
supplies the evolution during the time of LFG from biodegradable compounds, on the basis of
some parameters as temperature, moisture, etc.
In this chapter the attention is focused on four previously proposed models which are: the
Triangular model (Tchobanoglous et al., 1993; Bonori et al., 2001); the Scholl Canyon
equation model (Department of the Army U.S., 1995); the LandGEM model (EPA, 2005) and
a modified version of first order decay model (Van Zanten and Scheepers 1995).
The Triangular Model
In the triangular model, a linear growth of the LFG production rate is assumed until it
reaches a peak after which a linear decrement starts (Tchobanoglous et al., 1993), as shown in
