Environmental Engineering Reference
In-Depth Information
However, this type of model is of limited use because it provides an estimate of the total
amount of gas generated and does not provide information on the rate of generation. It also
requires knowledge of the chemical composition of the waste.
F. Other Models
El-Fadel et al. (1996b) developed a mathematical model to simulate the biodegradation of
solid waste and biogas generation in a sanitary landfill. The model was based on bio-kinetic
equations describing the microbial processes, time dependent transport and generation of gas
and heat. The biodegradation process occurred in three stages: hydrolysis, acidogenesis and
methanogenesis. They assumed hydrolysis to be the rate limiting step in the biodegradation
process and was represented by first order kinetics. Monod kinetics were used to simulate the
growth rate of acidogenic and methanogenic biomass. The effect of pH inhibition on the
methanogenic growth rate was also included. In El-Fadel et al. (1997) several runs were
performed to assess the model sensitivity to the hydrolysis rate constant, the kinetics constant
of acidogenic and methanogenic biomass (ยต, k
d
, k
s
) and initial carbon concentrations (solid,
aqueous, acetic acid, acidogenic and methanogenic). They concluded that the hydrolysis rate
is the most important parameter in gas generation in landfills. Gas generation showed greater
sensitivity to the methanogenic kinetics than to the acidogenic kinetics, and the initial
concentrations of acetic acid and methanogenic biomass had a more important impact on gas
generation than aqueous and acidogenic biomass.
In El-Fadel et al. (1996c) the model was used to simulate data from the Mountain View
controlled landfill. The results of the model showed good agreement with the field data. They
concluded that the model could be used to predict the rate and total production of biogases in
landfills.
Al-Yousfi et al. (1998) developed a numerical (PITTLEACH) model to predict the
leachate quality and quantity and biogas generation from municipal solid waste, for both
single pass and leachate recirculation. The model consisted of four phases. In the first phase,
the water budget method was used to estimate the net percolation rate of water flow into the
waste layer. In the second phase, leachate generation and transport rates were estimated by
using the theory of moisture flow through unsaturated porous media. The third step was to
simulate the anaerobic process involved in landfill stabilization, including hydrolysis, acid
formation and methane fermentation. In the fourth phase, the effect of acids on the pH was
modeled. The model was calibrated with experiments done by Pohland et al. (1992) and Yari
(1986). The model results were close to the experimental results for both scenarios (single
pass and leachate recirculation).
Pareek et al. (1999
)
developed a mathematical model to predict the methane and carbon
dioxide production from landfill reactors operated under sulfate reducing and methane
producing conditions. The model was based on biochemical processes responsible for the
degradation of solid waste in landfills. These processes were hydrolysis, acidogenesis,
methanogenesis and sulfidogenesis. Hydrolysis was assumed to be the rate limiting step in the
process and followed first order reaction, whereas the Monod kinetics were applied for the
growth of acidogenic and methanogenic
bacteria. A multiplicative model was applied to
estimate the growth rate of the sulfidogenic bacteria. The values of kinetics required were
taken from the literature. The model was calibrated with four experiments run for 700 days.
