Environmental Engineering Reference
In-Depth Information
The above equations are also useful to represent the substrate uptake and utilization
by a microbial cell, if we replace the Michaelis-Menten kinetics by Monod kinetics.
This forms the basis of the kinetic analysis of biological floc in activated sludge
processes for wastewater treatment. Powell (1967) developed a lumped parameter
model for substrate utilization and kinetics. The rate of substrate utilization was given
by the following equation:
⎡
⎣
1
/
2
⎤
⎦
, (6.261)
1
μ
max
y
s
2
1
2
+ σ +
[
S
0
]
K
s
+ σ +
[
S
0
]
K
s
4
[
]
0
K
s
S
r
s
=
−
+
σ
σ = μ
max
δ
R
2
/
3
y
s
K
s
D
s
(R
+ δ
)
.
where
is the thickness of the stagnant zone of liquid
around a cell of radius
R
. [S]
0
is the substrate concentration in the bulk fluid.
δ
μ
max
and
K
s
are the Monod kinetics constants. The above equation is identical to that derived
earlier using Michaelis-Menten kinetics.
6.5.1.5
In Situ
Subsoil Bioremediation
An important application of environmental bioengineering is the use of special
microbes to degrade recalcitrant and persistent chemicals in the subsurface soil and
sediment environment. Primarily bacteria, algae, and fungi bring this about. Bacteria
arefoundinlargenumbersinthesoil;theyaresmallinsizecomparedwithfungi.Fungi
are filamentous in nature and account for a large mass of microorganisms in the soil.
Microbesinthesoilcansurvivewithorwithoutoxygen.Accordingly,soilmicrobes
are classified as
aerobes
(requiring oxygen) and
anaerobes
(require no oxygen).
Facultativeanaerobes
arethosethatcangrowineitherenvironment.Aerobicmicrobes
break down chemicals into CO
2
and H
2
O. Anaerobes break down chemicals into
completely oxidized species in addition to some CO
2
,H
2
O, and other species (H
2
S,
CH
4
,SO
2
, etc.). The biotransformation of chemicals in soil involves several steps,
enzymes, and species of many types. Since enzymes are specific to a chemical, the
biodegradation of complex mixtures requires that several species co-exist and exert
their influence on individual chemicals in the mixture. Microbes act in two ways:
(i) they imbibe chemicals and the
intracellular enzymes
act to degrade the pollutant,
and (ii) the organism excretes enzymes (
extracellular enzymes
) which subsequently
catalyze the degradation.
The general pathway of degradation of many types of pollutants by specific
microbes has been elucidated over the past several decades. These aspects are
described extensively in a recent topic (Alexander, 1994). The specific mechanisms
are not the topic of this chapter, whereas we are more interested in the equilibrium
and kinetics of biodegradation in the subsoil.
In the earlier section, we noted the kinetics of enzyme catalysis and cell growth
in the aqueous environment. In the subsurface soil environment, which is a two-
phase system (soil
porewater), we should expect some similarities with the aqueous
system. The microbes are attached to the soil surface and they act as a fixed-film
(immobilized cell) reactor.
In the subsurface soil environment, the degradation of pollutants and micro-
bial growth are most conveniently handled using a Monod-type expression (see
+
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