Experimental Study of Biodegradation of Ethanol and Toluene Vapors

Because the feed media are sterile, C x0 = 0. When the system is at steady state
(dC x /dt = 0), then:
μ = D
(5-61)
In a continuous stirred tank bioreactor (CSTR), cells are removed at a rate equal
to their growth rate, and the growth rate of cells is equal to the dilution rate during
steady state operations. This phenomenon allows us to manipulate growth rate as an
independent parameter. Since growth rate is limited by at least one substrate in the
CSTR, a simple description of its performance can be made by substituting the Monod
equation for µ in Equation (5-61):
μ C
m e
μ D
(5-62)
=
=
K
s
+ C
e
If D is set at a value greater than μ , the culture cannot reproduce quickly enough to
m
maintain itself and is washed out. That is, when
dX
μ < D , = ( μ − D)
Cx
< 0 , then
dt
biomass would be washed out. Moreover, when the dissolved oxygen (DO) is higher
than the critical value (DO>
C cr
), the growth rate is independent of the DO value.
When the DO is lower than
C cr
, the growth approaches a first-order rate dependence on
the dissolved-oxygen concentration (Shuler and Kargi, 2002). The oxygen limitation
results in μ < D , thus biomass will also be washed out, and the operation will no longer
be steady state.
Using Equation (5-62) at steady state, when other nutrients are in excess, the
ethanol concentration can be described as a function of the dilution rate for
D < μ and
m
C
O
> C
2 cr
:
130