Experimental Study of Biodegradation of Ethanol and Toluene Vapors

coefficient (k L a) in the well-mixed bioreactor was determined for different conditions
by obtaining the coefficients associated with the best-fit (using Solver in Excel for nonlinear,
least square regression) of the dynamic oxygen concentration data using the
following equation (Merchuk et al., 1990):
C
t
*
−tkLa
( C
0
)( *
* o
− C e − t k
2
e
= Co
−
2
(1 − t * k a)
e
L
L
a * e
−t
/ te
)
(4-4)
where
*
C o2
is the equilibrium (maximum) oxygen concentration in the solution (mg/L),
C t is oxygen concentration in the solution at time t (mg/L), C 0 is initial oxygen
concentration in the solution at t = 0 (mg/L), t is time (s), and t e is response time for the
probe (s).
In order to study the influence of ethanol on k L a, four different concentrations were
used: 0, 1, 3 and 8 g/L. Also, four different impeller speeds (135, 300, 450 and 600
rpm) were studied for their effects on the oxygen mass transfer coefficient. The best
fitting mass transfer coefficients obtained with different impeller speeds and ethanol
concentrations are shown in Figure 4-3. The overall oxygen mass transfer coefficient
increases from 0.0020 to 0.0025 s -1 and from 0.0095 to 0.027 s -1 with increasing ethanol
concentration from 0 to 1 g/L at 135 and 300 rpm, respectively, but remains at the same
level with further increases of ethanol concentration. When the ethanol concentration
was increased from 0 to 8 g/L, k L a increased from 0.015 to 0.049 and from 0.017 to
0.076 s -1 when impeller speeds of 450 and 600 rpm, respectively, were applied. This
demonstrates that these low ethanol concentrations can increase the mass transfer
coefficients by as much as a factor of four. This is a considerable improvement
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