Environmental Engineering Reference
In-Depth Information
TABLE 3.18 (continued)
Summary of Studies of Biodegradation of 1,4-Dioxane
Vainberg et al. (2006): Biodegradation of DX with sucrose, lactate, yeast extract, 2-propanol, and propane, by
Pseudonocardia
sp. Strains ENV478 and ENV425
Fungi and other
Skinner (2007):
Graphium
sp., a i lamentous fungus, degrades DX by cometabolism after growth on THF or gaseous
alkanes, catalyzed by cytochrome P450 monooxygenase
Kelley et al. (2001): phytoremediation with 1-butanol
Notes:
DX = 1,4-dioxane; THF = tetrahydrofuran.
where
r
is the dioxane biodegradation rate,
μ
max
is the maximum growth rate,
X
is the bacteria con-
centration,
S
is the dioxane concentration,
Y
is the growth yield on dioxane,
K
s
is the half-saturation
constant, and
r
/
X
is the specii c dioxane biodegradation rate.
The minimum concentration that will maintain growth in a biological treatment system under
steady-state conditions (
S
S,min
; Alexander, 1994) is
Kb
s
S
=
.
(3.41)
S,min
m-
b
max
Table 3.19
summarizes published parameters of biodegradation kinetics for bacteria capable of
metabolizing 1,4-dioxane.
Information on kinetics is extremely important because it characterizes the concentration of the
chemical remaining at any time, permits prediction of the levels likely to be present at some future
time, and allows assessment of whether the chemical will be eliminated before it is transported to a
site at which exposure will occur (Alexander, 1994).
The relationship between bacterial growth rate and the concentration of the carbon substrate
increases, but only at low substrate concentrations. A concentration is reached above which the
growth rate falls as the substrate level rises further. This decline is a result of the antimicrobial
action (i.e., toxicity to microbes) of the chemical substrate at high concentrations (Alexander,
1994).
The Monod equation describes the growth of a population on a chemical substrate constrained
by the toxicity to the population of that substrate in high concentrations:
m
S
S
KS
m
max
max
m=
ª
,
(3.42)
+
KSSK
++
(/)
2
s
s
1
where
μ
max
is its maximum specii c growth rate,
S
is
the concentration of the chemical substrate,
K
s
is a constant that represents the concentration of the
chemical substrate at which the rate of growth is half the maximum rate, and
K
1
is the inhibition
constant that represents the suppression of the growth rate by the toxic substrate (Alexander, 1994).
This equation may apply to 1,4-dioxane, whose metabolic by-product, 2-hydroxyethoxyacetic acid
(2HEAA), is toxic to microorganisms, as described in research by Vainberg et al. (2006) (see
Section 7.6.3).
Bacterial growth requires that the substrate be present at sufi ciently high concentrations relative
to the cell population that acts on the substrate to allow the population to continue doubling. When
cell density is higher than the substrate concentration can sustain, growth is limited.
μ
is the specii c growth rate of the bacterium,
Search WWH ::
Custom Search