Biomedical Engineering Reference
In-Depth Information
systems (5-9) generally reflects the buffering capacity of the framework
carbonate or silicate minerals that make up groundwater aquifers
(Chapelle, 1992; King et al., 1992). This range of pH values in aquifers
is adequate for the biodegradation of methanol by indigenous sub-
surface microorganisms since most of the methanol-utilizing bacteria
that have been isolated from natural sources have been shown to grow in
the neutral pH range (Komagata, 1990). Even if the pH of groundwater
were to deviate significantly from the neutral pH range, some strains of
methanol-utilizing microorganisms have been shown to prefer acidic
pH values ranging from 2.0 to 5.5, while others have been shown to
prefer alkaline pH values of 7.0-9.5 (Komagata, 1990).
The temperature of an environment is also an important factor for the
growth and activity of microorganisms. Metabolism of subsurface
populations accelerates with increased temperatures up to an optimum
value at which growth is maximized. Most of the bacteria present in
subsurface environments operate most effectively in a temperature
range of 20-40
C. This is the range of many natural environments
(Chapelle, 1992) since subsurface temperatures within 100 m of the
surface are typically within 2
C of the mean annual surface tempera-
ture. Temperature should therefore not pose a significant problem for
the biodegradation of methanol by indigenous subsurface microorgan-
isms even at temperatures as low as 10
C.
R
EPORTS OF
M
ETHANOL
B
IODEGRADATION
U
NDER
V
ARIOUS
R
EDOX
C
ONDI-
TIONS
Simple alcohols such as methanol and ethanol can be degraded
in both aerobic and anaerobic environments at rates much faster than
gasoline components, such as benzene (Table 2.5). It is important to
note in Table 2.5 the difference between laboratory-derived and field-
derived degradation rates. Laboratory rates are often higher (an order of
magnitude or more) than field-derived rates due to a combination of an
elevated laboratory temperature and a larger microbial density in the
laboratory growth medium. For example, all the reported benzene rates
are laboratory-derived and, thus, likely represent faster degradation
rates than would be observed in the field.
Only large concentrations (
>
100,000 ppm) of alcohols are generally
considered to be toxic to most microorganisms and are therefore not
