Environmental Engineering Reference
In-Depth Information
spiral-shaped spirillium. Rods usually have diameters of 0.5 to 1 μm and lengths
of 3-5 μm. The diameter of spherical cells varies from 0.2 to 2 μm. Spiral-shaped
cells range from 0.3 to 5 μm in diameter and 6 to 15 μm in length. The cells grow in
clusters, chains, or in single form and may or may not be motile. The substrate of
the bacteria must be soluble. In most cases, classiication is according to the genus
and species (e.g.,
Pseudomonas aeruginosa
and
Bacillus subtilis
). Some of the most
common species are
Pseudomonas
,
Arthrobacter
,
Bacillus
,
Acinetobacter
,
Micrococcus
,
Vibrio
,
Achromobacter
,
Brevibacterium
,
Flavobacterium
, and
Corynebacterium
. Within
each species, we will have various strains. Each of these can behave differently.
Some strains can survive in certain conditions that others cannot. The ones that
are better adapted will survive. For survival, strains called mutants originate due
to problems in the genetic copying mechanisms. Some species are dependent on
other species for survival. Degradation of chemicals to an intermediate stage by
one species of bacteria may be required for the growth of another species that
utilizes the intermediate.
9.6.1.1 Alkanes, Alkenes, and Cycloalkanes
Alkanes, alkenes, and cycloalkanes, among others (PAHs, asphaltenes, etc.), are compo-
nents of petroleum hydrocarbons (PHCs). Low-molecular-weight alkanes are most easily
degraded by microorganisms. As the chain length increases from C
20
to C
40
, hydrophobic-
ity increases and both solubility and biodegradation rates decrease. Alkenes with a double
bond on the irst carbon may be more easily degradable than those alkenes with the double
bond at other positions (Pitter and Chudoba, 1990). Cycloalkanes are not as degradable as
alkanes due to their cyclic structure, and their biodegradability decreases as the number
of rings increase.
9.6.1.2 Polycyclic, Polynuclear Aromatic Hydrocarbons
As with cycloalkanes, the compounds become more dificult to degrade as the number of
rings of PAHs increases. This is due to decreasing volatility and solubility and increased
sorption properties of these compounds. They are degraded one ring at a time in a manner
similar to single ring aromatics.
9.6.1.3 Benzene, Toluene, Ethylbenzene, and Xylene
Benzene, toluene, ethylbenzene, and xylene (BTEX) are volatile, water-soluble com-
ponents of gasoline. Aromatic compounds with benzene structures are more dificult
to degrade than cycloalkanes. Aerobic degradation of all components of BTEX occurs
rapidly when oxygen is present. Aromatic compounds can also be degraded under
anaerobic conditions to phenols or organic acids to fatty acids to methane and carbon
dioxide (Grbic-Galic, 1990). Degradation is less assured and is slower than under aerobic
conditions.
9.6.1.4 Methyl Tert-Butyl Ether
Methyl
tert
-butyl ether (MTBE), which is an additive to gasoline, is highly resistant to bio-
degradation. It is reactive with microbial membranes.
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