Environmental Engineering Reference
In-Depth Information
of these higher-molecular-weight PAHs appeared to plateau in the slurry
reactor, whereas degradation was still occurring at the end of the solid phase
reactor experiment. Thus, the added time of treatment may be rewarded by
a greater extent of degradation.
7.1.3.4 The flask-to-field selected treatment option (land farming)
In order to avoid the possibility of long-term passive treatment or the use
of additional active treatment technologies, the initial, rapid phase of PAH
degradation must be expanded and the second, slower phase enhanced. This
will provide a greater and more predictable degradation of HMW PAHs.
Improvements in the degradation of HMW PAHs result from the
enhancement of the metabolic capabilities of natural microbial communi-
ties and the increased availability of the PAHs to the microorganisms. These
two considerations appear to have the greatest effect on the second phase
of degradation. Enhancing metabolic capabilities of microbial communities
will require an understanding of where the deficiencies originate, including
an appreciation for the metabolic pathways used by these organisms.
Enhancing bioavailability will require knowledge of the interactions
between the degrading microorganisms and the availability of the PAHs
from the bound state. To achieve these enhancements, chemicals and micro-
organisms can be added to soil. Any amendment additions must generate
enough cost savings in the end to pay for the additions, an aspect that was
considered carefully in this work. Each of these enhancement strategies is
presented in detail.
7.1.3.5 Microbiological studies
7.1.3.5.1 Isolation/characterization of PAH-degrading bacteria.
Bacterial
isolates that have been enriched for their ability to grow on low-molecu-
lar-weight PAHs (i.e., naphthalene, phenanthrene, fluorene, and, to some
extent, indan, acenaphthene, and anthracene) were studied. The common bac-
terial genera encountered include
Pseudomonas
,
Alicaligenes
,
Mycobacterium
,
Rhodococcus
,
Comamonas
, and
Sphingomonas
. This is a relatively small range of
genera considering the prevalence of PAHs in the environment; however, it
does show that the ability to degrade low-molecular-weight PAHs is common.
Recent studies have emphasized the potential importance of
Mycobacte-
rium
and
Sphingomonas
species (Bastiaens et al., 2000; Bouchez et al., 1995;
Fredrickson et al., 1995; Givindaswami et al., 1995; Kastner et al., 1994; Meyer
et al., 1999), with some indication that
Sphingomonas
strains are more likely
to degrade aqueous phase PAHs and
Mycobacterium
are more likely to
degrade solid PAHs (crystals) because of their hydrophobic cell surfaces
(Bastiaens et al., 2000). Quantitative polymerase chain reaction (PCR) has
been used to identify microorganisms able to degrade PAHs in soil. PCR is
a technique that increases the number of copies of a specific region of DNA.
From a single microorganism, enough DNA can be produced to allow precise
identification of that species. In some soil samples where PAH degradation
