Environmental Engineering Reference
In-Depth Information
and should be free of other contaminants, such as oils and heavy metals.
Although more heavily chlorinated PCB mixtures, such as Aroclor 1260,
could be treated, sequential inoculations would be necessary to obtain
extensive enough dechlorination, and this would add to the time required
for the anaerobic phase. Oils can be expected to slow both the anaerobic
and aerobic steps by serving as a phase, to which the PCBs can partition,
thus decreasing their availability to the microorganisms. The anaerobic
step is relatively immune to heavy metals because under anaerobic condi-
tions, they are precipitated as sulfides and thus not bioavailable. However,
they may be toxic when conditions are made aerobic for the second stage
treatment.
Also, soil should have a moderate (2 to 3%) amount of organic carbon
to provide substrates for the microorganisms. If the organic carbon level is
much higher, sorption of PCBs to the organic matter may begin to limit PCB
bioavailability, and growth of non-PCB-dechlorinating and -degrading
microorganisms could be favored (both anaerobic and aerobic stages). A
deficiency of organic carbon can easily be made up by the addition, prefer-
ably, of a simple carbon source, such as ethanol or trypticase soy broth, that
does not bind PCBs.
Many PCB-contaminated sites (Table 6.4) were evaluated. However, for
various reasons these sites were found unsuitable for our field test. Finally,
a GE site, located in Rome, GA, was found suitable for collecting the field
samples. Several samples were analyzed for PCB profile and contamination
levels (Table 6.5). The Aroclor 1242-contaminated soil from Rome, GA, met
all of the criteria mentioned above and was used for the pilot-scale evaluation
of the two-phase PCB bioremediation.
The pilot-scale reactor treatment sequence involves an anaerobic
phase, designed to dechlorinate the higher-chlorinated congeners, fol-
lowed by an aerobic phase, to treat the accumulated lower-chlorinated
congeners. To enhance the anaerobic treatment process, river sediment
inoculum, ferrous sulfate, and surfactants were added to the reactors.
During the aerobic treatment phase, genetically engineered microorgan-
isms were added using a vermiculite carrier along with a nutrient/sur-
factant solution (Figure 6.26).
Traditionally, low solids slurry reactors are used for creating the reduced
environment for PCB dechlorination. In these reactors mixing is needed for
maintaining the uniform moisture level throughout the system as well as
for uniformly distributing the nutrient amendments. Achieving a well-mixed
system is dependent on operational capacity of mixing systems, which in
turn is dependent on the slurry concentration. Most of the traditional mixing
systems can mix only slurries with consistencies up to 10 to 20% (sol-
ids:water, w/w). Once the treatment goals are achieved, the dewatering costs
become prohibitive for the disposal/reuse of the solid material. To overcome
these operational limitations, this study focuses on the economic feasibility
and cost-benefit analysis of three bioremediation options with different sol-
ids loadings.
