Biomedical Engineering Reference
In-Depth Information
Bioaugmentation can be used successfully to decrease the lag period for cometabolic treatment
in situ using Approaches II, III and IV. Field studies of McCarty et al. ( 1998b ) showed that the
stimulation of phenol or toluene utilizing microorganisms could be greatly reduced through the
addition of PR1 301 (Approach II). This finding has practical implications when the spread of
toluene or phenol are of concern. Semprini et al. ( 2007b , 2009 ) also showed that adding a
butane-utilizing culture reduced the lag time before effective cometabolism was measured
(Approach III). This process is easy to implement because large quantities of microorganisms
are not needed. In addition, strains can be enriched from the subsurface of the site, increasing
the likelihood that the introduced bacteria will survive in situ.
Approach I, adding strains for their biocatalytic transformation potential, has had some
success but has not proceeded to large-scale implementation. Field evaluations reviewed here
stopped at the pilot-scale stage. One of the limitations of Approach I is the need for effective
oxygen delivery when high concentrations of cells are added to the subsurface. Both studies
with Methylosinus trichosporium OB3b (Duba et al., 1996 ) and Burkholderia cepacia ENV435
(Steffan et al., 1999 ) resulted in lower transformation yields than demonstrated under labora-
tory conditions. It is possible that oxygen limitations can partly be responsible. It is not clear
from the demonstrations performed to date that this approach, and the situations where this
technology has potential, have been well established, or whether there is a niche where it can be
practically applied. One of the most interesting applications thus far was the test in which
cultures were added during pneumatic fracturing. The approach of adding microbes during the
fracturing process is a novel and interesting application of the process.
Research needs include determining how best to apply Approach I and whether there are
situations in which the process might have both application and cost advantages over competing
processes, such as in situ chemical oxidation (ISCO). An excellent review of ISCO is provided
by Huling and Pivetz ( 2006 ) and Seigrist et al. ( 2011 ). Reviews such as this one are helpful
when evaluating competing processes. For example, it would be valuable to know the cost of
delivering cells compared to Fentons Reagent to achieve the same degree of treatment.
Approach II, the least successful thus far, involves adding strains that express TMO
constitutively, with biostimulation accomplished by adding a benign substrate such as lactate.
The mutant PR1 301 of Burkholderia cepacia G4 has been the most studied for this purpose.
Studies performed by McCarty et al. ( 1998b ) showed that enhanced cometabolism of TCE could
not be maintained for extended periods, even with the continuous addition of microorganisms
(Figure 8.5 ). Competition for the easily utilized substrate by other organisms, predation and
transformation product toxicity potentially contributed to the poor performance.
Research on Approach II might focus on protecting the bioaugmented strains from
predation and competition for substrate utilization. As reviewed by Gentry et al. ( 2004 ), one
potential method is to encapsulate the bacteria in a material that creates a non-toxic environ-
ment through which gases and liquids can diffuse. Materials such as alginate, agarose, and
gellan gums might be used. The benign substrate also could be incorporated into the encapsu-
lant. Substrates, like phenol, that can be potentially formed slowly via a hydrolysis reaction as a
slow release substrate, also might be useful for maintaining the introduced bacteria for longer
times.
There are few peer-reviewed publications on Approach III, bioaugmentation with strains
selected for their enhanced cometabolic abilities. This approach requires a lower mass of cells
than Approach I and II, because growth occurs in situ through substrate addition. Recently,
Semprini et al. ( 2009 ) showed that transformation of 1,1,1-TCA could be enhanced by bioaug-
mentation with a Rhodococcus sp. butane-utilizing strain. The approach could be implemented
with previously demonstrated approaches for remediation, such as the recirculation well
technology used by McCarty et al. ( 1998a ) for the stimulation of indigenous toluene utilizers
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