Biomedical Engineering Reference
In-Depth Information
The cost for bioaugmentation includes the culture itself, as well as the labor and equipment
needed to inject the culture into the subsurface. It is important to realize that the cost is often lower
to bioaugment at the same time that electron donors are added, rather than to bioaugment at a later
date when it is proven to be necessary, particularly if separate mobilizations and drilling time are
needed. Bioaugmentation itself (i.e., the supplemental addition of dechlorinators) is not usually a
large cost item relative to the overall site remediation cost (Chapter 11 ), and bioaugmenting done
concurrently with biostimulating often can reduce the overall life cycle cost. However, project
managers need to justify the additional costs, particularly if there is a perception that bioaugmen-
tation is not truly required or is being used just as an insurance policy.
The difficulty in justifying the costs results from the uncertainty in predicting the benefits.
Biostimulation may be sufficient, and it requires less initial cost. So managers often are faced
with a choice between biostimulation only, relying on “more time and more electrons” to
eventually attain optimal performance (Koenigsberg et al., 2003 ), or bioaugmentation for
faster results and greater certainty, even if it is not absolutely required.
This chapter is intended to help site managers in the decision-making process regarding the
use of biostimulation or bioaugmentation at a given site. The discussion is structured around a
decision flowchart based on a series of diagnostic questions and parameters critical in deter-
mining whether to biostimulate and/or bioaugment. After introducing the decision guidance
and the key sources of uncertainty involved in the decision making, the remainder of the
chapter describes these diagnostic questions, parameters to be considered/monitored and key
sources of information useful in answering them. The final section summarizes key issues for
managers when deciding whether to bioaugment a specific site.
4.2 NEED FOR DECISION GUIDANCE
Bioaugmentation with Dhc has been successfully implemented at hundreds of sites
(see Chapter 1 ) . Many times the decision has been an easy one, but at other sites it has been
more difficult because of the uncertainties involved. The uncertainties ultimately stem from the
low numbers, slow growth, patchy distribution and genetic variability of the Dhc strains in the
subsurface. As a result, it can be difficult to be certain that bioaugmentation is necessary at a
particular site, or to predict the lag time before measurable ethene production occurs.
Even persistent stalling at cis -DCE or VC after biostimulation is not conclusive proof that
bioaugmentation is needed. There are several other possible explanations for stalling. These
include: (1) unknown sources providing a constant feed of parent compounds (TCE and/or
PCE); (2) the parent compounds (PCE and TCE) being dechlorinated faster than the daughter
products (DCE and VC) causing a temporary daughter product accumulation (Cupples et al.,
2004b ); (3) the differences in water solubility (VC > DCE > TCE > PCE) making the daugh-
ter products more prevalent in the dissolved phase; (4) unfavorable geochemical conditions,
such as acidic pH inhibiting complete dechlorination (Vainberg et al., 2006 ); and (5) an excess
of iron shunting electrons away from DCE and therefore inhibiting later stage dechlorination
(Koenigsberg et al., 2002 ).
One of the most useful diagnostic analyses for bioaugmentation decisions is the direct
measurement of Dhc numbers in the groundwater. But these measurements may not provide
definitive answers in every case. For example, the lack of detectable Dhc or key biomarkers
may be misleading because the organisms can be sparsely distributed and the samples analyzed
may not be representative (Koenigsberg et al., 2003 ). On the other hand, detection of such
biomarkers is not necessarily proof that biostimulation alone will be effective, because Dhc
capable of VC reduction can be eliminated by early stage dechlorinators that outcompete
Dhc for hydrogen (Becker, 2006 ; Huang and Becker, 2009 ).
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