Biomedical Engineering Reference
In-Depth Information
Another uncertainty is the frequency of incompetent sites - the probability that a given site will
require bioaugmentation for complete dechlorination in a reasonable timeframe. Practitioners
estimate that roughly 5-10% of all sites contaminated with chlorinated ethenes do not have the
microbial capacity for complete dechlorination (e.g., Koenigsberg et al.,
2002
), although there
has been no thorough study of the frequency of incompetent sites. However, recent surveys
indicate that complete dechlorination will eventually occur under favorable conditions at most, but
not all, solvent contaminated sites (Lu et al.,
2009
; van der Zaan et al.,
2010
).
While such uncertainties complicate bioaugmentation feasibility assessments, the scientific
foundation and experience base has increased dramatically in recent years (ESTCP,
2005
).
Researchers and practitioners have learned enough to provide useful guidance for those facing
bioaugmentation decisions at specific sites. The decision guidance presented in this chapter
builds on that existing knowledge and experience, provides practical recommendations and
discusses the rationale for them.
4.3 DECISION GUIDANCE OVERVIEW
This guidance is not intended to be prescriptive in nature. Rather it presents a systematic
approach to making a decision that generally requires both technical information on specific
site conditions, as well as non-technical judgments regarding risk tolerance and economic
assumptions. The guidance is depicted in Figure
4.1
as a flow chart, and begins with questions
intended as “off ramps,” to allow rapid screening of sites where the decision is relatively easy.
Later questions require more detailed information and testing, and the final questions require
consideration of management objectives and development of comparative cost estimates.
The guidance assumes that
in situ
bioremediation has been selected as a feasible technology
based on remedial investigations and feasibility studies. Although presented as a decision flow,
most or all of the following questions will have to be addressed at most sites.
4.4 IS COMPLETE DECHLORINATION OCCURRING?
The initial question in evaluating bioaugmentation is to determine whether complete
dechlorination is already occurring. Even if the rate and extent are not optimal, if
complete dechlorination (production of innocuous nonchlorinated compounds, notably ethene
and ethane) is already occurring, bioaugmentation is unlikely to be beneficial. Several
parameters deserve attention when assessing this issue, notably:
Oxidation-Reduction Potential (ORP)
. At some sites, there will be electron donors present
already (naturally or due to prior deliberate additions) and the ORP will be sufficiently reducing
to promote reductive dechlorination. Examples may include landfill sites or mixed-waste sites
with chlorinated solvents as well as reduced organic compounds. Biodegradation of these
reduced compounds can deplete any available oxygen as well as other electron acceptors and
cause the subsurface to become sufficiently anaerobic for reductive dechlorination to occur
(i.e.,
<
200 mV).
The influx of oxygen and other electron acceptors also should be considered when
evaluating inhibition. The electron acceptor influx may be so great that sufficiently reducing
conditions cannot be established, or cannot be sustained at reasonable expense. Maintaining an
ORP below about
100 millivolts [mV], or preferably
<
200 mV is
preferable (i.e., the geochemistry should indicate that the subsurface is at least sulfate-
reducing, and ideally some methane will be generated).
100 mV is necessary for complete dechlorination, and below
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