Biomedical Engineering Reference
In-Depth Information
There are many organic substrates that can be naturally degraded and fermented in the
subsurface to generate hydrogen. Examples of easily fermentable organic substrates include
alcohols, low-molecular-weight fatty acids (e.g., lactate), carbohydrates (e.g., sugars), vegetable
oils and plant debris (e.g., mulch). The substrates most commonly added for enhanced anaero-
bic bioremediation include lactate, molasses, Hydrogen Release Compound (HRC
®
), mulch
and emulsified vegetable oils. Substrates used less frequently include ethanol, methanol,
benzoate, butyrate, high-fructose corn syrup (HFCS), whey, chitin and gaseous hydrogen
(Parsons Corporation,
2004
). The physical nature of the substrate (i.e., liquid, solid or gas)
will influence the frequency of addition, the addition technique, and the potential system
configurations.
The
Dhc
-containing culture KB-1
®
has been demonstrated to work with most commonly
used electron donors, including sugars (e.g., glucose, molasses), alcohols (e.g., methanol,
ethanol), organic acids (e.g., lactate), vegetable oils (canola), emulsified oils (e.g.,
readily utilizes hydrogen but not formate, acetate, lactate, pyruvate, propionate, glucose,
ethanol or yeast extract as an electron donor (He et al.,
2003
). SDC-9
TM
also has been applied
with a wide variety of electron donors. Although hydrogen is used by methanogenic popula-
tions, several studies suggest that
Dhc
microorganisms competitively utilize hydrogen at
concentrations below those supporting methanogenesis (Smatlak et al.,
1996
; Yang and
McCarty,
1998
;L¨ffler et al.,
1999
).
The choice of electron donor will depend on the method of application and cost considera-
tions. For example, biobarriers typically lend themselves to the use of emulsified vegetable oil
or mulch whereas active recirculation systems favor soluble electron donors, such as lactate.
Regardless of the electron donor selected, sufficient electron donor must be provided to meet
the demand of the competing electron acceptors (most notably sulfate) so that sufficient
electron donor is available for dechlorination reactions. The addition of electron donor is
often required to reduce the ORP of the aquifer to the desired range for complete reductive
dechlorination (generally less than
100 mV).
Electron donor is typically added prior to or during bioaugmentation to provide a source of
fermentable substrate and lower the ORP of the groundwater. Depending on the bioremediation
system configuration, the electron donor can be added using either extracted groundwater or
municipal water. If either is oxygenated, it is generally recommended to wait for reducing
conditions to be established
in situ
after the electron donor is injected before bioaugmenting.
The time required for aquifer conditions to be appropriate for bioaugmentation after electron
donor addition varies from site to site; however, typical lag times are 4-8 weeks. Extracted water
can be reduced
ex situ
by amending a tank of water with a soluble electron donor and allowing time
(typically days to weeks) for the biomass to consume oxygen and lower the ORP to below
75 to
75mV.
Chemical reductants, such as sodium sulfite, also may be used to reduce the extracted water. It is
recommended that small-scale tests be conducted prior to field implementation to better estimate
the time required to produce anaerobic water. Another approach involves the addition of the
bioaugmentation culture in a “donut” of anaerobic water part way through the injection of the
electron donor as discussed further in Section
5.3.4
.
5.3.2.2 Selection and Addition of Buffers
As mentioned earlier,
Dhc
is most active between pH 6 and 8.3. Prior to bioaugmentation,
it is important to establish the aquifer pH in this range. To maintain growth and activity of
Dhc
following bioaugmentation, it is also necessary to maintain the pH of the groundwater in
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