Environmental Engineering Reference
In-Depth Information
5.1.6 Biotreatment (SERB) technology
Site characterization activities are used to identify the location and extent of
the source area and whether a separate phase DNAPL is present, but most
often these procedures are not conducted to give the level of detail required
for the successful application of many remediation technologies. Figure 5.2a
shows the complex distribution of DNAPL that can exist in the subsurface
and how the DNAPL moves through the aquifer due to gravity and pools
on less permeable layers. Site characterization, groundwater flow character-
ization, and optimization modeling must be conducted to determine the
number and placement of injection and recovery wells for the cosolvent
extraction process.
The selection of a cosolvent is based on the properties of contaminant
to be removed, cosolvency properties, and regulatory constraints concerning
injection of compounds. The cosolvent extraction process utilized in the
SERB demonstration was based on the principle of enhanced dissolution of
the contaminant into the cosolvent, which was then removed through the
recovery/extraction wells. PCE was the contaminant at the SERB demon-
stration site, and ethanol was selected as the cosolvent for enhanced disso-
lution. Some
in situ
extraction technologies are also designed to mobilize the
residual contaminant, which is also removed through the recovery/extrac-
tion wells. Mobilization of the contaminant requires excellent hydraulic con-
trol, especially in the case of DNAPL contaminants, to ensure the DNAPL
is not allowed to contaminate the aquifer further. Upon selection of the
cosolvent and determination of the injection/extraction design, the cosolvent
flushing test is conducted (Figure 5.2b). Maximum contact and mixing of the
cosolvent and residual contaminant is the most important criterion in the
design of the injection/extraction system for successful application of the
technology. Because of the complex nature of the subsurface environment,
it is known that some portions of the DNAPL will not be contacted by the
cosolvent, and the majority of the flow will be through layers of higher
hydraulic conductivity.
Following the application of the cosolvent extraction technology and
cessation of active pumping, it is envisioned that there will be separate areas
of DNAPL contamination and cosolvent remaining in the aquifer but that
there will also be areas where the DNAPL and cosolvent are mixed (Figure
5.2c). The areas where the cosolvent (electron donor) and DNAPL (electron
acceptor) are mixed contain the highest potential to stimulate bioremedia-
tion. The SERB technology application at the demonstration site was
designed to enhance the reductive dechlorination of the residual DNAPL
contaminant, PCE. Ethanol was selected as the cosolvent because it was both
an acceptable cosolvent and an acceptable electron donor for the reductive
dechlorination process.
Stimulation of the bioremediation processes should result in subsurface
bioactive zones that will remove the contaminant from the groundwater at
a rate faster than it dissolves from the remaining residual DNAPL in the
