Liquid-Phase Granular
Activated Carbon Adsorption
Description
Liquid-phase Granular Activated Charcoal (GAC) Adsorption
is
a treatment technology to remove contaminants
from groundwater.
Groundwater is pumped through one or more vessels containing GAC. The
thermal
processing of carbon, often derived from ground coconut shells, creates
small porous
particles
with a large internal surface area. This attribute makes it activated.
The
activated carbon attracts and adsorbs organic
molecules as well as certain metal and inorganic
molecules. Dissolved contaminants sorb
onto the
surfaces of the activated carbon. Water is passed through the vessels
relatively quickly. When the concentration of contaminants in the water
exiting
the vessels exceeds a certain level, the carbon must be replaced. Spent
carbon
can be regenerated in place, removed and regenerated at an off-site
facility,
or most commonly, removed and disposed.
Limitations and Concerns
Groundwater with suspended solids or oil and grease may cause
fouling of the
carbon. The chemical characteristics of the contaminants must be known
prior to
implementation. In many cases pretreatment may be required to ensure
the
treatment's effectiveness.
Costs are high if used as the primary treatment for groundwater with
high
concentrations of contaminants. Often, GAC is phased in after a
different
technology is used.
Some degradation products, such as vinyl chloride and smaller
molecules, are
not sorbed well. Consequently the effluent must be monitored carefully.
All spent carbon eventually needs to be disposed in landfills
or
regenerated. There are few regeneration facilities. Although activated
carbon
is a well-established technology for removing organic compounds, its
use in the
removal of inorganic contaminants has not been as widespread due to its
low
capacity as well as the difficulty of regeneration and cost of
disposal. Also,
the presence of iron may promote fouling of the carbon.
Most regeneration systems release sorbed contamination to the air.
There is
a concern by many communities that GAC, while removing contamination
from the
groundwater (or air in a vapor phase system), spreads the contamination
to
other areas. In the Western U.S., one regeneration facility is located
on an
Indian reservation.
Carbon used for some contaminants (e.g., explosives or metals) may
not be
regenerated.
If used to remove radioactive contamination from groundwater, GAC
does not
reduce radio-toxicity. In cases where the GAC is used to remove both
radionuclides and VOCs, the GAC may become mixed waste
that will be
difficult to dispose.
Applicability
This technology is used to treat groundwater contaminated with VOCs,
metals,
and explosives. GAC can also be used to treat certain radionuclides
such as
uranium, cobalt-60, and ruthenium-106.
Technology Development Status
The technology is well proven, and it is frequently included in
remedial
designs. Innovations in regenerating GAC contaminated with high
explosive
compounds are in the research phase.
Web Links
http://clu-in.org/download/citizens/activatedcarbon.pdf
http://www.frtr.gov/matrix2/section4/4-47.html
Other Resources and Demonstrations
See the related descriptions of Vapor Phase
GAC
and Sorption.
See Coupled Chemical and Biological Systems for Regenerating
Activated
Carbon Contaminated with High Explosives,
Knezovich et al., 1996, UCRL-ID-103483-95.
See "Engineer and Design: Adsorption Design Guide" (DG 1110-1-2),
published by the U.S. Army Corps of Engineers, March 2001. It provides
practical guidance for the design of liquid and vapor phase devices for
the
adsorption of organic chemicals. The adsorptive media addressed include
granular activated carbon (GAC) and other alternative adsorption carbon
media,
such as powdered activated carbon (PAC) and non-carbon adsorbents.