Liquid-Phase Granular Activated Carbon Adsorption
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.
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.
Other Resources and Demonstrations
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.