Environmental Engineering Reference
In-Depth Information
32.2.8.1 Liquid-Phase Carbon Adsorption
Liquid-phase carbon adsorption using granular activated carbon (GAC) has been used to
remove perchlorate from contaminated groundwater and surface water. In this technol-
ogy, GAC is the adsorbent to remove contaminant ions from water as it passes through
the GAC bed. However, GAC has a relatively small treatment capacity for perchlorate
removal. Some research is being done to identify methods to improve the treatment capac-
ity of a GAC system for perchlorate removal, including use of “tailored GAC.” Tailored
GAC technology was tested on a pilot-scale level. As discussed above, GAC also has been
used in conjunction with bioreactors, including the role of substrate for the biodegradation
processes.
Liquid-phase carbon adsorption involves use of adsorbent media (such as GAC, activated
alumina, or other proprietary materials) packed into a column (FRTR, 2005; Graham et al.,
2004). GAC sorbent has commonly been used to remove organic and metallic contaminants
from groundwater, drinking water, and wastewater. GAC media are usually regenerated
by thermal techniques to desorb and volatilize contaminants, and an off-gas unit captures
the volatilized contaminants and treats it before release into the atmosphere (Graham et al.,
2004). GAC media are generally considered cost-effective for water treatment when used
for removal of nonpolar contaminants with low water solubility (Graham et al., 2004; FRTR,
2005). Because of the issues discussed above, activated carbon is generally considered inef-
fective for removal of inorganic contaminants, such as perchlorate, from water.
Carbon adsorption technology can be used in multiple beds in series to reduce the need
for media regeneration. Multiple beds can also allow continuous operation because some
beds can be regenerated as others continue to treat water (Graham et al., 2004). Thermal
decomposition of perchlorate-contaminated GAC is a possible regeneration method for
spent GAC (USEPA, 2005).
Recently, there has been discussion among experts about the types of mechanisms and
effectiveness of tailored GAC for treatment of perchlorate. In addition, there have been ques-
tions raised about the potential use of tailored GAC for treatment of water contaminated with
perchlorate and explosives such as royal demolition explosives, cyclotetramethylene trinitra-
mine, and trinitrotoluene and volatile organic compounds (i.e., co-contaminated groundwater).
For co-contaminated groundwater sites, practitioners have suggested the use of treatment
trains consisting of standard GAC/ion-exchange resins or tailored GAC/standard GAC.
For example, a treatability study was conducted at the Massachusetts Military Reserva-
tion site investigating innovative options for
ex situ
removal of perchlorate and explosives
in groundwater (Weeks et al., 2004). Life cycle cost comparisons of these different tech-
nologies and treatment trains is being examined. Contaminants in the water can reduce
the effectiveness of these technologies. The performance of GAC is affected by low rate,
polarity, and water solubility as well as fouling by solid, organics, or silica (FRTR, 2005).
Waste streams with high amounts of suspended solids, oil, and grease may foul the car-
bon. Spent carbon from the adsorption unit may require treatment before ordinary or haz-
ardous waste disposal (FRTR, 2005; Graham et al., 2004). Contaminants with high water
solubility and polarity can reduce the ability of GAC to remove contaminants from water
(FRTR, 2005).
32.2.8.2 Surfactant-Modiied Zeolites
Surface-modiied zeolites have also been evaluated as a sorption media to remove perchlo-
rate. Natural zeolites are hydrated alumino silicate minerals with high internal/external
