Cometabolism
Description
Cometabolism involves the injection of a dilute
solution of nutrients such as methane and oxygen into the contaminated groundwater
or soil. The microbes
that metabolize these nutrients produce enzymes that react with the organic
contaminant
and degrade it to harmless minerals. The addition of methane or methanol has
been demonstrated to degrade chlorinated solvents, such as vinyl chloride (VC)
and trichloroethylene
(TCE). Toluene, propane and butane have also been used to support
the cometabolism of TCE.
Limitations
and Concerns
VC
concentrations may increase, as it is a breakdown byproduct of chlorinated
solvents.
Where
the subsurface is irregular (i.e., heterogeneous),
it is very difficult to deliver the methane solution to every portion of the
contaminated zone. Consequently, some contaminants sorbed
onto small particles may not be degraded and will remain in the subsurface.
Over the long term, these contaminants may desorb due to other environmental
factors and re-contaminate the groundwater.
To
use cometabolism in areas where there is clay, the area must be flooded with
the methane-oxygen solution to achieve greater contact with the contaminants.
This may increase the groundwater flow rate, and the addition of downstream
wells may be required to capture contaminated water. A surface treatment
system, such as air stripping or carbon adsorption,
may be required to treat extracted groundwater.
Higher
permeability
zones are cleaned up much faster because groundwater flow rates are greater.
Safety
precautions (such as removing all ignition sources in the area) must be used
when handling methane.
Certain
fuels can support the cometabolism of chlorinated solvents,
and it is speculated that at many sites where fuel and chlorinated volatile
organic
compounds (VOCs)
are combined, cometabolism may be ÒnaturallyÓ occurring.
To
use this technology, the subsurface must be well characterized.
Applicability
Cometabolism
degrades chlorinated VOCs in soil and groundwater. Current work is focusing on
expanding the list to include pesticides
and polychlorinated
biphenyls (PCBs).
Technology
Development Status
This
technology is commercial.
Web
Links
http://www.clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/AS-Cometabolic-2001.pdf
Other
Resources and Demonstrations
See
description of Enhanced
Bioremediation.
A
field demonstration project was conducted at Moffett Naval Air Station,
California, and another was conducted at the Energy DepartmentÕs Savannah River
site, South Carolina. A full-scale application is in place at Air Force Plant
44 in Tucson, Arizona.
See http://water.usgs.gov/pubs/wri/wri994285/text/chlorinated.html for a technical overview.