Co-metabolism is one form of secondary substrate transformation in which enzymes produced for primary substrate oxidation are capable of degrading the secondary substrate fortuitously, even though the secondary substrates do not afford sufficient energy to sustain the microbial population. An emerging application involves the injection of water containing dissolved primary substrate (e.g. methane, toluene) and oxygen into ground water to support the co-metabolic breakdown of targeted organic contaminants.
The addition of methane or methanol supports methanotrophic activity, which has been demonstrated effective to degrade chlorinated solvents, such as vinyl chloride and TCE, by co-metabolism.
Although toluene, propane and butane are used to stimulate a different class of microorganisms, not methanotrophs, they have been used successfully for supporting co-metabolism of TCE.
Co-metabolic technologies may be classified as long-term technologies which may take several years to clean up a plume.
The primary target contaminants for cometabolism are the chlorinated solvents. Current work is focussing on expanding the list to include such contaminants as pesticides and PCBs. It has been demonstrated that certain fuels can support cometabolism of chlorinated solvents and it is speculated that at many co-contaminated sites, cometabolism may be naturally occurring.
Factors that may limit the applicability and effectiveness of the process include:
- This technology is still under development.
- Regulatory approval for use of specific cometabolites may be required.
- Where the subsurface is heterogeneous, it is very difficult to circulate the methane solution throughout every portion of the contaminated zone. Higher permeability zones are cleaned up much faster because ground water flow rates are greater.
- Safety precautions (such as removing all ignition sources in the area) must be used when handling methane.
- A surface treatment system, such as air stripping or carbon adsorption, may be required to treat extracted ground water prior to re-injection or disposal.
- High copper concentrations affect methanotrophic cometabolism.
- Predation affects methanotrophic cometabolism.
Characteristics that should be addressed prior to system design include aquifer permeability, site hydrology, dissolved oxygen content, pH, and depth, type, concentration, and biodegradability of contaminants.
While ex situ bioreactors for methanotrophic TCE biodegradation are being used in full-scale remediation, in situ application has not yet been demonstrated at a practical scale. A field demonstration project has been conducted at DOD's Moffett Naval Air Station, and another is being conducted at DOE's Savannah River site.
The DOE pilot-scale demonstration was performed at the Savannah River site's abandoned seepage basin and process sewer line employed for disposal of solvents used to degrease nuclear fuel target elements. Contamination is mostly TCE and PCE with concentrations of 10,000 ppb in soil and 1,000 ppb in ground water. Extensive soil and ground water monitoring has demonstrated that when methanotroph densities increased five orders of magnitude, TCE and PCE concentrations declined to less than 2 ppb.
For the DOE Savannah River demonstration, capital investment costs were $150K and 200 man-hours for site preparation, setup, and assembly. The operation is low maintenance, requiring only one technician at 25% time (10 hours per week); other operational costs are for electricity, natural gas, and equipment maintenance.
O&M costs can be significant because a continuous source of methane solution must be delivered to the contaminated ground water.