Biofiltration is a technology in which vapor-phase organic contaminants, such as fuel hydrocarbons, are passed through a soil bed where they sorb to the soil surfaces and are broken down by microorganisms in the soil. Specific strains of bacteria may be introduced into the filter to preferentially degrade specific compounds. The biofilter provides several advantages over conventional activated carbon adsorbers. Biofilters are self-regenerating; thus they maintain maximum adsorption capacity. The greatest advantage is that the contaminants are destroyed, not just separated.
Limitations and Concerns
Low temperatures may slow or stop degradation. Moisture levels, pH, temperature, and other filter conditions such as fungi growth should be monitored to maintain high removal efficiencies.
Until recently, biofilters were not designed to treat chlorinated compounds. However, recent demonstrations have suggested that they can be used to remove these compounds, as well.
The accumulation of excess bacteria may plug filters, requiring periodic mechanical cleaning of the filter.
There is a potential for release of fugitive fungi.
As with most processes using biodegradation, some contaminants are difficult to break down. Because many contaminant plumes contain a mixture of chlorinated and non-chlorinated volatile organic compounds (VOCs), byproducts and the effluent stream must be carefully monitored.
Biofiltration is used to treat vapor emissions (i.e., off-gas) from remediation systems, including compost piles. Its effectiveness is dependent upon the biodegradability of the contaminants. Biofiltration is used primarily to treat non-chlorinated VOCs and fuel hydrocarbons. Halogenated VOCs and semi-volatile organic compounds (SVOCs) also can be treated, but the process may be less effective. Additionally, biofilters have been successfully used to control odors from compost piles.
Technology Development Status
The technology has been field tested in several locations.
Other Resources and Demonstrations
In a demonstration at Anniston Army Depot, trichloroethylene (TCE) contaminated air streams were destroyed, and no secondary waste streams were created. The biofilter used at Anniston was a scaled-up version of a system that operated at the Tennessee Valley Authority (TVA) for three years, testing different volatile compounds. The system used propane gas as the co-substrate to feed the microorganisms, alternately feeding propane and TCE or other solvents. The filter bed was composed of pelletized-composted chicken litter, pine bark, and chopped kenaf with pulverized limestone as a buffering agent. The bed at TVA had operated for three years with no addition of materials or changes.