Biofiltration

Biofiltration refers to the removal and oxidation of organic gases (i.e., volatile organic compounds, or VOCs) from contaminated air by vapor phase biodegradation in beds (biofilters) of compost, soil , or other materials such as municipal waste, sand, bark peat, volcanic ash, or diatomaceous earth. As contaminated air (such as air from a soil vapor extraction process) flows through the biofilter, the VOCs sorb onto surfaces of the pile and are degraded by microorganisms . Nutrient blends or exogenous microbial cultures can be added to a biofilter to enhance its performance. Moisture needs to be continually supplied to the biofilter to counteract the drying effects of the gas stream. The stationary support media that make up the biofiltration bed should be porous enough to allow gas flow through the biofilter and should provide a large surface area with high wetting and sorptive capacities. This support media should also provide adequate buffering capacity and may also serve as a source of inorganic nutrients. Biofilters, also used to treat odors as well as organic contaminants, have been used in Europe for over twenty years. Compared to incineration and carbon adsorption , biofilters do not require landfilling of residuals or regeneration of spent materials.

Waste gases are moved through the units by induced or forced draft. Biofilters are capable of handling rapid air flow rates (e.g., up to 90,000 cubic ft (2,700 cubic m) per minute (cfm) in filters up to 20,000 sq ft (1,800 sq m) in wetted area) and VOC concentrations greater than 1,000 ppm. However, biofiltration removal of more highly halogenated compounds such as trichloroethylene (TCE) or carbon tetrachloride, which biodegrade very slowly under aerobic conditions, may require very long residence times (i.e., very large biofilters) or treatment of very low flow rates of air containing the contaminants.

The soil-type biofilter is similar in design to a soil compost pile. Fertilizers are preblended into the compost pile to provide nutrients for indigenous microorganisms, which accomplish the biodegradation of the VOCs. In the treatment bed type of biofilter, the waste air stream is humidified as it is passed through one or more beds of compost, municipal waste, sand, diatomaceous earth or other materials. Another type of biofilter is the disk biofilter, which consists of a series of humidified, compressed disks placed inside a reactor shell. These layered disks contain activated charcoal, nutrients, microbial cultures, and compost material. The waste air stream is passed through the disk system. Collected water condensate from the process is returned to the humidification system for reuse .

[Judith L. Sims ]

RESOURCES

BOOKS

Baker, K. H., and D. S. Herson. Bioremediation. New York: McGraw-Hill, 1994.

King, B. R., G. M. Long, and J. K. Sheldon. Practical Environmental Bioremediation. Boca Raton, FL: Lewis Publishers, 1992.

Stoner, D. L., ed. "Organic Waste Forms and Treatment Strategies." Biotechnology for the Treatment of Hazardous Waste. Boca Raton, FL: Lewis Publishers, 1994.