Bioventing
Background
Bioventing is a process of stimulating the natural in situ biodegradation of contaminants in soil by providing air or oxygen to existing soil microorganisms. Bioventing uses low air flow rates to provide only enough oxygen to sustain microbial activity in the vadose zone. Oxygen is most commonly supplied through direct air injection into residual contamination in soil. In addition to degradation of adsorbed fuel residuals, volatile compounds are biodegraded as vapors move slowly through biologically active soil [1,2,3].
Applicability
Bioventing is applicable to any chemical that can be aerobically biodegraded. Techniques have been successfully used to remediate soils contaminated by petroleum hydrocarbons, non-chlorinated solvents, some pesticides, wood preservatives, and other organic chemicals [1,2,3].
Limitations
Factors that may limit the applicability and effectiveness of the process include: (1) low permeability soils (reduce bioventing performance); (2) air near the structure of concern has to be extracted in order to avoid vapor build up in basements within the radius of influence of air injection wells; (3) monitoring of off-gases at the soil surface may be required; (4) aerobic biodegradation of many chlorinated compounds may not be effective; and (5) low soil moisture content, which may be caused by bioventing, limits biodegradation [3,4].
Performance
Performance data for three bioventing projects are summarized in Table 1.Table 1. Performance data of bioventing systems [3,4].
Site Characteristics
Beginning Levels
Levels Attained
Disposal of solvents
10 ppm in soils and 1 ppm
in groundwater<2 ppb
Pilot-scale field test for volatile hydrocarbons in vadose zone
>1,000 mg TPH/kg of soil
<30 mg TPH/kg of soil
25,000 gallons of JP-4 spill to a depth of 60 feet
20,000 ppm TPH
98% reduction
TPH: total petroleum hydrocarbons
Data Requirements
Two basic criteria have to be satisfied for successful bioventing. First, air must be able to pass through the soil in sufficient quantities to maintain aerobic conditions. Second, natural hydrocarbon-degrading microorganisms must be present in concentrations large enough to obtain reasonable biodegradation rates. Soil gas permeability is significantly impacted by soil grain size and soil moisture. Bioventing is infeasible if sites have high water tables, high moisture, and fine-grained soils. Soil characteristics, such as pH, moisture, and basic nutrients, affect microbial activity. A pH range of 6 to 8 is optimal for microbial activity. Higher temperature favors microbial activity [2,3,4].
Cost
The total cost for in situ bioremediation using bioventing technology ranges from $10 to $60 per cubic yard. At sites with over 10,000 cubic yards of contaminated soil, costs of less than $10 per cubic yard are achievable [2]. Higher costs are associated with small sites.
Status of Technology
Bioventing has been used to remediate sites since the early- to mid-1980s, and is becoming more common. Most of the needed equipment is readily available. It is likely that the use of in situ bioventing will become more attractive to the remediation community, particularly its use in conjunction with soil vapor extraction. In the mid-1990s, soil venting vendors began to monitor bioventing degradation, design venting systems, and optimize biodegradation. More than 1,000 sites worldwide have applied bioventing technology [3].
References
1. U.S. Air Force Environics Directorate of the Armstrong Laboratory, U.S. Air Force Center for Environmental Excellence, 1995, Manual: Bioventing Principles and Practices, EPA/540/R-95/534a.2. U.S. Air Force Environics Directorate of the Armstrong Laboratory, U.S. Air Force Center for Environmental Excellence, 1995, Manual: Bioventing Principles and Practices, Volume II, EPA/540/R-95/534b.
3. Hinchee, R.E., 1993, Bioventing of Petroleum Hydrocarbons, in Handbook of Bioremediation, CRC Press, Boca Raton, FL.
4. Office of Research and Development, EPA, ATTIC Downloadable Documents, available at http://www.epa.gov/bbsnrmrl/attic/documents.html.