Bioventing is a promising new technology that stimulates the natural in situ biodegradation of any aerobically degradable compounds in soil by providing oxygen to existing soil microorganisms. In contrast to soil vapor vacuum extraction, bioventing uses low air flow rates to provide only enough oxygen to sustain microbial activity. 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.
The U.S. Air Force Bioventing Initiative is demonstrating that this technology is effective under widely varying site conditions. Initial testing has been completed at 117 sites, with more than 90 pilot systems now operating at 41 USAF installations. On smaller sites, many of these single-well pilot systems are providing full-scale remediation.
Regulatory acceptance of this technology has been obtained in 30 states and in all 10 EPA regions, and the use of this technology in the private sector is growing rapidly following USAF leadership.
Bioventing is a medium to long-term technology. Cleanup ranges from a few months to several years.
Typical Bioventing System
Bioventing techniques have been successfully used to remediate soils contaminated by petroleum hydrocarbons, nonchlorinated solvents, some pesticides, wood preservatives, and other organic chemicals.
While bioremediation cannot degrade inorganic contaminants, bioremediation can be used to change the valence state of inorganics and cause adsorption, uptake, accumulation, and concentration of inorganics in micro or macroorganisms. These techniques, while still largely experimental, show considerable promise of stabilizing or removing inorganics from soil.
Factors that may limit the applicability and effectiveness of the process include:
The water table within several feet of the surface, saturated soil lenses, or low permeability soils reduce bioventing performance.
Vapors can build up in basements within the radius of influence of air injection wells. This problem can be alleviated by extracting air near the structure of concern.
Extremely low soil moisture content may limit biodegradation and the effectiveness of bioventing.
Monitoring of off-gases at the soil surface may be required.
Aerobic biodegradation of many chlorinated compounds may not be effective unless there is a co-metabolite present, or an anaerobic cycle.
Low temperatures may slow remediation, although successful remediation has been demonstrated in extremely cold weather climates.
Two basic criteria must 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. Initial testing is designed to determine both air permeability of soil and in situ respiration rates.
Soil grain size and soil moisture significantly influence soil gas permeability. Perhaps the greatest limitation to air permeability is excessive soil moisture. A combination of high water tables, high moisture, and fine-grained soils has made bioventing infeasible at some Air Force test locations.
Several soil characteristics that are known to impact microbial activity are pH, moisture, and basic nutrients, (e.g., nitrogen and phosphorus), and temperature. Soil pH measurements show the optional pH range to be 6 to 8 for microbial activity; however, microbial respiration has been observed at all sites, even in soils that fall outside this optimal range. Optimum soil moisture is very soil-specific. Too much moisture can reduce the air permeability of the soil and decrease its oxygen transfer capability. Too little moisture will inhibit microbial activity. Several Air Force bioventing test sites have sustained biodegradation rates with moisture levels as low as 2 to 5% by weight. However, in extremely arid climates, it may be possible to increase the rate of biodegradation through irrigation, or humidifying the injected air.
Biological activity has been measured at Eielson AFB, Alaska, in soil temperatures as low as 0° C. Bioventing will more rapidly degrade contaminants during summer months, but some remediation occurs in soil temperatures down to 0° C.
Hydrocarbon degradation rates are almost always estimated from oxygen utilization rates using a simple stoichiometric relationship with the implicit assumption that all oxygen loss is due to the mineralization of hydrocarbons by microbes. However, simple stoichiometric relationships do not account for biomass production and inorganic oxidation reactions. Oxygen serves a terminal electron acceptor not only in the degradation of organic matter but also in oxidation of reduced inorganic compounds by microorganisms which obtain energy through chemical oxidation. In situ respiration tests can also be taken. Measurement of oxygen utilization in a nearby uncontaminated area is used to account for inorganic oxidation reactions. When used with other indicators of increased microbial activity or biodegradation, respiration tests can provide one of several convergent lines of independent evidence to at least qualitatively document biodegradation.
Bioventing is becoming more common, and most of the hardware components are readily available. Bioventing is receiving increased exposure to the remediation consulting community, particularly its use in conjunction with soil vapor extraction (SVE). The Air Force is sponsoring bioventing demonstrations at 135 sites. As with all biological technologies, the time required to remediate a site using bioventing is highly dependent upon the specific soil and chemical properties of the contaminated media.
Based on the Air Force Center for Environmental Excellence (AFCEE) and commercial applications of this technology, costs for operating a bioventing system typically are $10 to $70 per cubic meter ($10 to $50 per cubic yard). Factors that affect the cost of bioventing include contaminant type and concentration, soil permeability, well spacing and number, pumping rate, and off-gas treatment. This technology does not require expensive equipment and relatively few personnel are involved in the operation and maintenance of a bioventing system. Periodic maintenance monitoring is conducted.