IntroductionBioventing is a bioremediation technique in which the subsurface is aerated in order to promote biological activity of the microorganisms that are naturally present in soils. Bioventing provides for the degradation of any chemical that can be aerobically degraded. However, bioventing has only been utilized at petroleum-contaminated sites thus far.
To learn more about bioventing at petroleum-contaminated sites click on your topic of interest or scroll down:
Overview of
BioventingBacteria, fungi, and other microorganisms are found in vast numbers in most soils and significant numbers persist even at depths of several hundred feet. These microorganisms can adapt to and metabolize a wide range of petroleum hydrocarbons. The petroleum serves as the microbes' food source (electron donor), with oxygen serving as their preferred electron acceptor. The bacteria degrade petroleum hydrocarbons by converting these organics into energy, new cellular material, and oxidizing the rest to CO2. Oxygen is used by the microorganisms to convert the organics into the products previously mentioned while the oxygen is reduced to water in the process (Bedient, 1994).
At most fuel contaminated sites, the lack of oxygen is the only rate limiting factor for biodegradation(Ratz, 1997). The microbial oxygen utilization rate exceeds the rate of oxygen diffusion into the soil and therefore the oxygen supply is quickly exhausted. Anaerobic decomposition of the contamination then occurs at a rate which is usually an order of magnitude slower than under aerobic conditions. Some chemicals, however, are not able to be degraded both aerobically and anaerobically. The goal of in situ bioventing is to supply the subsurface microorganisms with an adequate supply of oxygen so that they will degrade the fuel as quickly as possible.
Air InjectionThere are two ways to provide oxygen into the subsurface:
Air injection can provide higher concentrations of oxygen to areas where contaminant concentrations are highest. It also pushes out volatile hydrocarbons into the neighboring soils increasing the contaminated area and allowing more microorganisms to assist in biodegradation. Air injection may extract volatile hydrocarbons from the subsurface requiring emissions monitoring at the soil surface. Below is an animation of bioventing using air injection. The injected oxygen stimulates microbial activity which degrades the contamination in the soil. Suggested monitoring points are also shown. The water table level is below the contaminent. If significant amounts of contaminent are found below the water table, dewatering may have to be performed initially (Leeson, 1997).
Extracting soil gas draws oxygen from clean soils into the contaminated soils at concentrations sufficient to allow for aerobic degradation of the fuel. However, this method can not provide oxygen concentrations to the subsurface as high as air injection can.
Soil Gas
Monitoring
Soil gas monitoring is necessary to ensure that
vapor degradation is occuring. Monitoring points are used for
pressure and soil gas monitoring. These points should be located
in contaminated soils of levels greater than 1,000 mg/kg of total
petroleum hydrocarbon. They are usually used to collect soil gas
for carbon dioxide (a byproduct of degradation) and oxygen
analysis, and for high levels of hydrocarbons (Leeson, 1997).The
photo on the left, provided by of Dr. Widdowson, shows what the
sampling devices look like. Temperature monitoring is also
completed by connecting thermocouples to monitoring points. These
temperatures are usually constant at varying lateral distances,
but will change with depth.
The wiring and screening is done at different
lengths of the casing. Monitoring points must occcur at depths
and distances determined from soil gas permeability tests and
radius of influence determination. Probes are installed here so
that measurements at specified depths can be taken; a minimum of
three depths is recommended. In more permeable soil, the
monitoring point is screened closer to the water table. A
close-up picture of a probe, courtesy of Dr. Widdowson, is below.
If the vapors are not being consumed by the microorganisms then the vapors are escaping to the atmosphere. The possibility of volatile hydrocarbons moving through the subsurface without being degraded will mean that surface monitoring will need to be implemented.
Air SourcesThe three most common sources of oxygen are:
Atmospheric air is the preferred oxygen source since it is free and readily available in unlimited quantities at any site. However, sparging the groundwater with air (or pure oxygen) can only raise the dissolved oxygen level to moderate levels (8 to 40 mg/L). Hydrogen peroxide is infinitely soluble in water, dissociating into water and oxygen. However, hydrogen peroxide can be toxic to microorganisms at low concentrations and must be properly introduced into the subsurface environment (Bedient, 1994).
Bioventing
Initiative and the Extended Bioventing ProjectThe U.S. Air Force Center for Environmental Excellence ran a project called the Bioventing Initiative from April 1992 to June 1996. This program provided consulting, and field testing in order to test the applicability of bioventing for the remediation of petroleum-contaminated soils. A wide range of petroleum contaminants were studied at various environmental conditions. Four parameters were determined necessary to measure: aromatic hydocarbons (BTEX), total petroleum hydrocarbons (TPH), moisture content, and particle size. Since the commencement of the Bioventing Initiative and the Extended Bioventing Project, over fifty sites have continued bioventing activities (Bedient, 1994). Several case studies will be cited to represent this methodology and to explain its usefulness and shortcomings when applied in real life situations. Figure 1 below shows levels of BTEX (in mg BTEX per kg soil) that were found at the Bioventing Initiative Sites. The high levels found at many of them demonstrate the need for some type of remediation.
Figure 1. Initial BTEX levels at Bioventing
Initiative Sites Data from Leeson, 1997
To learn more about the case studies click on a selection below or scroll down.
Hill Air Force Base
Two 12,000 gallon underground storage tanks that had been used to store diesel fuel were removed from Hill AFB. A site investigation was performed and revealed 50 feet of fuel contamination within the soil. Groundwater at the site occurs at a depth of 150 feet below the ground surface and had not yet been impacted.
Immediate action included the installation of a single vent well and three monitoring points. Air was injected continuously during treatment. Though oxygen is already available in the vadose zone, this amount was not sufficient for the desired amount of microbial activity. Installation occurred in May 1992 and in May 1993 soil and soil gas sampling was performed. Soil total petroleum hydrocarbon concentrations showed no noticeable reductions. However, soil BTEX (benzene, toluene, ethyl benzene, xylene) concentrations were lower by 98%. BTEX levels were measured through surface emissions using groundwater well measurements. The Utah Department of Environmental Response and Remediation decided that no further remedial action was necessary (Bedient, 1994).
TOTAL COST: $37,700 (preliminary testing, system
operation, site closure, vent well and monitoring point
abandonment)
COST / YD3 of CONTAMINATED SOIL:
$8.60
Kelly Air Force Base
One site at Kelly AFB was used from the 1950s to the 1980s for fire training exercises. This often included setting and extinguishing fires around a simulated airplane. No containment system was used and fuel eventually infiltrated into the soil. Groundwater was located about 15 feet below the surface and was affected by the fuel.
One vent well and four monitoring wells were installed in December 1992. Air was injected at a rate which provided a sufficient radius of oxygen influence. The radius of oxygen influence, RI , is defined as the radius to which oxygen must be supplied which will sustain maximum biodegradation. It is a function of air flowratess as well as ozygen utilization rates. The biodegradation rate was estimated to be around 0.044 oz of fuel per pound of soil per year. Soil and gas samples were collected after one year and the BTEX concentrations had been reduced to very low levels. The soils were determined to no longer contaminate the groundwater. To complete the remediation of approximately 30,000 cubic yards, six vent wells and three monitoring wells were then installed (Bedient, 1994).
TOTAL COST: $125,000 (full scale upgrade and system
operation)
COST / YD3 of CONTAMINATED SOIL:
$4.15
Williams Air Force Base
Williams AFB contained a site that served as a jet fuel and aviation gasoline storage area from 1942 until 1991. The ground was contaminated through over 200 feet below the ground surface, into the vadose zone soils to the groundwater table. Floating free product was also found floating on the water table surface and a plume of contaminate was dissolved into the groundwater.
During August and September 1996, one vent well and four monitoring wells were installed. Specialized drilling equipment was necessary due to the increased depth. Air was injected into the soil. Within this and other desert environmental sites, low rates of biodegradation were evident (Bedient, 1994). This is due to the correlation between moisture and soil gas permeability. Particle size was also proportional to soil gas permeability. The microbes at this site were not capable of consuming the hydrocarbons at the Williams AFB and a different method of remediation had to be used (Leeson, 1997).
TOTAL COST:: $88,000 (drilling and pilot testing)
COST / YD3 of CONTAMINATED SOIL:
$0.67
The figure below shows the initial BTEX levels at the Bioventing Initiative Sites and the final BTEX levels after remediation. It is observed that bioventing is more advantageous at sites with large initial volumes of BTEX. Microbial activity at less contaminated sites is less efficient and the BTEX is not decreased. Increases at these sights may even occur, possibly do to a larger volume of contaminent continuing to spill than is degraded by the microbes.
Figure 2. Initial and Final BTEX concentrations at Bioventing
Initiative Sites Data from Leeson, 1997
ConclusionsBioventing is a very complicated and intricate method of remediation. Numerous factors determine the suitability of a site for this method. These factors are interconnected and changing. Microbial kinetics and heterogeneous soil characteristics must be carefully analyzed to predict success. One disadvantage of in-situ bioventing is the larger time required, compared to ex-situ methods such as excavation and disposal. But, bioventing eliminates the contaminant rather than just moving its location. In addition, the cost of the two methods are very similar. The cost per yard for bioventing decreases considerably with larger contamination volumes, from $20 to $5 per cubic yard. Figure 3 compares the cost of bioventing to other common remediation techniques. The volume of contaminant greatly impacts this cost.
Figure
3. Costs of Several Remediation Techniques Data from
Leeson, 1997
Lack of well-documented field demonstrations and long term data is a major problem in choosing bioventing. But, around 90% of bioventing pilot tests have been successful and BTEX and TPH levels have been greatly reduced. Some environments are more suited to this method than others, but overall results encourage its use.
References
Student authors: Kellyn Roth and Michael Shrader
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
