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| Resource Library > Technology Transfer > Programs and Initiatives > Bioventing > Development History |
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Bioventing Initiative Exhibit, Smithsonian Museum in Washington, DC
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The AFCEE Bioventing Initiative
Bioventing is the process of aerating soils to stimulate in situ biological activity and promote bioremediation. Bioventing typically is applied in situ to the vadose zone and is applicable to any chemical that can be aerobically biodegraded, but to date has been implemented primarily at petroleum-contaminated sites. The Armstrong Laboratory Environics Directorate (an element of the Air Force Human Systems Center now part of the Air Force Research Laboratory) began its research and development (R&D) program in bioventing in 1988 with a study at Hill Air Force Base (AFB), Utah. Follow-on efforts included field research studies at Tyndall AFB, Florida; Eielson AFB, Alaska; and F.E. Warren AFB, Wyoming, to monitor and optimize process variables. Results from these research efforts led to the AFCEE Bioventing Initiative.
The AFCEE's Bioventing Initiative has involved conducting field treatability studies to evaluate bioventing feasibility at more than 125 sites throughout the US. At those sites where feasibility studies produced positive results, pilot-scale bioventing systems were installed and operated for 1 year. Results from these pilot-scale studies have culminated in production of Volume 1 of the Bioventing Principles and Practice Manual.
The US EPA contributed to the AFCEE Bioventing Initiative in the development of the test plan for conducting the pilot-scale bioventing studies and assisted in the development of Volume 1 of the Manual. The US EPA did this through their own Bioremediation Field Initiative. This initiative was established to provide the US EPA and state project managers, consulting engineers, and industry with timely information regarding new developments in the application of bioremediation at hazardous waste sites. This program sponsored field research to enable US EPA laboratories to more fully document newly developing bioremediation technologies such as bioventing.
The results from bioventing R&D efforts and from the pilot-scale bioventing systems via the AFCEE Bioventing Initiative have been used to produce Volume 1 of the Manual. Although the design Manual was written based on extensive experience with petroleum hydrocarbons (and thus, many examples use this contaminant), the concepts of bioventing should be applicable to any aerobically biodegradable compound. The Manual provides details on bioventing principles; site characterization; field treatability studies; system design, installation, and operation; process monitoring; site closure; and optional technologies to combine with bioventing if warranted. Volume 2 of the Bioventing Principles and Practice Manual focuses on bioventing design and process monitoring.
Through the efforts of the AFCEE Bioventing Initiative and the US EPA Bioremediation Field Initiative, bioventing has been implemented at more than 150 sites and has emerged as one of the most cost-effective and efficient technologies currently available for vadose zone remediation of petroleum-contaminated sites.
The Development of Bioventing
Addressing Oxygen Supply to Contaminated Areas
One of the main driving forces behind the development of bioventing was the difficulty in delivering oxygen in situ. Many contaminants, especially the petroleum hydrocarbons found in fuels, are biodegradable if oxygen is available. Traditionally, enhanced bioreclamation processes used water to carry oxygen or an alternative electron acceptor to the contaminated zone. This was common whether the contamination was present in the groundwater or in the unsaturated zone. Media for adding oxygen to contaminated areas have included pure oxygen-sparged water, air-sparged water, hydrogen peroxide, and air.
In all cases where water is used, the solubility of oxygen is the limiting factor. At standard conditions, a maximum of 8 to 10 mg/L of oxygen can be obtained in water when aerated, while 40 to 50 mg/L can be obtained if sparged with pure oxygen, and up to 500 mg/L of oxygen theoretically can be supplied utilizing 1,000 mg/L of hydrogen peroxide.
Due to the low aqueous solubility of oxygen, hydrogen peroxide has been tested as an oxygen source in laboratory studies and at several field sites. If 500 mg/L of dissolved oxygen can be supplied via hydrogen peroxide, the mass of water that must be delivered is reduced by more than an order of magnitude. Initially, this theory made the use of hydrogen peroxide appear to be an attractive alternative to injecting air-saturated water.
Hydrogen peroxide is miscible in water and decomposes to release water and oxygen. Many substances commonly present in groundwater and soils act as catalysts for the decomposition of peroxide. Important among these are aqueous species of iron and copper, and the enzyme catalase, which has significant activity in situ. If the rate of oxygen formation from hydrogen peroxide decomposition exceeds the rate of microbial oxygen utilization, gaseous oxygen may form due to its limited aqueous solubility. Gaseous oxygen may form bubbles that may not be transported efficiently in groundwater, resulting in ineffective oxygen delivery.
Phosphate is commonly used in nutrient formulations in an effort to decrease the rate of peroxide decomposition in groundwater applications. However, the effectiveness of phosphate addition in stabilizing peroxide injected into an aquifer has not been well established and conflicting results have been reported by different researchers.
A field experiment was conducted to examine the effectiveness of hydrogen peroxide as an oxygen source for in situ biodegradation. The study was performed at a JP-4 jet fuel-contaminated site at Eglin AFB, Florida. Site soils consisted of fine- to coarse-grained quartz sand with groundwater at a depth of 2 to 6 ft (0.61 to 1.8 m). Previous studies at the same site had shown that rapid decomposition of hydrogen peroxide occurred, even with the addition of phosphate as a peroxide stabilizer. In subsequent studies, hydrogen peroxide was injected at a concentration of 300 mg/L both with and without the addition of a phosphate-containing nutrient solution. As in previous studies, hydrogen peroxide decomposition was rapid, resulting in poor distribution of oxygen in groundwater. Addition of the phosphate-containing nutrient solution did not appear to improve hydrogen peroxide stability.
Other attempts have been made using hydrogen peroxide as an oxygen source. Although results indicate better hydrogen peroxide stability than achieved by the study at Eglin AFB, it was concluded that most of the hydrogen peroxide decomposed rapidly. Some degradation of aromatic hydrocarbons appears to have occurred; however, no change in total hydrocarbon contamination levels was detected in the soils.
In contrast to hydrogen peroxide use, when air is used as an oxygen source in unsaturated soil, 170 ft3 (4,800 L) of air must be delivered to provide the minimum oxygen required to degrade 1 lb (0.45 kg) of hydrocarbon. Since costs associated with water-based delivery of oxygen can be relatively high, the use of gas-phase delivery results in a significant reduction in the cost associated with supplying oxygen.
An additional advantage of using a gas-phase process is that gases have greater diffusivity than liquids. At many sites, geologic heterogeneities cause fluid that is pumped through the formation to be channeled into the more-permeable pathways (e.g., in an alluvial soil with interbedded sand and clay, all of the fluid flow initially takes place in the sand). As a result, oxygen must be delivered to the less permeable clay lenses through diffusion. In a gaseous system (as found in unsaturated soils), this diffusion can be expected to take place at rates at least three orders of magnitude greater than rates in a liquid system (as is found in saturated soils). Although it is not realistic to expect diffusion to aid significantly in water-based bioreclamation, diffusion of oxygen in a gas-phase system is a significant mechanism for oxygen delivery to less-permeable zones.
Given the advantages of using air rather than water as the oxygen source, the feasibility of an air-based oxygen supply system as a remedial option was explored.
Research and Development into Bioventing
The first documented evidence of unsaturated zone biodegradation resulting from forced aeration was reported by the Texas Research Institute, Inc., in a 1980 study for the American Petroleum Institute. A large-scale model experiment was conducted to test the effectiveness of a surfactant treatment to enhance the recovery of spilled gasoline. The experiment accounted for only 8 gallons (30 L) of the 65 gallons (250 L) originally spilled and raised questions about the fate of the gasoline. Subsequently, a column study was conducted to determine a diffusion coefficient for soil venting. This column study evolved into a biodegradation study in which it was concluded that as much as 38% of the fuel hydrocarbons were biologically mineralized. It was concluded that venting not only would remove gasoline by physical means, but also would enhance microbial activity and promote biodegradation of the gasoline.
The first actual field-scale bioventing experiments were conducted by Jack van Eyk for Shell Research. In 1982, a series of experiments was initiated to investigate the effectiveness of bioventing for treating hydrocarbon-contaminated soils. These studies were reported in a series of papers.
In 1986 it was suggested that using air as a carrier for oxygen could be 1,000 times more efficient than using water, especially in deep, hard-to-flood unsaturated zones. The connection was made between oxygen supply via soil venting and biodegradation by observing that "soil venting uses the same principle to remove volatile components of the hydrocarbon." In a general overview of the soil venting process, it was concluded in 1987 that soil venting provides large quantities of oxygen to the unsaturated zone, possible stimulating aerobic degradation. Water and nutrients addition was suggested as a ptential requirement for significant degradation and encouraged additional investigation into this area.
Biodegradation enhanced by soil venting was observed at several field sites. Investigators claimed that at a soil venting site for remediation of gasoline-contaminated soil, significant biodegradation occurred (measured by a temperature rise) when air was supplied. Investigators pumped pulses of air through a pile of excavated soil and observed a consistent rise in temperature, which they attributed to biodegradation. They claimed that the pile was cleaned up during the summer primarily by biodegradation. However, natural volatilization from the aboveground pile was not controlled, and not enough data were published to critically review their biodegradation claim.
Other researchers observed a decrease in the toluene concentration in unsaturated zone soil gas, which they measured as an indicator of fuel contamination in the unsaturated zone. They assumed that advection had not occurred and attributed the toluene loss to biodegradation. The investigators concluded that because toluene concentrations decayed near the oxygenated ground surface, soil venting was an attractive remediation alternative for biodegrading light volatile hydrocarbon spills.
Air Force Bioventing Research and Development
The US Air Force initiated its R&D program in bioventing in 1988 with a study at Site 9141, Hill AFB, Utah. The site initially was operated as a soil vapor extraction unit, but was modified to a bioventing system after 9 months of operation because there was evidence of biodegradation and in an effort to reduce costs by reducing off-gas. Moisture and nutrient addition were studied at this site; however, while moisture addition appeared to improve biodegradation, nutrient addition did not. Final soil sampling demonstrated that benzene, toluene, ethylbenzene, and xylenes (BTEX) and total petroleum hydrocarbon (TPH) levels were reduced to below regulatory levels, and this site became the first Air Force site that was closed through in situ bioremediation. During this study, it became apparent that bioventing had great potential for remediating JP-4 jet fuel-contaminated soils. It also was apparent that additional research would be needed before the technology could be applied routinely in the field.
Following the Site 914, Hill AFB study, a more controlled bioventing study was completed at Tyndall AFB, Florida. This study was designed to monitor specific process variables and the subsequent effect on biodegradation of hydrocarbons. Several important findings resulted from this work, including the effect of air flowrates on removal by biodegradation and volatilization, the effect of temperature on biodegradation rates, the lack of microbial stimulation from the addition of moisture and nutrients, and the importance of natural nitrogen supply through nitrogen fixation. In addition, initial and final contaminant measurements showed over 90% removal of BTEX. Although this study was short-term, it illustrated the effectiveness of bioventing.
The studies conducted at Hill and Tyndall AFBs provided valuable information on bioventing. However, it was apparent that long-term, controlled bioventing studies were necessary to fully evaluate and optimize the technology. In 1991, long-term bioventing studies were initiated at Site 280, Hill AFB, Utah, and at Site 20, Eielson AFB, Alaska. These studies were joint efforts between the US EPA and the US Air Force Environics Directorate of the Armstrong Laboratory (now the Air Force Research Laboratory). These studies involved intensive monitoring of several process variables, including the effect of soil temperature on biodegradation rates, surface emission analyses, and optimization of flowrate. Based on the success of these previous studies, in 1992, AFCEE initiated the Bioventing Initiative where pilot-scale bioventing systems were installed at 125 contaminated sites located throughout the continental US and in Hawaii, Alaska, and Johnston Atoll. The sites varied dramatically in climatic and geologic conditions. Contaminants typically were petroleum hydrocarbons from JP-4 jet fuel, heating oils, waste oils, gasoline, and/or diesel; however, some fire training areas also were studied where significant concentrations of solvents were present. Volume 1 of the Bioventing Principles and Practice Manual is a product of the AFCEE Bioventing Initiative and represents the culmination of data collected from these sites and other projects.
Bioventing Field Design
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AFCEE Bioventing Initiative Sites
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The Structure of the AFCEE Bioventing Initiative Field Treatability Studies and Bioventing System Design
The design of the field treatability studies and final bioventing system was developed based on experience at previous studies at Hill, Tyndall, and Eielson AFBs. The Test Plan and Technical Protocol for a Treatability Test for Bioventing was written to standardize all field methods from treatability tests to well installations. The document allowed for collection of consistent data from 125 sites, which provided a strong database for evaluating bioventing potential. At all sites, the following activities were conducted:
- Site characterization, including a small-scale soil gas survey and collection of initial soil and soil gas samples for analysis of BTEX, TPH, and soil physicochemical characteristics;
- Field treatability studies, including an in situ respiration test and a soil gas permeability test;
- Identification of a background, uncontaminated area for comparison with the contaminated area of background respiration rates and nutrient levels;
- Installation of a blower for 1-year of operation (typically configured for air injection), if results of field treatability studies were positive;
- Conduct of 6-month and 1-year in situ respiration tests at sites where a blower had been installed; and
- Collection of final soil and soil gas samples for analyses of BTEX and TPH.
Of particular significance were the use of the in situ respiration test to measure microbial activity and the use of air injection instead of extraction for air delivery. The in situ respiration test was developed to rapidly measure aerobic biodegradation rates in situ at discrete locations. Biodegradation rates calculated from the in situ respiration test are useful for: (1) assessing the potential application of bioremediation at a given site, (2) estimating the time required for remediation at a given site, and (3) providing a measurement tool for evaluating the effects of various environmental parameters on microbial activity and ultimately on bioventing performance. The actual effect of individual parameters on microbial activity is difficult to assess in the field due to interference and interactions among these parameters. The in situ respiration test integrates all factors to simply assess whether the microorganisms are metabolizing the fuel. Data from the in situ respiration test and site measurements were used to conduct a statistical analysis of the observed effects of the site measurements on microbial activity in the field. The statistical analysis was constructed to account for parameter interactions.
Also of note is that 120 of the 125 bioventing systems installed were configured for air injection. Prior to the bioventing studies conducted at Hill (Site 280) and Eielson AFBs, bioventing systems typically were operated in the extraction configuration, similar to soil vapor extraction systems. However, research at Hill and Eielson AFBs demonstrated that air injection was a feasible and more efficient alternative to air extraction, resulting in a greater proportion of hydrocarbon biodegradation rather than volatilization and reduced air emissions. Therefore, the air injection configuration was selected for the basic bioventing system at Bioventing Initiative sites.
The results generated from the Bioventing Initiative are summarized in detail in Volume 1 of the Bioventing Principles and Practice Manual and are used to illustrate the basic principles of bioventing and microbial processes discussed in the manual. The design guidelines presented in the manual were culminated primarily from the experience of installing and operating the 125 Bioventing Initiative sites. The design guidelines presented in the manual represent the basic bioventing system, which is applicable to the majority of sites suitable for bioventing. The following section addresses emerging techniques for modifications to the basic bioventing system described in the manual for sites that are not amenable to standard bioventing methods.
Emerging Techniques for Modifications to Bioventing Systems
Several techniques have been investigated as a means of modifying the conventional bioventing system. They are briefly presented here to illustrate their potential application to address specific challenges in bioventing:
- Injection of pure oxygen instead of air for treatment of low-permeability soils. Because only low flowrates are possible in low-permeability soils, injection of pure oxygen may be useful for providing larger oxygen concentrations for a given volume than is possible with air injection.
- Soil warming for bioventing in cold climates. Soil warming can be used to increase biodegradation rates, thus decreasing remediation times. This technique has been studied in detail at Site 20, Eielson AFB, Alaska, but might be an option only in extreme environments.
- Remediation of recalcitrant compounds through ozonation. Ozonation may be used to partially oxidize more recalcitrant contaminants, making them more susceptible to biodegradation. This technique would not be necessary at petroleum-contaminated sites, but may be considered at sites contaminated with compounds such as polycyclic aromatic hydrocarbons (PAHs) or pesticides.
- Remediation of contaminated saturated soils through air sparging. Air sparging is being investigated as a means of aerating saturated soil to enhance biodegradation, as well as volatilization. However, studies to date have been inconclusive concerning its effectiveness due to a lack of adequate controls and measurement techniques.
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