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Treatment of Petroleum-Contaminated Soils

Treatment Options (continued)

Ex Situ Vapor Extraction

The volatile fraction of petroleum products can be removed by passing air through the contaminated soil. Vapor extraction works well on the highly volatile fraction of petroleum hydrocarbons (those with a boiling point below 300 °C). Contaminated soil is spread out on a network of aboveground piping. A vacuum is applied to pull air through the contaminated soil. A vacuum can be created by a power vacuum blower or by a wind-actuated exhaust fan (no external power source is required). Other variations of this process include piling the contaminated soil into a mound aboveground or in the excavated pit, placing slotted piping into and below the mound, and covering the mound with an impermeable liner. Once again, a vacuum is applied to the piping and air is drawn through the contaminated soil. Constructed reactors can also be used for vapor extraction. The contaminated soil is loaded into a reactor and air is drawn through the soil by forcing air into the reactor or by creating a vacuum that draws air through the soil. Whichever process is used, once the flow of air is established in the porous medium, volatile components contained in the soil preferentially partition into the flowing air from:

• The liquid phase volatile petroleum compounds contained in the pore space
• The portion of the compound that is dissolved in the soil water
• The portion of the compound that is adsorbed onto soil particles

Compounds with high vapor pressures, high Henry's Law constants, and low sorption characteristics are best suited for this type of treatment. Aromatics (the most toxic of the petroleum hydrocarbons) are for the most part amenable to venting. However, higher molecular weight PAH compounds such as benzo(a)anthracene and benzo(a)pyrene will not be effectively removed by venting.

Under the right conditions, biological treatment in the soil aided by the movement of air through the soil during vapor extraction may also reduce the level of petroleum hydrocarbons. The common thought is that while biological treatment may be occurring during the extraction process, the dominant removal process is vapor extraction. The extracted vapors may be treated to remove the hydrocarbons by vapor-phase granular-activated carbon or by vapor-phase incineration.

Ex situ vapor extraction is similar to the in situ process of soil vapor extraction that will be described later in this report. There are some advantages to performing vapor extraction ex situ. The main advantage is that there is more control over the process. Having more control means that the diffusion limitation caused by heterogeneous soil may be less of a problem. Also, the temperature of the extraction air can be more efficiently controlled. Increasing the extraction air temperature is advantageous because volatility increases with increasing temperature.

Vapor extraction is a simple process requiring only a blower (or blowers), piping, and possibly an impermeable liner or a constructed reactor. The time required to treat contaminated soil depends on soil type, temperature, and moisture content.

Use in Cold, Wet, Remote Regions

Because ex situ vapor extraction offers more control over the extraction process, it seems that ex situ vapor extraction would be more effective in cold regions than in situ vapor extraction. Simple engineering solutions to the process for operating in cold regions include heating the air before passing it through the soil and containing the soil to decrease heat loss. Also, soil that is excavated and treated ex situ may achieve higher temperatures during the summer months than soil below grade. This is particularly true for excavated soil that is spread in a relatively thin layer above grade, as would be the case for one of the vapor extraction scenarios described.

The effect temperature has on vapor extraction in the cold with no engineered controls (heating) can be illustrated with an example. The Henry's Law constant for benzene at 25 °C (table 4) is 0.00476 atmosphere cubic meter per mole and the reported value for KH is 0.0033 atmosphere cubic meter per mole at 10 °C. Calculating the mass of benzene that will partition to the air phase at the two temperatures shows that 37 percent more time will be required to remove the same amount of benzene dissolved in the soil water at 10 °C as at 25 °C. Obviously, heating the soil in these conditions will increase the efficiency of the removal process. However, heating may not be cost effective.

High soil moisture in contaminated soil will limit the effectiveness of vapor extraction. Water contained in the soil will decrease the permeability of the media. In heterogeneous soil, water may severely slow the removal process in discrete volumes of soil within the volume of soil to be treated. Covering the soil and using reactors will help decrease the effects of precipitation in wet regions. If vapor extraction is to be performed on soil piles or on soil that has been spread on the surface, liners and berms may be required to contain runoff water during storms.

This treatment process seems to be applicable to remote regions. In addition to the issues associated with soil excavation in remote regions, the only other issue is the provision of three-phase electrical power (unless wind-actuated fans are used) and the need for visits to the site to check on the operation and conduct maintenance. The required number of visits to the site for operation and maintenance depends on factors such as the volume of soil to be treated, the complexity of the venting system, and requirements that might be imposed by regulatory agencies. Accommodations during the installation of the treatment system will also need to be accounted for in a cost estimate.

Ex situ vapor extraction has been successfully used in Alaska. Engineers contacted for this study have used vapor extraction to treat petroleum-contaminated soil in the Arctic during summer months (information provided by Shannon and Wilson, Inc., appendix C). Soil piles (placed above grade or in the excavation pit) and constructed reactors seem to work best in this region. Extraction air is heated to extend the season of operation. Alaskan engineers have also had success using wind-actuated exhaust fans. Ex situ vapor extraction using wind-actuated exhaust fans might noticeably reduce the rate of contaminant mass removal compared to a powered blower system capable of moving larger volumes of air.

Cost Estimate

Table 12 shows the items to be considered in developing a cost estimate for vapor extraction. Assumptions include:

• After treatment, the soil can be used as fill material.
• The Forest Service owns the backhoe.
• The backhoe used for soil excavation can be used for placing the soil into the area or in the reactor to be used for vapor extraction and can be used to remove treated soil after treatment is complete.
• No hazardous waste is produced.

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This page last modified September 23, 2002
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