Ex situ soil vapor extraction (SVE) is a full-scale technology in which soil is excavated and placed over a network of aboveground piping to which a vacuum is applied to encourage volatilization of organics. Soil piles are generally covered with a geomembrane to prevent volatile emissions and to prevent the soil from becoming saturated by precipitation. The process includes a system for handling off-gases. Advantages over its in situ counterpart include that the excavation process forms an increased number of passageways, shallow ground water no longer limits the process, leachate collection is possible, and treatment is more uniform and easily monitored. The major disadvantage over in situ SVE is the increased excavation costs.
The length of operations and maintenance for ex situ SVE process is medium- to long-term.
The target contaminant group for ex situ SVE is VOCs.
Factors that may limit the applicability and effectiveness of the process include:
- Air emissions may occur during excavation and materials handling, possibly requiring treatment.
- High moisture content, high humic content, or compact soil inhibits volatilization.
- In addition to air emission/off-gas treatment(s), residual liquid and spent activated carbon will require treatment, increasing the project costs.
- A large amount of space is required.
Soil characteristics that need to be determined include the concentration of the contaminants, radius of influence, soil type and properties (e.g., texture, moisture content, particle size, permeability, porosity, and TOC), and the presence of oil and grease. Key operating parameters include air flow rate and vacuum pressure required.
An advantage of the technology over its in situ counterpart is the increased number of passageways formed by the excavation process; however, as an ex situ remedy, the excavation associated with SVE poses a potential health and safety risk to site workers through skin contact and air emissions. Personal protective equipment, at a level commensurate with the contaminants involved, is normally required during excavation operations. The time required to remediate a site using ex situ SVE is highly dependent upon the specific soil and chemical properties of the contaminated media. Cleanup of a typical site, consisting of 18,200 metric tons (20,000 tons) of contaminated media, would require 12 to 36 months. Generally, most of the hardware components are relatively well developed with repair parts readily available to minimize downtime. Typical ex situ SVE systems can be operated without a full time operator.
The overall cost for ex situ SVE is under $110 per metric ton ($100 per ton), including the cost of excavation but excluding treatment of off-gases and collected ground water.