Dual-phase vacuum-extraction system design theory
Dual-Phase Vacuum-Extraction (DPVE) is an aggressive remedial technique, that has gained wide acceptance by EPA as an effective method for remediating soil and groundwater impacted by VOCs. IMT, a provider of new and used remediation equipment, provides information on the design and use of dual-phase vacuum-extraction systems. The basic concept is similar to conventional soil vapor extraction in that a vacuum is applied to an extraction point to facilitate removal of VOCs...
• IMT shares valuable information on the design of dual-phase vacuum-extraction systems and equipment used in the effective remediation of contaminated soil and groundwater.
BIRMINGHAM, AL, March 12, 2008 -- Dual-Phase Vacuum-Extraction (DPVE) is an aggressive remedial technique, which has gained wide acceptance by the EPA as an effective method for remediating soil and groundwater impacted by volatile organic compounds (VOCs). IMT, a provider of new and used remediation equipment provides information on the design and use of dual-phase vacuum-extraction systems.
The basic concept is similar to conventional soil vapor extraction in that a vacuum is applied to an extraction point to facilitate removal of VOCs. However, with this process, the extraction wells are screened above and below the water table to allow soil gases drawn into the well under the influence of the vacuum to entrain the liquid phase (groundwater and free product), so that both phases are brought to the surface. When applied correctly the process results in rapid, simultaneous remediation of both soil and groundwater.
The components of a DPVE system must be capable of extracting both subsurface vapor and groundwater at a rate that will adequately remediate the site. The DPVE system consists primarily of liquid and vapor extraction, phase separation, and air and water treatment equipment. Typical components include a vacuum pump, an oil/water separator, air stripper, phase separation vessels, transfer pumps, and associated instrumentation, wiring, and piping. Specialized equipment, depending on site-specific requirements, may include filtration equipment for suspended solids, vapor and liquid adsorption vessels for VOC removal, sequestering agent and pH balance to reduce precipitation of dissolved metals, and thermal or catalytic oxidizers for VOC destruction in the vapor stream.
The best tool for the proper design of a DPVE system is a pilot study. IMT recommends a 24-hr pilot study with readings taken on defined time intervals to establish response curves. Information that should be collected during the pilot study should include the following:
• Applied vacuum
• Air flow, temperature, and VOC concentrations
• Groundwater drawdown
• Subsurface pressure (inches of water)
• Liquid recovery rate
• Dissolved concentrations
• Water quality
Applied vacuum, airflow, and temperature are essential in the design of the DPVE vacuum pump. There are several types of vacuum pumps on the market today, and depending on the site conditions, careful consideration must be taken in selecting the proper pump.
Pneumatic and Hydraulic radius of influence are important in determining the number of extraction wells and placement. The more wells that are required to recover vapors and liquids from the subsurface, the larger the DPVE equipment will need to be. Keep in mind that it is expensive to treat air and water that is extracted from the subsurface. Therefore, if air treatment and water treatment is required to meet discharge limits, and you will be extracting large volumes of air and water, then it may be more cost effective to use another technology. Typical vapor extraction rates are less than 400 standard cubic feet per minute (SCFM) at 20 inHg. Typical liquid extraction rates are less than 30 gallons per minute (GPM).
Contaminant concentrations in both the liquid and vapor phase provide the necessary information needed to size the air and water treatment equipment. Sometimes carbon adsorption vessels can be used to treat the vapor stream. Higher concentrations will require other means of air treatment such as an oxidizer because of the amount of carbon required to treat the air. Another consideration when using carbon is the temperature of the air stream. The efficiency of carbon decreases as the vapor temperature increases. We recommend that the temperature of the air stream be less than 110°F.
For water treatment, information on the type of contaminant and its concentration is used to size the air stripper and carbon vessel if used to polish the water discharge. If you are using carbon, we recommend that a bag filter be used to filter precipitants subsequent to the air stripper and prior to the carbon vessel. If the groundwater contains high levels of dissolved phase metals such as iron, calcium and phosphorous, then a sequestering agent may be necessary hold the dissolved metals in solution, which want to combine with the oxygen that is introduced to the groundwater discharge stream by the DPVE system and air stripper.
IMT assists in the design and implementation of site-specific pilot testing at project sites to help evaluate remedial alternatives in order to create the optimal remedial equipment solution.