Abs: Multi-Phase Extraction (MPE) is a rapidly emerging, in-situ remediation technology for simultaneous extraction of vapor phase, dissolved phase and separate phase contaminants from vadose zone, capillary fringe, and saturated zone soils and groundwater. It is a modification of soil vapor extraction (SVE) and is most commonly applied in moderate permeability soils.
Multi-Phase Extraction (MPE) is a rapidly emerging, in-situ remediation technology for simultaneous extraction of vapor phase, dissolved phase and separate phase contaminants from vadose zone, capillary fringe, and saturated zone soils and groundwater. It is a modification of soil vapor extraction (SVE) and is most commonly applied in moderate permeability soils.
In-situ soil and groundwater remediation techniques are being relied on more and more frequently as methods that are less expensive than excavation and that do not simply move the contamination to another location. However, the limitations of many solitary in-situ technologies are becoming more apparent, especially longer-than-expected remediation times. In addition, solitary technologies may only treat one phase of the contamination when, in fact, the contamination is often spread through multiple phases and zones. For example, SVE and bioventing treat only the vadose zone and groundwater pump-and-treat removes dissolved material only from the saturated zone. Most separate (free) phase [Lighter (than water) Non-Aqueous Phase Liquid (LNAPL)] recovery systems rely on gravity alone to collect and pump the LNAPL. In contrast, MPE can extract:
- Groundwater containing dissolved constituents from the saturated zone.
- Soil moisture containing dissolved constituents from the unsaturated zone.
- LNAPL floating on the groundwater.
- Non-drainable LNAPL in soil.
- Perched or pooled Dense Non-Aqueous Phase Liquid (DNAPL), under some conditions.
- Soil gas containing volatile contaminants.
It is therefore a technology that finds its widest use in source areas.
In general, MPE works by applying a high vacuum (relative to SVE systems) to a well or trench that intersects the vadose zone, capillary fringe and saturated zone. Because the resulting subsurface pressure is less than atmospheric, groundwater rises and, if drawn into the well, is extracted and treated aboveground before discharge or reinjection. If liquid and gas are extracted within the same conduit (often called a suction pipe or drop tube), this form of MPE is often called “bioslurping” (when used for vacuum-enhanced LNAPL recovery), or “two-phase extraction” (TPE, often when used to address chlorinated solvents). If separate conduits for vapor and liquids are used, some call the technology “dual-phase extraction” (DPE). (These terms, “twophase extraction” and “dual-phase extraction” more commonly refer to situations where there is no LNAPL.) LNAPL floating on the water table will also flow into the well screen and be removed. Due to the imposed vacuum, soil moisture and NAPL retained by capillary forces within the soil can, to some degree, also move to the well for collection and removal. The groundwater level may be lowered, thereby creating a larger vadose zone that can be treated by the SVE aspect of MPE. The soil gas that is extracted is, if necessary, conveyed to a vapor-phase treatment system (i.e., activated carbon, catalytic oxidation, etc.), prior to its discharge.
Because air movement through the unsaturated zone is induced during MPE, oxygen can stimulate the activity of indigenous aerobic microbes, thereby increasing the rate of natural aerobic biodegradation of both volatile and nonvolatile hydrocarbon contamination.
One of the difficulties encountered with MPE is the tendency to form emulsions of LNAPL and groundwater that may need to be “broken” or separated before subsequent treatment or disposal.
In dual-phase extraction (DPE), soil gas and liquids are conveyed from the extraction well to the surface in separate conduits by separate pumps or blowers. A common “pipe within a pipe” configuration is depicted in Figure 2-1. It shows that a submersible pump suspended within the well casing extracts liquid, which may be NAPL and/or groundwater, and delivers it through a water extraction pipe to an aboveground treatment and disposal system. Soil gas is simultaneously extracted by applying a vacuum at the well head. The extracted gas is, in turn, conveyed to a gas-liquid separator prior to gas phase treatment. DPE is in essence a rather straightforward enhancement of SVE, with groundwater recovery being carried out within the SVE well. Other DPE configurations are also common, such as use of suction (e.g., exerted by a double-diaphragm pump at the ground surface) to remove liquids from the well, rather than a submersible pump (Blake and Gates 1986). A line-shaft turbine pump could also be employed to remove liquids from the well, provided the water table is shallow enough.
NAPL Recovery. If a subsurface zone containing NAPL (i.e., a source zone) is present at a site, the most efficient way to remove contaminant mass is direct extraction of the NAPL itself, if it is amenable to recovery. Furthermore, free-product recovery to remove the bulk of the floating product is generally considered a prerequisite to the application of in-situ technologies, such as BV, that require a well-aerated soil for spatially distributed microbial growth and hydrocarbon degradation (Baker 1995). The successful removal of NAPL depends greatly on the method of free-product recovery that is selected.
Conventional LNAPL Recovery. Where floating product forms a continuous, free-phase layer on the water table, and especially in coarsetextured soils (e.g., sand and gravel), conventional modes of free-product recovery using submersible and skimmer pumps in wells/trenches are generally effective (API 1996; USEPA 1996). Submersible pumps generally extract NAPL and water, whereas skimmer pumps can extract LNAPL only. Submersible single- or double-pump systems (Figure 2-6a and b) extract groundwater and product and thus create a cone of depression in the water table. The resulting drawdown produces a hydraulic gradient, causing floating product to flow into the well. Because water that has been in contact with NAPL is also recovered, it must be treated prior to discharge. Skimmer systems (Figure 2-7) recover floating product only and do not usually induce a significant cone of depression. Floating filter scavenger systems, for example, can remove product down to thin layers as they track fluctuations in the water table. Although recovery rates are generally smaller, skimmer systems have the advantage that treatment of water is not required. Such systems tend to be most suitable for highly permeable formations, or where recovery rates would not be sufficient to justify operation of more costly combined water and product recovery systems. Absorbent bailers and belt skimmers also fall within this category, but are suitable only when very low rates of product recovery are acceptable. Table 2-1 presents a range of free-product recovery approaches and relative advantages and disadvantages of each. Note that pneumatic transfer of flammable liquids by air pressure (in direct contact with the liquid) is prohibited by EM 385-1-1. If pneumatically operated pumps are used, it must be ensured that the air supply is 100% isolated from free product. Most pneumatic remediation pumps sold today and/or operating today keep the motive air separate from the pumped liquid; therefore, they do not violate this prohibition.
Vacuum-Enhanced LNAPL Recovery. Vacuum-enhanced free-product recovery (Blake and Gates 1986; Hayes et al. 1989; API 1996) is employed, usually in medium-textured soils, to increase recovery rates of LNAPL relative to those that can be obtained using conventional means. The application of a vacuum to a recovery well increases the extraction flow rate without inducing a physical cone of depression (Blake and Gates 1986). In cases where physical drawdown is used in combination with vacuum enhancement, the effective drawdown, by superposition, is the sum of the induced vacuum (expressed in water equivalent height) and the physical drawdown (Figure 2-8). The gradient of hydraulic head that is the driving force for flow of liquid to the well is thus increased. Consequently, the volume of water extracted typically increases to an even greater extent than does the volume of LNAPL. Vacuumenhanced recovery may also mobilize some of the LNAPL that would not otherwise be able to drain into a well because it is retained by capillary forces (Baker and Bierschenk 1995). Offsetting the increase in LNAPL removal is the necessity to treat and/or discharge a larger volume of extracted groundwater and an extracted gas stream.