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Multiphase Extraction (MPE)
This is a well used common approach, combining the removal of dissolved phase contaminants in groundwater in the saturated zone, together with vapour in the unsaturated zone by vacuum (slurping) also known as vacuum enhanced recovery (VER) and non-aqueous phase liquid (NAPL) or free phase product, from recovery well points. The associated drawdown of the water table towards the well by dewatering creates a cone of depression, which enhances recovery of vapours by from the originally saturated smear zone.
The application of high vacuum can also aid the recovery of NAPL and water from fine grained soils, with induced air flow through the contaminated zone helping to promote aerobic biodegradation.
LNAPL is recovered by skimmer units as it flows towards the recovery point, with oil/water separators used at the surface to separate out the water. Product removal is important when considering remedial design, as it can act as a secondary source of contamination on a site and its recovery can account for a significant amount of contaminant mass reduction. Product recovery can be enhanced by surfactant / cosolvent flushing. This method involves the injection and subsequent extraction of either biodegradable detergent or solvent into the saturated zone, within target areas to solubilise and/or mobilise DNAPLs. This would provide greater recovery than would be achieved using abstraction wells and excavation sumps. Extracted DNAPL can then be disposed off site or can go for recycling at special facilities. The vapour extracted is generally treated by stripping GAC units, with dissolved phased contaminants in water treated on site using a variety of physical, chemical and biological techniques, prior to disposal to foul sewer or recirculation into the site via infiltration trenching, depending on the quality required.