![]() A mesh cylinder is designed to float in the LNAPL layer inside a recovery well. Active skimming is typically achieved using pneumatic or electric pumps equipped with fixed intakes equipped with timer operation or a floating filter of oleophilic/hydrophobic mesh having a high affinity for nonpolar hydrocarbons and the ability to reject polar molecules such as water. Active skimming uses a mechanical device or system designed to promote LNAPL migration into a recovery well and to remove it as it enters the well.However, as noted above, NSZD may be a more appropriate technology to apply when these conditions are present at a site. Passive skimmers are ideal in areas where free product contamination is minimal, the LNAPL recharge rate is slow, and where a source of power is not readily available. If the element collects water, it can be removed and dried out to remove the water from the pores, which allows the element to revert to hydrophobic operation. If the hydrophobic element is subjected to hydrostatic pressure from being submerged in water, the element will permit water to pass through it. To recover the LNAPL, the skimmer is pulled out of the well and a valve on the bottom is opened to drain it. The LNAPL is usually collected in a reservoir at the bottom of the unit. ![]() Some systems include a floatation device that allows the skimmer to move up and down as the water elevation changes. The media is periodically pulled from the well and either disposed or the LNAPL is squeezed from the media and placed into a container for disposal.īailer-type skimmer systems are lowered to the elevation of the LNAPL-water interface. The media length can vary, but typically is 3 ft, which allows LNAPL to continue to be recovered if the groundwater level raises or lowers to some extent. When using LNAPL absorbent media, the media is placed inside a supporting canister or cage and then lowered into the well such that the media extends across the LNAPL-water interface. They typically consist of two types of devices: 1) LNAPL absorbent media (e.g., absorbent socks), or 2) a specially designed bailer that consists of a hydrophobic element that will allow LNAPL to pass but not water. Passive skimmers selectively recover LNAPL floating on the surface of the water table within a well or trench without the use of any hoses, external electric or pneumatic power. Hence, an alternative technology, such as natural source zone depletion (NSZD), should be evaluated in lieu of passive skimmer methods. It should be noted, however, that skimming methods don't recover a substantial volume of LNAPL and LNAPL generally is not migrating at times when passive skimming technologies are used. Similar to hand bailing, passive skimmers generally are used to recover LNAPL when little LNAPL remains at a site. Passive skimming techniques are designed to remove LNAPL as it flows naturally (without pumping) into a recovery well or trench.Bailing can be performed daily, weekly, or monthly an appropriate frequency is determined based on the time it takes for the well to recharge as well as any applicable regulatory requirements/agreements.įree product skimming can be accomplished in a variety of methods that can be categorized as active skimming and passive skimming, described as follows: A check valve at the bottom of the bailer allows LNAPL (and some water) to enter, but prevents the fluids from exiting as the bailer is raised from the well. Bailing is performed using either a reusable or disposal bailer that is lowered into the well to remove remaining LNAPL from the well casing. It may be performed after a more aggressive (and costly) recovery technology has been implemented. Hand bailing generally is performed when there is little LNAPL remaining at the site as evidenced by relatively thin but measurable LNAPL thicknesses in one or more wells. ![]() Other Technology Namesįree product recovery systems in order of increasing complexity and cost include the following: The objective of these recovery techniques is to remove light nonaqueous phase liquid (LNAPL) to the extent practicable, prevent its migration and reduce its impact to dissolved phase contaminants in groundwater. Single and Dual Pump Free Product Recovery Technologiesįree product recovery consists of several technologies ranging from simple hand bailers and passive skimmer systems to more complex active skimming systems and large-scale total fluids recovery systems. Skimming Free Product Recovery Technologies This information may be reproduced without restriction as long as the source attribution is included.
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