Public Services and Procurement Canada
Skimming is a technology that can recover free products which are generally found on the surface of the water table. Passive skimming makes it possible to manually recover the product in free phase flowing freely towards a well or a trench. Active skimming requires the use of a mechanical or pneumatic system or device to promote the recovery of the free phase product at a recovery point.
The passive method consists of three classes of techniques. The first class consists of a skimmer where the free phase is introduced into the skimmer by gravity. The skimmer must be calibrated to the density of the product in order to float on the water, such as the inlets will be at level with the product and allow the product to be recovered. Another variant of this is an oleophilic and hydrophobic absorbent or “skimming sock” installed in the free phase. The absorbent is either removed and disposed of or reused once the product has been removed. Finally, the third type involves the introduction of a reservoir with an oleophilic and hydrophobic wall at the free phase level.
Active skimming is used to pump the free phase using a pump (pneumatic or peristaltic). The product in free phase is accumulated in a container and then pumped periodically inside a tank placed on the surface of the ground. The pumping cycles can be controlled by a timer or using high and low level probes.
In surface wells, skimming can also consist of oleophilic and hydrophobic bands which allow for the removal of the free phase by immersion and extraction during the bands’ rotations. The band, saturated with free phase, is lifted from the well with a motorized pulley and passes through scrapers where the product is removed and transferred to storage drums. In trenches, the same principle can be used with rotating disk skimmers, which their dimensions prevent their use in wells.
The implementation of this type of system may include:
Skimming can also be done inside an excavation.
If hydraulic control of the contamination is required, skimming should be done within the hydraulic downstream recovery trenches and across the width of the contamination plume. Trenches must reach a depth below the lowest annual groundwater level and be filled with sand and gravel. An impermeable liner should also be installed on the downstream wall of the trench to limit a potential migration of contamination beyond this trench.
Skimming can be done in a northern environment under low temperature conditions. The viscosity of the products in the free phase is, however, dependent on the temperature. Conclusive tests were carried out using drum skimmers. Those with a grooved surface compared to those with a smooth surface resulted in more conclusive results. The presence of ice mixed with the oil reduces the recovery efficiency, since it accumulates in the grooves of the drums.
Note: Free-phase products are generally composed of several different products that belong to more than one family of contaminants.
Free-product levels and thicknesses should be monitored when the recovery work is stopped, and recovery devices may be required once more if free-phase product is again measured in the wells.
Transmissivity tests (bail down or skimming tests) generally determine whether the recovery device should be put back in place if free-phase product is detected again.
The following site provides an application example:
Main Exposure Mechanisms
Applies or Does Not Apply
Monitoring and Mitigation
Emissions monitoring at the source (choice of parameters, types of samples and type of intervention [source, risk or local requirements]).
Atmospheric / Steam Emissions—Point Sources or Chimneys
Does not apply
Atmospheric / Steam Emissions—Non-point Sources
Air / steam—by products
Modelling and monitoring using pressure sensors
Groundwater—chemical/ geochemical mobilization
Accident/Failure—damage to public services
File checks and licensing prior to drilling or digging, development of drilling or digging procedures and emergency response.
Accident/Failure—leak or spill
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.
Other—handling of contaminated soils
Composed by : Martin Désilets, B.Sc., National Research Council
Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada
Updated Date : April 29, 2014
Latest update provided by : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée
Updated Date : March 22, 2019