Fact sheet: Skimming

From: Public Services and Procurement Canada

On this page

Description

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.

Sources:

Implementation of the technology

The implementation of this type of system may include:

  • Mobilization, access to the site and setting up temporary facilities.
  • The development of wells or salvage trenches (including temporary trenches or for emergency operations).
  • Implementation of free product recovery devices.
  • Installation of free product recovery tanks and lines.
  • Disposal of the product in the free phase recovered.
  • Dismantling of wells or trenches prepared for recovery.

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.

Materials and Storage

  • Wells or trenches are constructed using traditional/common and readily available methods and equipment.
  • Pumps that allow free product flow to the recovery tank require energy.
  • A free-product recovery tank usually stays on-site.
  • Unless a significant number of trenches or wells are required, the site layout has little impact and requires minimal storage.

Residues and Discharges

  • Pit or trench development generally requires drilling or excavation activities in contaminated areas. This results in the handling of contaminated soils that will have to be removed off-site.
  • Absorbents used should be disposed off-site at the end of their usage.
  • The recovered free phase product will accumulate inside a tank. It will then have to be managed off-site.

Recommended analyses for detailed characterization

Chemical analysis

  • Contaminant concentrations:
    • identification and concentration of all contaminants (sorbed, dissolved, and free phase)
  • Presence of free-phase product
  • Petroleum product identification

Physical analysis

  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
    • interfacial tension

Recommended trials for detailed characterization

Hydrogeological trials

  • Permeability test
  • Free product recovery tests

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient
  • The state of the free product (confined, perched, free)

Applications

  • Skimming allows for the recovery of free phase hydrocarbons situated on the groundwater.
  • Passive skimming is generally applied for sites with a small volume of free-phase product, where recharge in the well is low and where power sources are not available.
  • Skimming systems can be used in shallow trenches to recover free phase located near ground level or installed within wells to recover free phase product located at greater depths.
  • Efficient system when the soil permeability is greater than 10-4 cm/s.
  • Skimming is often employed after other free phase recovery techniques have been used, as a primary intervention after a spill or when the thickness of the free phase significant.

Applications to sites in northern regions

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.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Does not exist
Free Phase
Applies
Physical
Applies
Residual contamination
Does not exist
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
With restrictions
Chlorobenzenes
Does not apply
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Does not apply
Pesticides
Does not apply
Petroleum hydrocarbons
With restrictions
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
Does not apply

Notes:

Note: Free-phase products are generally composed of several different products that belong to more than one family of contaminants.

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Applies
More than 5 years
Applies

Long-term considerations (following remediation work)

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.

Secondary by-products and/or metabolites

None.

Limitations and Undesirable Effects of the Technology

  • The time for free phase recovery increases when soil permeability is low.
  • Free-phase product recovery is impaired when there are significative fluctuations in the water table level.
  • Offers no hydraulic confinement to prevent migration of free phase.
  • Risk of off-site migration.
  • Low radius of influence.
  • Risk of clogging of hydrophobic/oleophilic membranes.
  • The greater the viscosity of the product in the free phase, the more difficult it is to recover.
  • Does not allow the recovery of free-phase product located in the unsaturated zone.
  • This technology does not apply for a site where control of the dissolved contamination plume is required.
  • Free-product recovery rates are generally low.

Complementary technologies that improve treatment effectiveness

  • Thermal soil enhancement.
  • Applying a hydraulic or pneumatic gradient towards the recovery wells (double-phase or triple-phase recovery).
  • The use of gravel in the trenches allows a faster recovery of the product in the free phase.

Required secondary treatments

  • According to the design of the skimming system, a phase separator may be required followed by treatment of the collected aqueous phase.

Application examples

The following site provides an application example:

Performance

  • Allows for recovery of the free phase until less than 1 mm of product is left on the groundwater.
  • Little, if any, contaminated groundwater to manage.
  • Low operational costs.

Measures to improve sustainability or promote ecological remediation

  • Choice of equipment and optimization of the size of each.
  • Optimization of the calendar to reduce the number of days of mobilization.
  • Use of renewable energy and low-energy equipment.
  • Recycling of recovered free-phase product for use as fuel.
  • Cyclic rather than continuous operation to improve recovery.
  • Operation in favourable seasonal conditions (during low water periods, for example).

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Does Not Apply

Monitoring and Mitigation

Dust

Applies

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

N/A

Atmospheric / Steam Emissions—Non-point Sources

Does not apply

N/A

Air / steam—by products

Does not apply

N/A

Runoff

Does not apply

N/A

Groundwater—displacement

Applies

Modelling and monitoring using pressure sensors

Groundwater—chemical/ geochemical mobilization

Does not apply

N/A

Groundwater—by-product

Does not apply

N/A

Accident/Failure—damage to public services

Applies

File checks and licensing prior to drilling or digging, development of drilling or digging procedures and emergency response.

Accident/Failure—leak or spill

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.

Accident/failure—fire/explosion 

Does not apply

N/A

Other—handling of contaminated soils

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.

References

Author and update

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

Version:
1.2.1