The objective of the dispersed oil removal or primary deoiling stage is to remove about 90% of the dispersed oil. Effluent from this stage typically vary from 50 to 200 ppm oil.

Options for primary treatment include:

  • Skim tank
  • Voraxial Separator
  • Plate Separators
  • Static Hydro-cyclones

Skimming tanks or vessels are gravity-based separation equipment that is designed for continuous separation of oil droplets from the water. Skimmers are often provided for surge protection and large slug removal. Droplets above a specified diameter will rise to the surface and be skimmed off the water surface. Skimmers include vertical and horizontal variations each with their own advantages and dis

Advantages of Vertical Skimmers

  • May be preferred to the horizontal skimmer where significant solids are to be handled by the skimmer. More efficient in removing accumulated solids.
  • Are sometimes preferred to the horizontal skimmer where large surges in liquid levels are likely.

Disadvantages of Vertical Skimmers

  • The oil droplets must rise upwards against the flow of water.
  • Even if fitted with distributors and baffles, large vertical skimmers may be relatively inefficient due to internal turbulence and short circuiting. Large skimmers should not be relied upon for efficient separation.

Advantages of Horizontal Skimmers

  • Oil droplets rise perpendicular to the flow of water.
  • More efficient at oil removal. 

Operational & Design Considerations

  • Horizontal skimmers are typically horizontal cylindrical vessels or rectangular tanks sized using Stokes law.
  • Plate packs can often be installed inside the tanks to improve performance.
  • A siphon leg on the water outlet may be installed to control the water level inside the tank. But this will prevent the tank from being used as a surge vessel and may not protect the downstream equipment from surges.
  • In order for the skimmer to be used as a surge vessel, the water level must be allowed to vary somewhat.
  • Due to the changing levels of a skimmer during surges, oil cannot be continuously removed by a weir or fixed skimming position. Oil must be removed manually or continuously removed by a floating skimmer.