Effect of Surfactants on Oil Water Separators

Basic Facts & Theory;

  • Most oils (hydrocarbons) will separate out from water naturally, given enough time, and rise to the surface forming a layer.
  • Large droplets rise quickly, tiny droplets rise very slowly.
  • Coalescing Plates intercept the path of these oil droplets and effectively reduce the time needed to achieve separation (residence time) by a factor of up to 10.
  • This is all based on Stokes’ Law with clean fresh water (SG=1.0) and clean oils (SG=<1.0) The greater the difference in SG between water and oil the more efficient (and faster) the process is.


  • Detergents, degreasers, solvents etc. are designed to aid in the removal of oils from a surface or item. They do this by a complex chemical reaction.
  • Their purpose is the exact opposite to what oil water separators are trying to achieve.
  • A very simple analogy is that the detergents form a slippery coating around the oil droplet so that it slides off the surface to be cleaned. As a result, this slippery oil droplet remains in the water as an “emulsion” (surface tension/pressure of oil droplet is altered)
  • A strong degreaser will hold the oil droplet in solution indefinitely, but a “quick break” detergent may “release” the droplet after a short period of time.

Practical Application;

Try asking your detergent supplier two definitive questions;

a)    How long (in minutes) will it take for the detergent to release the oil droplet? This has to occur in a large Holding Tank before the oily water enters the Separator.

b)    How effective will the release be? (95%, 99%, or ?%). If they claim 99% release, then 1% may still be held in suspension. Remember that 1% = 10,000 ppm.


  1. A chemical emulsion requires chemicals to break the emulsion. Detergents cause a chemical emulsion. Our RM-10 chemical treatment systems remove surfactants, heavy metals, oils, solids, etc. in one step process.
  2. A physical emulsion (pumps, pressure, shearing forces) should break with time (could be minutes/hours/days?). This can allow an oil water separator to function correctly.
  3. Conduct a simple “Jar Test” on approx. 200ml of oily water emulsion, by letting it stand still in a clear container for 45 to 60 minutes. Whatever settles out, or rises to the surface, is representative of what an oil water separator will remove.
  4. A very small amount of a good quality “Quick Break” detergent may have little effect on the quality of discharge water, but it needs to be carefully tried in practice to determine what amounts can be accommodated.
  5. Adding more plates or slowing the flow rate down will NOT remove or counter the detergents.

If discharge is to go to sewer, then it may not be too critical, however, if discharge is to go to storm water, then a final “polishing” type of filter may be needed after the Separator. Baldwin have polishing filters available to reduce the hydrocarbon content from oil water separator outlets – both new and existing separators.

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