Question 2 of 18: Removing Emulsified Oils from an Industrial Wastewater

Question 2: Removing Emulsified Oils from an Industrial Wastewater (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Emulsified oil is stabilized as fine droplets by a surfactant film and does not separate by gravity alone the way free (non-emulsified) oil does, so removal is a two-stage process: break the emulsion, then separate the freed oil. The emulsion is broken chemically — lowering the pH (acid cracking) to destabilize the surfactant film, and/or dosing a coagulant such as alum or ferric chloride, or a cationic polymer, to neutralize the droplets' surface charge and allow them to coalesce into larger, gravity- or flotation-separable globules. Heat can assist by lowering the oil's viscosity and further destabilizing the film. Once the emulsion is broken and coalesced, the freed oil is removed by dissolved air flotation (DAF): fine air bubbles attach to the coalesced oil droplets and float them to the surface as a scum layer that is mechanically skimmed off, while the clarified water passes underneath to further treatment. Where a very fine, stable emulsion resists chemical breaking, ultrafiltration membranes provide an alternative that physically rejects oil droplets above the membrane's pore size regardless of emulsion stability.