Refinery wastewater treatment — removal of free oil In petroleum refinery effluents, oil is present in both free and emulsified forms. Which unit process is used specifically to remove free oil from wastewater?

Introduction / Context:
Refinery wastewater typically contains oil in free, dispersed, and emulsified states. Treatment trains are staged so that free oil is removed early to prevent interference with downstream biological or physicochemical processes.

Given Data / Assumptions:

• Influent contains separable free oil droplets larger than typical emulsion droplet sizes.
• Gravity separation is feasible due to density difference between oil and water.
• Subsequent steps may include DAF, API separators, and biological treatment.

Concept / Approach:
Free oil separates by gravity because its droplet size is large enough for buoyant rise at practical residence times. API or parallel-plate separators create quiescent zones and short settling paths; skimmers collect the oil layer. Biological units (oxidation ponds, lagoons, trickling filters) are not designed for primary free-oil removal and can be upset by oil carryover.

Step-by-Step Solution:
Classify oil: free vs. emulsified.Match mechanism: gravity separation for free oil; coalescers or chemical treatment for emulsions.Select “gravity separator with skimming” as the proper primary unit.

Verification / Alternative check:
Standard refinery wastewater flow diagrams show API separators upstream of biological treatment precisely for free oil removal.

Why Other Options Are Wrong:

• Oxidation pond / aerated lagoon / trickling filter: biological systems targeting dissolved and colloidal organics, not primary oil separation.

Common Pitfalls:

• Confusing dispersed/emulsified oil (needing DAF or emulsion breaking) with free oil removable by simple gravity.

Final Answer:
Gravity separator with oil skimming devices