Preliminary treatment of sewage — what is the unit process called that removes free oil and grease from wastewater?

Introduction / Context:
Preliminary treatment protects downstream processes by removing large solids, grit, and floatables. Free oils and grease can foul equipment, inhibit biological processes, and cause odours. A dedicated step is used to remove this fraction before primary settling and biological treatment.

Given Data / Assumptions:

• Wastewater contains free (non-emulsified) oil and grease that naturally float.
• Flow conditions allow surface accumulation and removal.
• Coarse solids have already been intercepted by screens/grit chambers.

Concept / Approach:

Skimming is the process of removing floating matter from the surface. In skimming tanks (or API separators in industrial settings), oils accumulate at the surface and are scraped or skimmed off. Screening removes coarse debris via bar racks; filtration is a separate solids removal process typically used for tertiary polishing, not for bulk oil and grease removal in the headworks.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Design references specify skimming devices in primary clarifiers and separate separators for oily wastewater—confirming the terminology.

Why Other Options Are Wrong:

Screening targets rags and large debris; filtration is not the primary headworks method for oil; “none” contradicts standard practice; “floatation” is a different enhanced process (e.g., DAF) but the basic term here is skimming.

Common Pitfalls (misconceptions, mistakes):

Confusing skimming with dissolved air flotation; attempting to remove emulsified oils by simple skimming (requires coagulation/DAF).

Final Answer:

skimming