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Skimming tank: purpose and construction

Short answer

Purpose is to prevent oil and grease from forming scum, clogging filters and disturbing biological treatment. Construction: rectangular tank about 1.5 m deep, 3 min detention, diffusers supply 0.3 m³ air/m³ sewage; submerged baffle holds scum, sewage exits below; scum skimmed by trough or mechanical skimmer.

With neat sketches, describe the purpose and construction of a skimming tank.

A skimming tank removes floating grease, oil, fat and soap scum from sewage before it reaches the sedimentation tank.

Purpose

  • To stop oil and grease forming scum in sedimentation tanks, which causes odour, clogs trickling-filter media and disturbs sludge digestion and aeration.
  • Needed mostly for sewage from hotels, restaurants, slaughterhouses, dairies and workshops.

Construction

  Section                              Plan
  air ->     ~~~~~~~~~~ water level    +----------------+
  inlet -> |  o  o  o   scum/ baffle   | inlet ->  |    |
  ->       |  air rises  grease -> trough  diffusers | outlet
           |__diffusers____|__ outlet below baffle --> |
  • A rectangular tank about 1.5 m deep with a detention time of about 3 minutes.
  • Diffusers along one side supply air (about 0.3 m³ per m³ of sewage). The rising bubbles lift grease and oil to the surface and give a spiral flow.
  • A submerged baffle (scum board) keeps floating matter back, and the sewage is drawn off below it.
  • Scum is collected in a trough at the surface and is removed by hand or a mechanical skimmer, then dried, buried or burnt.

✓ Answer checked by Claude Opus 5.5 (high effort)

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