What is the main function of a skimming tank?

To remove floating oil, grease and scum from the surface

How do the air diffusers and the baffle work together?

Air bubbles lift grease to the surface; the dip baffle keeps it from leaving with the effluent

Air settles grease; the baffle sends it to the bottom

Air dissolves grease; the baffle supplies oxygen

Air and baffle only mix the sewage

Which statement correctly separates a grit chamber from a sedimentation tank?

Grit chamber removes heavy inorganic particles (G about 2.65) at high loading; sedimentation tank settles light organic solids at low loading

Grit chamber has low surface loading of 25-40 m^3/m^2/day

Sedimentation tank needs velocity control by a proportional weir

Both remove particles of G about 1.2

2070 Magh · 5+3 marks

Skimming tank; grit vs sedimentation tank

Short answer

Skimming tank removes floating oil, grease and scum by air bubbles and a submerged baffle; placed after grit chamber before primary sedimentation. Grit chamber removes heavy inorganic grit at 0.2-0.3 m/s in 30-60 s; sedimentation tank removes settleable organics at very low velocity in 2-3 hours.

With neat sketch, describe briefly about the skimming tank. Also enlist differences between grit chamber and sedimentation tank.

Skimming tank

A skimming tank is a small chamber in which floating matter such as oil, grease, fat, soap scum and wood pieces is held back and removed from the surface of sewage, while the sewage flows out from below the surface. It is provided where sewage contains much grease (hotels, restaurants, slaughterhouses, dairies, workshops) and is placed after the grit chamber and before the primary sedimentation tank.

Purpose

To prevent grease and oil from forming scum in sedimentation tanks and clogging the filter media of trickling filters.
To avoid interference with oxygen transfer in biological units and with sludge digestion.
To remove floating material that otherwise causes odour and an unsightly surface.

Construction and working

 Plan                     Section
 +---------------+        air ---+ (diffusers)
 |  inlet ->     |        ~~~~~~~~~~~~~~~~~~~~~~ water level
 |   air bubbles |   inlet->  |  floating grease collects
 |   rise, grease| baffle(dip plate)  | -> skimmer/ trough
 |   -> skim trough       outlet below baffle ->
 +---------------+

It is a long rectangular (or circular) tank, with detention of about 3 minutes at average flow; depth about 1.5 m.
Compressed air is released through diffusers on one side. The air bubbles rise, carry the grease and oil up and make the particles coalesce and rise; this also gives the spiral flow that keeps heavier solids from settling.
A baffle (scum board) dipping below the surface stops the floating matter from leaving with the effluent, which leaves below the baffle.
The accumulated scum is collected in a trough or removed by a hand skimmer or a mechanical rotating skimmer and is disposed of with the sludge, burnt or buried. Air requirement is about 0.3 m³ per m³ of sewage.
Sometimes the skimming tank and grit chamber are combined (aerated grit chamber with grease removal).

Differences between grit chamber and sedimentation tank

Point	Grit chamber	Sedimentation tank
Purpose	Removes heavy inorganic grit (sand, gravel, cinders)	Removes settleable organic and fine suspended solids
Particle size / $G$	About 0.15 mm and above, $G\approx 2.65$	Fine particles, $G\approx 1.2$ or less
Position	Before the sedimentation tank (after screens)	After the grit chamber
Flow velocity	About 0.2-0.3 m/s, kept constant	Very low, about 0.3 m/min (about 0.005 m/s)
Detention time	30-60 seconds	2-3 hours (primary)
Size	Small, narrow, long channel	Large rectangular or circular tank
Velocity control	Needed (proportional weir or Parshall flume)	Not critical
Sludge	Dry, inert grit; removed by hand or scraper	Wet putrescible sludge (about 95% water) needing digestion
Surface loading	High, 800-1200 m³/m²/day	Low, 25-40 m³/m²/day

✓ Answer checked by Claude Opus 5.5 (high effort)

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Skimming tank; grit vs sedimentation tank

Skimming tank

Purpose

Construction and working

Differences between grit chamber and sedimentation tank

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