--- title: "Skimming tank; grit vs sedimentation tank" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:skimming-tank-differences" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/skimming-tank-differences site: BCT Engineering --- # Skimming tank; grit vs sedimentation tank Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2070 Magh, 5+3 marks - Video: [SKIMMING TANK AND PRIMARY SEDIMENTATION TANK (WASTE WATER ENGINEERING)](https://www.youtube.com/watch?v=itx_50DzOL0) — SUJIT KUMAR SAHOO - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/skimming-tank-differences - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question With neat sketch, describe briefly about the skimming tank. Also enlist differences between grit chamber and 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. ## Answer #### 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 ```text 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 | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main function of a skimming tank? A. To remove sand and gravel B. To remove floating oil, grease and scum from the surface C. To remove dissolved BOD D. To digest sludge Answer: B. It holds back floating matter like grease and soap scum while sewage leaves from below the surface. (Key point: What a skimming tank removes) 2. How do the air diffusers and the baffle work together? A. Air settles grease; the baffle sends it to the bottom B. Air dissolves grease; the baffle supplies oxygen C. Air bubbles lift grease to the surface; the dip baffle keeps it from leaving with the effluent D. Air and baffle only mix the sewage Answer: C. Bubbles carry grease up and make it coalesce, and the baffle stops floating matter, since the outlet is below it. (Key point: Role of air and baffle) 3. Where is the skimming tank placed? A. After the grit chamber and before the primary sedimentation tank B. After the trickling filter C. Before the screens D. After secondary clarifier Answer: A. It follows screens and grit chamber so grease is removed before primary settling. (Key point: Position in the train) 4. Which statement correctly separates a grit chamber from a sedimentation tank? A. Grit chamber has low surface loading of 25-40 m^3/m^2/day B. Sedimentation tank needs velocity control by a proportional weir C. Grit chamber removes heavy inorganic particles (G about 2.65) at high loading; sedimentation tank settles light organic solids at low loading D. Both remove particles of G about 1.2 Answer: C. Grit chamber: G about 2.65, 800-1200 m^3/m^2/day. Sedimentation tank: fine organic solids, 25-40 m^3/m^2/day. 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