--- title: "Skimming tank: purpose and construction" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:skimming-tank-purpose-construction" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/skimming-tank-purpose-construction site: BCT Engineering --- # Skimming tank: purpose and construction Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2074 Bhadra, 4 marks - Video: [Skimming Tanks | Treatment of Sewage - Skimming Tank IN HINDI by easy civil](https://www.youtube.com/watch?v=8mga4ig_0Ms) — EASY CIVIL - 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-purpose-construction - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question With neat sketches, describe the purpose and construction of a skimming tank. ## 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. ## Answer 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is grease and oil removed before the sedimentation tank? A. It would make the grit too heavy B. It raises the pH of the sewage C. It prevents scum, odour and clogging of trickling filter media and disturbs digestion D. It reduces chlorine demand only Answer: C. Grease forms scum, causes odour, clogs filter media and interferes with aeration and sludge digestion. (Key point: Why grease must be removed) 2. What is the purpose of the diffusers in a skimming tank? A. Air lifts grease and oil to the surface and gives spiral flow B. Air oxidises BOD completely C. Air makes grease sink D. Air cools the sewage Answer: A. Rising bubbles carry grease up, with about 0.3 m^3 air per m^3 sewage. (Key point: Aeration in the tank) 3. What does the submerged baffle (scum board) do? A. Slows grit settling B. Holds back floating matter while sewage is drawn off below it C. Skims the sludge from the bottom D. Measures the flow Answer: B. Floating scum cannot pass beneath the baffle, so it stays at the surface for removal. (Key point: Submerged baffle) 4. What are typical design values for a skimming tank? A. Depth 4 m, detention 3 hours B. Depth 1.5 m, detention about 3 minutes C. Depth 0.3 m, detention 30 seconds D. Depth 10 m, detention 1 day Answer: B. A rectangular tank about 1.5 m deep with roughly 3 min detention. (Key point: Typical dimensions) ## Related in Sanitary Engineering - [Skimming tank; degree of treatment factors](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Askimming-tank-degree-treatment) — Sanitary Engineering, chapter 7 - [Skimming tank; grit vs sedimentation tank](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Askimming-tank-differences) — Sanitary Engineering, chapter 7 - [Grit chamber: purpose, construction, design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-chamber-purpose-design) — Sanitary Engineering, chapter 7, asked 4× - [Pit privy: purpose and construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Apit-privy-purpose-construction) — Sanitary Engineering, chapter 9 - [Septic tank and soak pits, 15 users, Terai](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-15-users-terai) — Sanitary Engineering, chapter 9 - [Drop manhole: need and construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adrop-manhole-purpose-sketch) — Sanitary Engineering, chapter 4, asked 4× ## Related in other subjects - [Service reservoirs: purpose and construction](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aservice-reservoir-purpose-construction) — Water Supply Engineering, chapter 7, asked 2× - [Break pressure tank: need and construction](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abreak-pressure-tank) — Water Supply Engineering, chapter 9, asked 9× - [Excavation equipment and sketches](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aexcavation-equipment-sketch) — Construction Management, chapter 4, asked 6× - [Earth compaction equipment with sketches](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aearth-compaction-equipment) — Construction Management, chapter 4, asked 3×