--- title: "Skimming tank; degree of treatment factors" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:skimming-tank-degree-treatment" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/skimming-tank-degree-treatment site: BCT Engineering --- # Skimming tank; degree of treatment factors Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2073 Magh, 8 marks - 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-degree-treatment - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question With the neat sketch, describe the purpose and location of skimming tank. Explain the factors that govern the degree of treatment required of municipal wastewater. ## Short answer Skimming tank removes floating oil/grease; placed after screens/grit chamber, before primary sedimentation. Degree of treatment depends on effluent standards, sewage characteristics, receiving water flow and uses, disposal on land, climate, cost/resources, and public health/aesthetic requirements. ## Answer #### Skimming tank: purpose and location A skimming tank is a small aerated chamber that removes floating oil, grease, fat and scum. The purpose is to prevent these materials from forming scum in the primary tanks, clogging filters and disturbing biological treatment and sludge digestion. **Location:** in the treatment train it is placed after the screens and grit chamber and just before the primary sedimentation tank. ```text Screen -> Grit chamber -> SKIMMING TANK -> Primary sedimentation | grease/scum removal air -> ~~~~~ water level ~~~~~ baffle (scum board) | outlet below baffle ``` Aeration (about 0.3 m³ air/m³ sewage) lifts the grease, and a dip baffle holds it back so it can be skimmed off by hand or a mechanical scraper. It is sometimes combined with the grit chamber. #### Factors governing the degree of treatment of municipal wastewater The degree of treatment (percent removal of pollutants needed) is fixed by the quality required at the point of disposal. 1. **Quality standards:** the effluent standards laid down by the authority (for Nepal, the Generic Standards for wastewater discharge of the Government of Nepal). 2. **Characteristics of sewage:** BOD, suspended solids, and presence of industrial wastes or toxic compounds. 3. **Receiving water body:** its flow (dilution), DO, existing BOD and its self-purification capacity. 4. **Uses of the receiving water:** drinking-water source, bathing, fishing, irrigation or industrial use downstream need higher treatment. 5. **Disposal on land:** soil type, groundwater level, crop to be irrigated and health risk. 6. **Climate and temperature:** higher temperature speeds up oxidation but lowers dissolved oxygen. 7. **Cost and available resources:** land, funds, power and skilled operators; the choice between simple (pond) and complex (ASP) plants. 8. **Public health and aesthetic requirements:** odour, nuisance, protection of aquatic life. The more sensitive the use of the water and the smaller the dilution, the higher the degree of treatment (preliminary, primary, secondary or tertiary). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which sequence is correct for the treatment train? A. Skimming tank, screen, grit chamber, primary sedimentation B. Grit chamber, primary sedimentation, skimming tank C. Screen, grit chamber, skimming tank, primary sedimentation D. Primary sedimentation, skimming tank, grit chamber Answer: C. The skimming tank sits after screens and grit chamber and just before primary sedimentation. (Key point: Location of skimming tank) 2. What problem in later units does a skimming tank prevent? A. Excess nitrogen in effluent B. Grit abrasion of pumps C. Low pH D. Scum in primary tanks, filter clogging and disturbed biological treatment and digestion Answer: D. Floating oil and grease would form scum and clog filters and interfere with biological action. (Key point: Purpose of skimming) 3. What primarily fixes the degree of treatment required? A. The quality required at the point of disposal, based on standards, receiving water and its uses B. Only the population served C. Only the cost of land D. The length of the sewer Answer: A. Required percent removal depends on the effluent standards and the receiving water or land that takes the discharge. (Key point: Basis for degree of treatment) 4. How does the receiving water affect the degree of treatment? A. Greater dilution and sensitive uses both reduce treatment B. Smaller dilution and more sensitive uses (drinking, bathing) call for higher treatment C. It has no effect D. Only its colour matters Answer: B. Low flow, low DO or sensitive downstream use means the effluent must be cleaner. (Key point: Receiving water dilution) ## Related in Sanitary Engineering - [Degree of treatment, industrial wastewater](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-industrial-waste) — Sanitary Engineering, chapter 6 - [Skimming tank: purpose and construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Askimming-tank-purpose-construction) — Sanitary Engineering, chapter 7 - [Is treatment necessary? Town wastewater 300 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atreatment-needed-town-300-ls) — Sanitary Engineering, chapter 6 - [Skimming tank; grit vs sedimentation tank](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Askimming-tank-differences) — Sanitary Engineering, chapter 7 - [Degree of treatment, city 1200 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-city-1200ls) — Sanitary Engineering, chapter 6 - [Degree of treatment, town with dairy and industries](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-city-industries-71) — Sanitary Engineering, chapter 6 ## Related in other subjects - [Municipal wastewater treatment plant](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Amunicipal-wastewater-treatment-plant) — Environment and Pollution Control, chapter 4 - [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6 - [Surge tank objectives and economic penstock diameter factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-objectives-economic-diameter) — Hydro Power, chapter 5 - [Estimating, purpose and data required](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aestimating-purpose-data-required) — Estimating and Costing, chapter 1, asked 6×