--- title: "ASP working, advantages and disadvantages" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:asp-flow-diagram-advantages" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-flow-diagram-advantages site: BCT Engineering --- # ASP working, advantages and disadvantages Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2077 Chaitra, 4+4 marks; 2071 Magh, 8 marks (asked 2 times) - Video: [(Part-1)Activated Sludge Process|Working of Activated Sludge Process](https://www.youtube.com/watch?v=7btREr5DcPw) — ISHA MISHRA - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-flow-diagram-advantages - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question What are activated sludge processes and how do they work? Explain with flow diagram. What are its advantages and disadvantages? ## Short answer Activated sludge process is an aerobic suspended-growth treatment where settled sewage mixes with a flocculent mass of micro-organisms in an aeration tank; after oxidation the mixed liquor settles, and a portion of settled sludge is returned. It achieves 85–95% BOD removal, needs less land and has no odour nuisance, but involves high power cost and skilled operation. ## Answer #### Activated sludge process (ASP) The activated sludge process is an aerobic, suspended-growth biological treatment in which settled sewage is mixed and aerated with a flocculent mass of micro-organisms called **activated sludge**. The organisms oxidise organic matter, and the mixed liquor is then settled; a part of the settled sludge is returned to the aeration tank to maintain the biomass. #### Working 1. **Primary treatment** (screens, grit chamber, primary sedimentation) removes about 30-35% of the BOD. 2. **Aeration tank:** the settled sewage is mixed with return activated sludge (RAS). Air (diffused or mechanical) supplies oxygen and keeps the flocs in suspension for 4-8 hours. Bacteria and protozoa in the floc adsorb and oxidise organic matter to CO₂ and water and form new cells. 3. **Secondary sedimentation tank:** the flocs settle and the clear effluent (BOD 20-30 mg/l) overflows. 4. **Sludge return:** 25-50% of the settled sludge is recycled to the aeration tank to keep MLSS at about 2000-4000 mg/l. The excess (waste) sludge is sent to thickening and digestion. ```text Raw +--------+ +-----+ +---------+ +---------+ Effluent sewage |Screen/ | |PST | |Aeration | |Secondary|--> ------>|Grit |>| |>|tank+air |>|settling | +--------+ +--+--+ +----^----+ +----+----+ | | RAS | | +-----------+ primary sludge waste sludge -> thickener -> digester ``` #### Advantages - High BOD removal efficiency (85-95%) and a good quality effluent. - Needs less land than a trickling filter or oxidation pond. - No fly or odour nuisance if properly run; no head loss problem. - Flexible: loading can be adjusted (conventional, extended aeration, step aeration, etc.). - Capable of nitrification with long sludge age. #### Disadvantages - High capital and operating costs, especially for power for aeration. - Needs skilled and continuous operation and control (F/M, MLSS, DO, sludge return). - Sensitive to shock loads and toxic substances. - Large quantity of sludge is produced; it needs treatment and disposal. - Sludge bulking (high SVI) and foaming problems may occur. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is activated sludge? A. Sludge from primary tank B. Chemical precipitate C. A flocculent mass of micro-organisms that oxidises organic matter in the aerated sewage D. Digested sludge Answer: C. Activated sludge is the biologically active floc in the aeration tank. (Key point: Definition) 2. Why is part of the settled sludge returned to the aeration tank? A. To maintain the biomass (MLSS about 2000-4000 mg/l) B. To reduce the pH C. To remove grit D. To add chlorine Answer: A. Return activated sludge keeps enough micro-organisms for treatment. (Key point: Sludge return) 3. What is the correct sequence? A. Primary treatment, aeration tank, secondary settling tank, effluent B. Secondary settling, aeration, primary C. Digester, aeration, screens D. Aeration tank, screens, grit Answer: A. Settled sewage is aerated and then the flocs settle in the secondary tank. (Key point: Flow sequence) 4. Which is a disadvantage of ASP? A. High power cost, skilled operation, sensitivity to shock loads and sludge bulking B. Needs more land than ponds C. Gives poor effluent D. Produces fly nuisance Answer: A. ASP is costly to run and sensitive, with large sludge production and bulking risk. 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