--- title: "Trickling filter: use and construction" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:trickling-filter-construction" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/trickling-filter-construction site: BCT Engineering --- # Trickling filter: use and construction Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2072 Magh, 8 marks - Video: [Trickling filter design guideline - How do trickling filters work?](https://www.youtube.com/watch?v=r-0wRzGbSqw) — Karl-Uwe Schmitz - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/trickling-filter-construction - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question What is a trickling filter? Why is it used? Explain the construction of a trickling filter with a neat sketch. ## Short answer Trickling filter is a secondary biological treatment unit where settled sewage is sprayed over stone/plastic media covered by biofilm that oxidises organics. It removes 65-90% BOD. Construction includes circular tank, 40-80 mm media depth 1.8-3 m, rotating distributor, underdrain tiles, ventilation, and secondary clarifier. ## Answer A **trickling filter** (percolating or biological filter) is a secondary treatment unit in which settled sewage is sprayed over a bed of coarse media; a biological film (slime) of micro-organisms grows on the media and oxidises the organic matter as the sewage trickles down in contact with air. #### Why it is used - Primary settling removes only 30-35% of BOD; dissolved and colloidal organics remain. - A trickling filter removes 65-90% of the BOD with simple operation, low power need and low skill, and with good tolerance to shock loads. - Suitable for small and medium towns where land and a little head are available. #### Construction ```text rotating distributor arm (sprinklers) inlet pipe | +------+-------+------------------+ wall | o o o o o o o o o o o o o o o o | <- filter media (brick/| o o o o stones 40-80 mm o o o | 1.8 - 3 m deep conc.) |_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o| | underdrain tiles (slope 1:100) | -> collecting +---- air ventilation channels ---+ channel -> to secondary clarifier ``` 1. **Circular (or rectangular) tank** of brick masonry or concrete, with walls about 0.3 m above the media; diameter up to 60 m. 2. **Filter media:** hard, durable, rough stones, crushed rock, slag or plastic modules of 40-80 mm size (25-40 mm at the top in some designs), depth 1.8-3 m (standard rate), up to 12 m for plastic media. 3. **Distribution system:** a rotating arm distributor with nozzles driven by the jet reaction of the sewage (or fixed nozzles in rectangular beds) that spreads the effluent evenly. 4. **Underdrainage system:** perforated or half-round tile channels on the floor with a slope of about 1 in 100 to collect the effluent and also admit air. 5. **Ventilation:** natural draft through the underdrains and side openings, so the film stays aerobic. 6. **Dosing and recirculation arrangement:** a dosing tank or siphon for constant head and a pump for recirculation in high-rate filters. 7. A **secondary settling tank** follows the filter to remove the sloughed film (humus). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens in a trickling filter? A. Sewage is strained through sand and chlorinated B. Air is blown through the sewage in a tank C. Settled sewage is sprayed over coarse media where a slime film oxidises organics D. Sludge is digested without air Answer: C. The biological film on the media oxidises organic matter as sewage trickles down in contact with air. (Key point: Definition of trickling filter) 2. What does the underdrainage system do? A. Spreads sewage on top B. Collects effluent and admits air C. Holds the biofilm D. Removes sloughed film Answer: B. Perforated or half-round tiles with slope about 1:100 collect effluent and allow ventilation. (Key point: Function of the underdrain) 3. How does a rotating-arm distributor work? A. It is driven by an electric motor only B. It is a fixed pipe with no nozzles C. It is turned by a blower D. It is driven by jet reaction of the sewage and spreads it evenly Answer: D. Sewage leaving the nozzles gives reaction that rotates the arm. (Key point: Distributor) 4. Why is a secondary settling tank placed after the filter? A. To add oxygen B. To remove the sloughed-off film (humus) C. To remove grit D. To remove grease Answer: B. Old film sloughs off the media and must be settled out for a clear effluent. (Key point: Role of secondary settling tank) ## Related in Sanitary Engineering - [Standard rate trickling filter, 1.2 MLD, NRC](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-standard-1p2mld-nrc) — Sanitary Engineering, chapter 7, asked 3× - [Standard rate trickling filter, 3 MLD, 300 mg/l](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-standard-3mld-300) — Sanitary Engineering, chapter 7, asked 3× - [HR trickling filter, 5 MLD, RR 1.5](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-hr-5mld-rr1p5) — Sanitary Engineering, chapter 7, asked 2× - [Two-stage trickling filter diameters, 4 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atwo-stage-trickling-4mld-200) — Sanitary Engineering, chapter 7, asked 2× - [Two-stage trickling filter diameters, 5 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atwo-stage-trickling-5mld-180) — Sanitary Engineering, chapter 7, asked 2× - [Unit operation, process; need of trickling filter](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aunit-operation-process-trickling-need) — Sanitary Engineering, chapter 7 ## Related in other subjects - [Equipment used in hydropower construction](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Ahydropower-equipment) — Construction Management, chapter 4 - [Terminology used in stair construction](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-terminology) — Building Technology, chapter 5 - [Rapid sand filter construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-construction-working) — Water Supply Engineering, chapter 6, asked 5× - [Slow sand filter construction and design](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-construction-design) — Water Supply Engineering, chapter 6