--- title: "Recirculation in trickling filters; low vs high rate" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:trickling-recirculation-low-high-rate" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/trickling-recirculation-low-high-rate site: BCT Engineering --- # Recirculation in trickling filters; low vs high rate Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2069 Bhadra, 8 marks - Video: [Calculate Trickling Filter Recirculation Ratio](https://www.youtube.com/watch?v=sOh-sxHzT3g) — Wastewater Dan - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/trickling-recirculation-low-high-rate - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question Why is recirculation necessary in trickling filters? Compare the low rate and high rate trickling filters. ## Short answer Recirculation returns filter effluent to inlet to steady hydraulic load, dilute strong sewage, increase contact, prevent ponding and seed microbes. Low-rate filter: hydraulic loading 1-4 m³/m²/day, no recirculation, 80-90% BOD removal, larger area. High-rate: 10-40 m³/m²/day, recirculation R=0.5-3, 65-85% removal, smaller area. ## Answer #### Need for recirculation **Recirculation** means returning a part of the filter (or secondary clarifier) effluent to the filter inlet, so that the filter receives $Q(1+R)$, where $R = Q_R/Q$ is the recirculation ratio. It is necessary because: 1. It keeps the **hydraulic load** and flow over the distributor steady, even at night and in low-flow periods, so the film does not dry out. 2. It **dilutes** strong sewage, reducing the organic load per pass and avoiding odours and overloading. 3. It increases the **contact** between sewage and biological film and so improves BOD removal (recirculation factor $F=\dfrac{1+R}{(1+0.1R)^2}$ in the NRC equation). 4. It flushes the film and controls excess growth, which reduces **clogging (ponding)** and fly breeding. 5. It **seeds** the incoming sewage with active micro-organisms and returns dissolved oxygen. 6. It makes high loading rates possible in a smaller volume. #### Comparison of low rate and high rate trickling filters | Point | Low (standard) rate | High rate | |---|---|---| | Hydraulic loading | 1-4 m³/m²/day | 10-40 m³/m²/day | | Organic loading | 0.08-0.32 kg BOD/m³/day | 0.32-1.0 kg BOD/m³/day | | Recirculation | Not used | Used (R = 0.5-3) | | Depth | 1.8-3 m | 1.8-2.5 m (up to 3 m) | | BOD removal | 80-90% | 65-85% | | Effluent | Well nitrified and stable | Not fully nitrified, less stable | | Dosing | Intermittent | Continuous | | Area needed | Large | Smaller | | Fly / odour nuisance | More (ponding, Psychoda flies) | Less | | Sloughing | Periodic, heavy | Continuous, light | | Capital cost | Higher (per unit flow) | Lower | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. If the recirculation ratio is R = Q_R/Q, what flow does the filter receive? A. Q x R B. Q / (1+R) C. Q(1+R) D. Q - Q_R Answer: C. The filter gets the raw flow plus the recycled part, Q(1+R). (Key point: Meaning of recirculation) 2. Which is a reason for recirculation? A. It dilutes strong sewage, keeps the film wet at low flow and helps reduce clogging B. It removes the need for a secondary clarifier C. It lowers the contact between sewage and film D. It reduces dissolved oxygen Answer: A. Recirculation keeps flow steady, dilutes organic load, improves contact and flushes the film. (Key point: Reasons for recirculation) 3. Which statement on loading is correct? A. Low rate: 10-40 m^3/m^2/day; high rate: 1-4 B. High rate filters have 10-40 m^3/m^2/day hydraulic loading and use recirculation; low rate 1-4 without C. Both have the same loading D. Low rate uses recirculation R = 0.5-3 Answer: B. High rate filters take much higher loading and use recirculation; low rate filters do not. (Key point: Loading comparison) 4. How does effluent compare between the two types? A. High rate gives better nitrified effluent B. Both give identical effluent C. Low rate gives lower BOD removal (65-85%) D. Low rate gives 80-90% removal and well nitrified stable effluent; high rate is not fully nitrified Answer: D. Low rate filters are larger and more efficient and nitrify; high rate filters trade quality for compactness. 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