In a trickling filter, recirculation is commonly provided to
- AIncrease sludge age and control solids retention time
- BRemove dissolved oxygen before biological treatment
- CPrevent formation of anaerobic zones in primary clarifier
- DImprove contact between wastewater and biofilm and dilute influent strengthCorrect
Solution
Concept
Recirculation in high-rate trickling filters
Formula
Recirculation ratio R = Qr / Q; Recirculation factor F = (1 + R) / (1 + 0.1·R)2
Step-by-step solution
1. High-rate trickling filters employ recirculation of settled or unsettled effluent. 2. Recirculation reduces organic shock loads by diluting high BOD influent. 3. It maintains continuous wetting of the filter bed and enhances biofilm-liquid contact. 4. Therefore, option D correctly describes the operational purpose of recirculation. Answer: D
Explanation
In high-rate trickling filters, recirculating treated effluent back to the filter inlet dilutes high-strength incoming wastewater, keeps the biological slime wet during low inflow periods, increases fluid shearing to facilitate sloughing of aging biofilm, and increases contact time between wastewater and active microorganisms.
Why the other options are wrong
A) Sludge age and SRT control apply to activated sludge, not fixed-film trickling filters. B) Trickling filters are strictly aerobic and require DO, not its removal. C) Recirculation flow does not enter or affect primary clarifier anaerobic zones.
Shortcut
Recirculation in trickling filters = Dilutes influent strength + Enhances biofilm contact.
Common mistake
Confusing trickling filter recirculation with activated sludge RAS (solids retention time control).
Correct answer: D) Improve contact between wastewater and biofilm and dilute influent strength