Which one of the following bioreactor configurations is the basis for a trickling biological
filter?
Options:
(A) Stirred tank
(B) Packed bed
(C) Air lift
(D) Fluidized bed
GATE 2018 Biotechnology
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.23)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Bioprocess Engineering / Bioreactor Design (Immobilized Film Reactors)
**2. Detailed Step-by-Step Analysis**:
- A **trickling biological filter** (commonly used in wastewater treatment and aerobic bio-oxidation) operates by passing liquid wastewater downward over a static, stationary packing bed of rock, slag, plastic rings, or ceramic media coated with a microbial biofilm.
- The supporting matrix/bed remains stationary while the fluid trickles over it under gravity, and air circulates naturally or under forced draft through the void spaces.
- This configuration is classical **packed bed bioreactor** (fixed-bed reactor).
- In a packed bed, the biocatalyst/biofilm is held stationary within the column while nutrient-bearing liquid flows through the interstices of the packing.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Air-lift bioreactors utilize density differences driven by gas sparging for liquid circulation, having no static packing bed.
- **Option (B) [CORRECT]**: A packed bed reactor is the physical and engineering basis of a trickling biological filter.
- **Option (C) [INCORRECT]**: Bubble column bioreactors lack mechanical packing; liquid and gas mix without a fixed bed.
- **Option (D) [INCORRECT]**: In a fluidized bed reactor, solid particles are suspended and maintained in dynamic motion by upward fluid flow, whereas a trickling filter uses a stationary (fixed) bed.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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