Q.19 In a chemostat operating under steady state, a bacterial culture can be grown at dilution rate higher than maximum growth rate by
- (A) Partial cell recycling
- (B) Using sub-optimal temperature
- (C) pH cycling
- (D) Substrate feed rate cycling
In chemostat steady state, dilution rate (D) equals specific growth rate (μ). Normally, D cannot exceed maximum growth rate (μmax) without washout. This MCQ tests advanced bioprocess control strategies enabling higher productivity.
Correct Answer: (A) Partial cell recycling
Partial cell recycling retains biomass by separating cells from effluent and returning them to the culture vessel, allowing D > μmax while maintaining steady state. Cell concentration (X) becomes decoupled from growth-limited levels.
Chemostat Steady State Principle
D = μ = μmax × S/(Ks + S)
At D ≥ μmax, washout occurs (dX/dt < 0).
Cell recycling: Effluent passes through a filter/membrane; concentrated cells recycle back. Effective growth rate becomes μ × (1 + R), where R = recycle ratio, enabling higher D while keeping X stable.
Explanation of All Options
| Option | Enables D > μmax? | Mechanism | Industrial Use |
|---|---|---|---|
| (A) Partial cell recycling | Yes | Biomass retention | High-density cultures |
| (B) Using sub-optimal temperature | No | Reduces μmax | None |
| (C) pH cycling | No | Cell stress | None |
| (D) Substrate cycling | No | Transient growth | Fed-batch only |
Bioprocess Engineering Applications
- Higher cell density (10-50 g/L vs 1-5 g/L)
- Increased productivity (D × X)
- Reduced substrate waste
Example: Penicillin production uses cell recycle chemostats. Model via Monod kinetics with recycle ratio.
Where: R = recycle ratio, kd = cell death rate, τ = residence time
Exam Essential:
Only partial cell recycling maintains chemostat steady state above μmax.
