Continuous culture control: a turbidostat is most appropriate when operating at what region of dilution rates relative to the washout point?

Introduction / Context:
Chemostats and turbidostats are continuous culture devices but differ in control strategy. Chemostats fix dilution rate D and let biomass reach a steady value set by D and μ(S). Turbidostats fix biomass (optical density) by feedback-adjusting D. This question asks where turbidostats excel relative to washout.

Given Data / Assumptions:

• Washout occurs when D ≥ μmax for the prevailing conditions.
• Turbidostat measures turbidity and adjusts inflow to maintain constant cell density.
• Rapid feedback can track growth changes at high D.

Concept / Approach:
Near washout, small perturbations in μ or substrate cause large biomass swings in a chemostat because D is fixed. A turbidostat mitigates this instability by modulating D to keep turbidity constant, enabling operation closer to μmax with higher specific productivity for fast-growing cultures.

Step-by-Step Solution:

Verification / Alternative check:
Control theory analyses show closed-loop OD control stabilizes operation near μmax compared to open-loop D control.

Why Other Options Are Wrong:

• Low or moderate D: chemostats already operate stably here.
• Any D: overbroad; turbidostat advantage is most pronounced near washout.
• Zero D: contradicts continuous operation.

Common Pitfalls:
Assuming turbidostat eliminates washout entirely; sensor lag and controller tuning still matter.

Final Answer:
High dilution rates near the washout point