Continuous Culture Terminology — What do we call a continuous bioreactor in which the growth rate (and thus productivity) is controlled solely by the flow rate via the dilution rate?

Introduction:
Continuous reactors enable steady production and constant physiological states. Nomenclature matters because control strategies differ between designs, affecting productivity and stability. The reactor controlled solely by flow rate is the classical chemostat.

Given Data / Assumptions:

• Single limiting substrate in the feed.
• Perfect mixing and constant reactor volume.
• Dilution rate D = F / VR determines the steady-state specific growth rate μ, provided D < μm.

Concept / Approach:

In a chemostat, the operator fixes D through the flow rate. At steady state, μ equals D and the biomass concentration is set by the substrate balance and yield. In contrast, a turbidostat uses turbidity feedback to adjust D dynamically; pH-stat manipulates acid/base addition; level-stat maintains volume but does not directly set μ.

Step-by-Step Solution:

Verification / Alternative check:

Material balances in a chemostat confirm that, at steady state, concentrations are time-invariant and set by D and stoichiometric yields.

Why Other Options Are Wrong:

A: Turbidostat relies on optical density feedback. C and D: These are control strategies with different objectives. E: Fed-batch lacks continuous outflow and true steady state.

Common Pitfalls:

Confusing turbidostat with chemostat; the former varies D to hold biomass constant, the latter holds D constant to set μ.

Final Answer:

Chemostat (fixed dilution rate sets the specific growth rate below washout).