CSTF (continuous stirred tank fermenter): Cellular productivity typically rises with dilution rate up to a maximum. What happens to productivity if the dilution rate is increased beyond the point of maximum productivity?

Introduction / Context:
In a CSTF, volumetric productivity often shows a peak versus dilution rate because biomass concentration falls as D increases, while specific rate terms may rise. Recognizing the post-peak behavior is key to safe operation and avoiding loss of culture.

Given Data / Assumptions:

• Steady-state operation with growth-limited substrate.
• Productivity refers to biomass (or product tied to growth) per reactor volume per time.
• Maximum occurs below washout (D < μm).

Concept / Approach:
As D increases, X decreases because μ(S) = D and S rises, but near μm the system approaches washout: X → 0. Hence the volumetric productivity (proportional to D * X for growth-associated products) declines steeply beyond the optimum and can collapse as D → μm.

Step-by-Step Solution:
Note productivity trend: rises with D at first due to increased throughput.Approaching peak: falling X offsets gains in D.Beyond the maximum: X drops quickly; productivity decreases abruptly.Near washout (D ≈ μm): productivity approaches zero.

Verification / Alternative check:
Plots from chemostat experiments show a dome-shaped curve of productivity versus D, with a sharp decline on the high-D side as cultures destabilize.

Why Other Options Are Wrong:
“Increases” or “increases drastically”: contradict empirical and theoretical trends.

“Becomes zero immediately”: only becomes zero at washout; beyond the maximum it declines toward zero, not instantly zero.

Common Pitfalls:

• Operating too close to μm; small disturbances can cause washout.
• Ignoring that product formation linked to growth mirrors biomass behavior.

Final Answer:
Decreases abruptly (approaching washout as D nears μm)