Airlift versus stirred-tank: what key operational advantage(s) do airlift bioreactors typically have over stirred tanks of similar size?

Introduction / Context:
Airlift bioreactors use gas-induced circulation rather than mechanical agitation. For shear-sensitive cultures, they are attractive alternatives to stirred tanks. This question focuses on two commonly cited advantages—oxygen transfer and shear environment—and asks whether both apply.

Given Data / Assumptions:

• Similar working volumes and gas flow rates.
• Well-designed internal or external loop airlifts.
• Target organisms may be sensitive to impeller-generated shear.

Concept / Approach:
Airlifts can reach competitive kLa values because circulation increases gas residence time and dispersion, while the absence of impellers yields more uniform, lower peak shear. Although foam control varies with medium and antifoam strategy, shear environment and oxygen transfer together often favor airlifts for delicate cells.

Step-by-Step Solution:
1) Compare hydrodynamics: airlifts use buoyancy-driven loops, reducing localized high shear zones.2) With proper geometry, gas holdup and interfacial area improve, sustaining high kLa and OTR.3) Conclude that both higher OTR (in many designs) and more uniform shear are typical advantages.

Verification / Alternative check:
Literature reports show mammalian and filamentous cultures benefiting from airlift operation due to gentler hydrodynamics, while achieving target oxygenation without high mechanical power input.

Why Other Options Are Wrong:
(a) or (b) alone understate the combined benefit; foam behavior (d) is medium-dependent; (e) is incorrect—airlifts can outperform for certain applications.

Common Pitfalls:
Assuming airlifts always have lower OTR; geometry and gas rates matter. The statement is a typical trend, not an absolute law.

Final Answer:
Both (a) and (b)