Membrane separations: In pressure-driven processes (e.g., ultrafiltration or reverse osmosis), concentration polarization at the membrane surface can be reduced most effectively by which single operational change?

Introduction:
Concentration polarization (CP) is the build-up of rejected solute near a membrane surface, which lowers the effective driving force and can precede irreversible fouling. This question tests practical understanding of how hydrodynamics, not just pressure, control CP in cross-flow membrane operations used in bioprocessing and water treatment.

Given Data / Assumptions:

• Pressure-driven membrane process (UF/NF/RO) operated in cross-flow.
• Same chemistry and membrane; only operating conditions change.
• Goal: reduce the thickness of the concentration boundary layer and solute back-diffusion limitation.

Concept / Approach:
At steady state, convective solute transport toward the membrane is balanced by back-diffusion away from the wall. CP severity depends on shear at the surface, which sets boundary-layer thickness. Increasing cross-flow velocity and introducing turbulence promoters (spacers, Dean vortices) intensify mixing at the wall, thin the boundary layer, and lower the solute concentration at the interface, thereby restoring flux at a given pressure.

Step-by-Step Solution:
Recognize CP as a mass-transfer limitation governed by Sherwood number ~ f(Re, Sc, geometry).Increase wall shear via higher cross-flow velocity or spacers to raise Sh and decrease boundary-layer thickness.Result: lower interfacial solute concentration ⇒ higher effective Δp − σΔπ ⇒ higher solvent flux at fixed pressure.Secondary aids (periodic backflush, prefiltration) help fouling control but the primary CP lever is hydrodynamics.

Verification / Alternative check:
Flux–pressure curves flatten near the CP limit. Raising cross-flow (or adding spacers) shifts the limiting flux upward; simple pressure increases past the limiting flux give diminishing returns, confirming CP control is hydrodynamic, not purely pressure-driven.

Why Other Options Are Wrong:
Reducing feed flow rate: lowers shear, thickens the boundary layer, and worsens CP.

Raising pressure indefinitely: past the limiting flux this mainly compresses the layer and may accelerate fouling with little flux gain.

Creating dead zones: promotes stagnant boundary layers and intensifies CP.

Common Pitfalls:

• Confusing reversible CP with irreversible fouling; hydrodynamic control targets CP first.
• Assuming pressure alone overcomes CP; mass transfer sets the limiting flux.

Final Answer:
Installing turbulence promoters or increasing cross-flow velocity along the membrane surface