Question

Which one of the following represents the CORRECT effects of concentration polarization in a reverse osmosis process?

• A. Reduced water flux and reduced solute rejection
• B. Increased water flux and increased solute rejection
• C. Reduced water flux and increased solute rejection
• D. Increased water flux and reduced solute rejection

Hint:

Concentration polarization is one of the main operational challenges in RO systems.
It can be minimized by enhancing turbulence at the membrane surface (e.g., using spacers, crossflow operation).
Reduced flux and rejection lower both productivity and quality of RO systems.

Answer 1

The Correct Option is A

Step 1: Understanding concentration polarization
In reverse osmosis (RO), water permeates through the membrane while solutes are rejected. However, solutes accumulate near the membrane surface, creating a concentrated boundary layer — this phenomenon is called concentration polarization.

Step 2: Effect on driving force (water flux)
Due to solute accumulation, the effective osmotic pressure at the membrane surface increases. Since net driving pressure is \[ \Delta P_{\text{net}} = \Delta P - \Delta \pi, \] where \(\Delta \pi\) is osmotic pressure difference, the higher osmotic pressure reduces the net driving force. Thus, water flux decreases.

Step 3: Effect on solute rejection
At higher solute concentration near the membrane surface, more solute tends to diffuse through the membrane. This reduces the overall solute rejection capability of the process.

Conclusion:
Concentration polarization leads to: \[ \text{Reduced water flux and reduced solute rejection.} \]