In nanofiltration and reverse osmosis, membrane compaction rates increase when membranes are operated at which conditions?

• A. high flows, high pressures
• B. higher pressures, higher temperatures
• C. high pressures, the water contains high concentrations of iron
• D. high flows, the water has high organic concentrations

Answer: (B)

Membrane compaction in nanofiltration and reverse osmosis is driven by mechanical compression of the dense polymer matrix under applied pressure, and temperature influences how easily the material deforms. When you push more pressure through the membrane, the compressive stress increases, squeezing the polymer chains closer together and reducing the free volume in the thin film. This densification slows permeate flow and can alter rejection.

Temperature adds another layer: higher temperature makes the polymer more flexible and water acts as a plasticizer, increasing chain mobility. With the membrane’s chains more able to rearrange under the same pressure, the material deforms more, enhancing compaction. So the combination of higher pressures and higher temperatures accelerates compaction rates.

Situations like high flow or high concentrations of iron or organics mainly affect fouling and resistance from deposits rather than changing the intrinsic tendency of the polymer to densify under pressure and heat, so they don’t drive compaction as directly as pressure and temperature do.