Question

As per the pore size of membrane, state the correct sequence:

• A. NF > MF > UF > RO
• B. MF > UF > NF > RO
• C. UF > MF > NF > RO
• D. RO > MF > UF > NF

Hint:

Remember the order by pore size: {Micro $\rightarrow$ Ultra $\rightarrow$ Nano $\rightarrow$ RO}.

Answer 1

The Correct Option is B

Step 1: Understanding membrane filtration.
Membrane processes are classified based on pore size, which determines the size of particles or solutes removed from water.
Step 2: Relative pore sizes.
Microfiltration (MF) has the largest pore size, followed by Ultrafiltration (UF), then Nanofiltration (NF), while Reverse Osmosis (RO) has the smallest effective pore size.
Step 3: Evaluating the options.
(A) Incorrect, NF does not have larger pores than MF.
(B) Correct, this follows decreasing pore size from MF to RO.
(C) Incorrect, UF pores are smaller than MF.
(D) Incorrect, RO has the smallest pores, not the largest.
Step 4: Conclusion.
The correct sequence according to decreasing pore size is MF > UF > NF > RO.

Answer 2

Membrane processes are best compared using their approximate pore-size ranges, since the smaller the pore, the finer the particle or ion the membrane can reject.

1. NF > MF > UF > RO: This places nanofiltration above microfiltration, but nanofiltration membranes are built to reject much smaller species than microfiltration membranes, so their pores must be smaller, not larger, than microfiltration pores; this ordering is inconsistent.
2. MF > UF > NF > RO: Microfiltration membranes have the coarsest pores, typically rejecting suspended solids and bacteria; ultrafiltration pores are finer, rejecting macromolecules and viruses; nanofiltration pores are finer still, rejecting multivalent ions and larger organic molecules; and reverse osmosis has the finest pores of all, capable of rejecting even monovalent ions, so pore size strictly decreases across this sequence.
3. UF > MF > NF > RO: This places ultrafiltration above microfiltration, but microfiltration is the coarser of the two processes, so this ordering reverses the correct relationship between them.
4. RO > MF > UF > NF: Reverse osmosis has by far the finest pores of the four processes since it is the only one capable of rejecting dissolved salts, so placing it first (largest) contradicts its actual role.

Ranking the four processes by what size of particle or ion each one is actually capable of rejecting confirms that pore size decreases from microfiltration through ultrafiltration and nanofiltration down to reverse osmosis.

Therefore, the correct answer is MF > UF > NF > RO.