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: Microfiltration membranes have the largest pores, generally in the range of roughly 0.1 to 10 micrometres, suited to removing suspended solids and larger microorganisms.
Step 2: Ultrafiltration pores are considerably finer, in the range of roughly 0.01 to 0.1 micrometres, fine enough to reject viruses and large dissolved macromolecules.
Step 3: Nanofiltration pores are finer again, down near 0.001 to 0.01 micrometres, capable of rejecting multivalent ions and smaller organic molecules that pass straight through ultrafiltration membranes.
Step 4: Reverse osmosis has the finest effective pore structure of all, able to reject even monovalent dissolved ions, placing it last in the ranking.

Arranging these four ranges from largest to smallest gives the sequence

\[ \text{MF} > \text{UF} > \text{NF} > \text{RO} \]

Answer 2

Another reliable way to rank these membrane types is by what each one is actually able to remove from water, since removal capability and pore size are directly linked, finer pores catch smaller things.

1. NF > MF > UF > RO: Nanofiltration is able to remove far smaller contaminants, such as hardness ions, than microfiltration, which only removes suspended solids, so its pores cannot be larger than microfiltration's; this option is inconsistent.
2. MF > UF > NF > RO: Microfiltration removes suspended particles and bacteria only; ultrafiltration additionally removes viruses and colloids; nanofiltration goes further and removes hardness-causing divalent ions and larger organics; and reverse osmosis removes even monovalent salts, the smallest species of all, so this order of increasing removal capability matches decreasing pore size perfectly.
3. UF > MF > NF > RO: Since microfiltration removes only larger particles than ultrafiltration does, its pores must be the coarser of the two, so placing ultrafiltration ahead of microfiltration is incorrect.
4. RO > MF > UF > NF: Reverse osmosis removes the smallest species of any of these processes, so it cannot have the coarsest pores as this ordering implies.

Matching each process's real-world removal capability to its pore size confirms the same decreasing order from microfiltration down to reverse osmosis.

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