Question

Which of the following statements is/are CORRECT regarding reverse osmosis (RO) as a water/wastewater treatment process?

• A. RO removes dissolved gases like CO2 more effectively than dissolved salts
• B. RO can remove monovalent ions such as sodium and chloride from water
• C. RO membrane rejects dissolved solutes while allowing water to pass through under applied pressure exceeding osmotic pressure
• D. RO does not require any external pressure to drive the separation process

Hint:

Recall RO needs applied pressure greater than osmotic pressure to force water through the membrane while rejecting solutes (including monovalent ions); dissolved gases like CO2 pass through more easily than dissolved salts, the opposite of option A.

Answer 1

The Correct Option is B, C

Step 1: Recall the working principle of reverse osmosis. Reverse osmosis works by applying an external hydraulic pressure to the feed (saline/impure) side that exceeds the natural osmotic pressure of the solution, forcing water molecules to pass through a semi-permeable membrane from the concentrated side to the dilute side, opposite to the natural direction of osmosis, while dissolved solutes are largely rejected and retained on the feed side. This matches option (C).

Step 2: Examine what RO removes effectively (option B). RO membranes are highly effective at rejecting both monovalent ions (Na+, Cl-) and multivalent ions, along with organic molecules, bacteria, and viruses, because the pore size and the solution-diffusion transport mechanism through the membrane strongly discriminate against dissolved ionic and molecular species. This makes option (B) correct.

Step 3: Examine dissolved gas removal (option A). Dissolved gases like CO2 are small, uncharged, and highly diffusive; they pass through RO membranes relatively easily compared to dissolved ionic salts, so RO is actually LESS effective at removing dissolved gases than it is at removing dissolved salts, making option (A) the reverse of the truth and hence incorrect.

Step 4: Examine option D. Reverse osmosis, by definition, requires an external applied pressure greater than the osmotic pressure of the feed solution to force water against its natural concentration gradient; without this applied pressure, no reverse flow would occur (natural osmosis would instead occur in the opposite direction). So option (D), claiming no external pressure is needed, is false.

Step 5: Conclude. The correct statements are (B), that RO removes monovalent ions, and (C), the correct description of the RO mechanism using applied pressure exceeding osmotic pressure.