With reference to the role of biofilters in Recirculating Aquaculture System, consider the following statements : 1. Biofilters provide waste treatment by removing uneaten fish feed. 2. Biofilters convert ammonia present in fish waste to nitrate. 3. Biofilters increase phosphorus as nutrient for fish in water. How many of the statements given above are correct?
- (a) Only one
- (b) Only two ✓ UPSC's answer
- (c) All three
- (d) None
Why the answer is (b)
• A Recirculating Aquaculture System reuses water in tanks by passing it through mechanical filters, biofilters and oxygenation before returning it to the fish.
• Statement 2 is correct and central: biofilters host nitrifying bacteria that convert toxic ammonia excreted by fish into nitrite and then relatively harmless nitrate.
• Statement 1 is treated as correct in the official key in the broad sense that the filtration train, including biofilters, provides waste treatment by removing feed residues and dissolved wastes (though solid uneaten feed is chiefly removed by mechanical filters).
• Statement 3 is wrong: biofilters do not add phosphorus; excess phosphorus is a pollutant that RAS seeks to remove, not supply.
• Hence two statements, option (b).
Why the other options are wrong
- (a) Only one
- Statements 1 and 2 are treated as correct, giving two.
- (c) All three
- Statement 3 is wrong: biofilters do not increase phosphorus.
- (d) None
- Statement 2 on ammonia-to-nitrate conversion is certainly correct.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2023, held on 28 May 2023. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.