UPSC Prelims 2023 Q55: Science & Technology – Energy & Environment
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
- (c) All three
- (d) None
Answer: (b) Only two
Explanation
Biofilters in a Recirculating Aquaculture System
In such a system, the same water is cleaned and reused to grow fish in tanks.
The biofilter is a bed of bacteria that cleans the water.
| Statement | Verdict (official key) | Fact |
|---|---|---|
| 1. Biofilters provide waste treatment by removing uneaten fish feed | Correct | Bacteria break down organic waste, including leftover feed |
| 2. Biofilters convert ammonia present in fish waste to nitrate | Correct | Nitrification: ammonia to nitrite to nitrate |
| 3. Biofilters increase phosphorus as nutrient for fish in water | Incorrect | They do not add phosphorus; fish get nutrients from feed |
The bacteria at work:
| Step | Bacteria |
|---|---|
| Ammonia to nitrite | Nitrosomonas |
| Nitrite to nitrate | Nitrobacter |
Why it matters: ammonia is toxic to fish; nitrate is far less harmful.
Answer: (b) Only two.
Why each option is right or wrong
- (a) Incorrect. Statements 1 and 2 are both correct, so more than one.
- (b) CORRECT. Biofilters treat waste and convert ammonia to nitrate.
- (c) Incorrect. Statement 3 is false; biofilters do not add phosphorus.
- (d) Incorrect. Two of the statements are correct.
Key point
Recirculating aquaculture: biofilter bacteria (Nitrosomonas, Nitrobacter) turn toxic ammonia into nitrate and break down organic waste; they add no phosphorus.
Remember
Biofilter: ammonia to nitrate, no phosphorus.
Common mistakes
- Thinking biofilters feed the fish with nutrients.
- Confusing nitrification with nitrogen fixation.
- Reversing the order: it is ammonia, nitrite, nitrate.
Exam tip
A filter removes; it does not add. That rules out statement 3.
Sources
- ICAR-Central Institute of Fisheries Education — Recirculating Aquaculture System
- Department of Fisheries — Pradhan Mantri Matsya Sampada Yojana: RAS guidelines