UPSC Prelims 2023 Q55: Biofilters in RAS Aquaculture — Explained

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?

  1. Only one
  2. Only two
  3. All three
  4. None

Answer: (b) Only two

Statement-wise explanation

  • 1: correct — A recirculating aquaculture system (RAS) reuses the same water. It keeps the water usable by treating waste in stages, and the biofilter is one of those treatment stages. The official key counts this statement as correct. Biofilter bacteria include heterotrophs that break down the fine organic matter left by uneaten feed and faeces, so the biofilter does part of the job of clearing feed waste. Strictly speaking, though, whole pellets of uneaten feed are caught by the mechanical filter that sits before the biofilter.
  • 2: correct — This is the main job of a biofilter. Fish give off ammonia through their gills and in their waste, and ammonia is toxic to them. Nitrifying bacteria living as a film on the filter media turn it into nitrite (Nitrosomonas) and then into nitrate (Nitrobacter/Nitrospira). This two-step process is called nitrification. Nitrate is far less toxic, and the system controls it by replacing some water or by denitrification.
  • 3: incorrect — Biofilters work on nitrogen compounds. They do not add phosphorus, and fish do not take phosphorus from the water as a nutrient. Fish get phosphorus from their feed. Phosphorus that builds up in RAS water comes from feed and faeces and is treated as a pollutant to remove. It is not a nutrient that the biofilter supplies.

How to eliminate

Statement 3 can be ruled out with basic knowledge. Phosphorus is the classic pollutant behind eutrophication. A water-treatment unit exists to remove pollutants, not to 'increase' one for fish. Fish also take their nutrients from feed, not from dissolved phosphorus. That rules out 'All three'. Statement 2 is textbook nitrification: the 'bio' in biofilter means bacteria, and the bacterial nitrogen cycle turns ammonia into nitrate. That rules out 'None'. You are left choosing between one and two correct statements, which depends on Statement 1. Read it as UPSC did: the biofilter is part of the system's waste treatment, and its bacteria break down organic matter from leftover feed. That makes two statements correct. Fact to remember: in RAS, the mechanical filter catches solids, the biofilter's nitrifying bacteria change ammonia into nitrate, and phosphorus is something to remove, never something added.

Know more

Recirculating Aquaculture System (RAS) is fish farming in tanks where 90% or more of the water is cleaned and reused. Only a small share is replaced each day. Because so little water is replaced, RAS can stock many fish on little land. It can also hold species in conditions that differ from the local climate, such as rainbow trout kept cool in tropical Telangana.

The treatment loop, in order

Unit What it removes or does
Mechanical filter (drum filter, settling tank) Solids: faeces, uneaten feed pellets
Biofilter (moving-bed, trickling, submerged media) Dissolved ammonia → nitrite → nitrate; heterotrophs break down fine organic matter
Degasser / aerator Strips CO₂ and adds oxygen
UV / ozone Kills pathogens
Denitrification unit (optional) Converts nitrate to N₂ gas

Nitrification in two steps

  • NH₃/NH₄⁺ → NO₂⁻ by ammonia-oxidisers (Nitrosomonas, and also ammonia-oxidising archaea)
  • NO₂⁻ → NO₃⁻ by nitrite-oxidisers (Nitrobacter, Nitrospira)

Both steps use up oxygen and alkalinity. This is why RAS operators watch dissolved oxygen and pH. A new biofilter needs weeks to build up its bacterial film. This start-up stage is called 'cycling'.

Phosphorus comes in with feed. Some leaves the system with the sludge, and the rest builds up in the water. Some farms remove it, or use the nutrient-rich water to grow plants. Growing plants this way is aquaponics, where crops take up the nitrate and phosphate.

India's angle: the Pradhan Mantri Matsya Sampada Yojana (PMMSY, launched in 2020) funds RAS and biofloc units as intensive farming options that use little water.

In the news

RAS is one of the intensive farming technologies the Pradhan Mantri Matsya Sampada Yojana (launched in 2020) funds, so it was in fisheries news in the run-up to 2023. The question itself tests static science: how nitrification works and what each filter does.

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