Question 6 from UPSC CSE 2023 - Environme nt

Question

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 a nutrient for fish in water.
How many of the statements given above are correct?

IASCODEX · Accepted Answer

Correct Option: B. Only two

Strategy: *   Focus on the primary biological filtration function of biofilters, specifically the conversion of nitrogenous waste.
*   Validate Statement 2, which correctly identifies the main job of biofilters in Recirculating Aquaculture Systems (RAS) as converting toxic ammonia into less harmful nitrate through nitrification.
*   Note Statement 1 as partly correct; biofilters can assist in removing uneaten feed through microbial breakdown, though their primary action is on dissolved nitrogenous wastes.
*   Reject Statement 3 as incorrect, as biofilters do not increase phosphorus; instead, filtration systems aim to reduce it.
*   Conclude that two statements (1 and 2) are correct based on their alignment with known biofilter functions.

Key Info: Recirculating Aquaculture Systems (RAS) are closed-loop tank systems used to rear aquatic organisms under controlled conditions. Water is continuously treated and recycled to minimize consumption and environmental impact. Biofilters are integral in RAS, consisting of surfaces colonized by beneficial bacteria (mainly Nitrosomonas and Nitrobacter) that perform biological oxidation.

### Key Functions

*   **Ammonia removal**: Fish metabolize protein, excreting ammonia (NH3/NH4+) as waste, which is toxic even at low concentrations. Biofilters support nitrifying bacteria that convert ammonia to nitrite and then to nitrate, a process known as nitrification.
    *   **Step 1**: Ammonia → Nitrite (Nitrosomonas)
    *   **Step 2**: Nitrite → Nitrate (Nitrobacter)
*   **Uneaten feed and solid organic matter**: These are removed primarily through solids filters and mechanical filtration. However, biofilters, through heterotrophic bacterial activity, partially aid in the mineralization of dissolved organic matter from leftover feed.
*   **Phosphorus removal**: Phosphorus compounds from feed and fish excreta accumulate as waste in RAS. Standard biofiltration does not increase phosphorus; instead, it may reduce phosphorus through microbial uptake and removal of solid waste. Excess phosphorus can contribute to algal blooms if not managed.

RAS technology is critical for sustainable aquaculture, minimizing effluent discharge and conserving water. Biofilters are central to water quality management, primarily for nitrogen compounds and some aspects of organic matter processing, but not for phosphorus enrichment.

IASCODEX · Answer

Option A: Only one

IASCODEX · Answer

Option C: All three

IASCODEX · Answer

Option D: None