Water Pollution

Microbial fuel cells (MFCs) indeed use living organisms, such as bacteria or algae, as catalysts to generate electricity from certain substrates. These microorganisms break down organic matter and transfer electrons to an electrode, creating an electric current. While MFCs primarily use organic substrates for electricity generation, they may also utilize some inorganic materials, but this is less common. MFCs can indeed be installed in wastewater treatment plants to cleanse water through microbial degradation of organic pollutants, while simultaneously producing electricity. This dual functionality makes them a sustainable source of energy.

Arsenic, fluoride, and uranium are naturally occurring elements that can leach into groundwater in some regions of India. Sorbitol is a sugar alcohol not typically found as a pollutant in drinking water. Formaldehyde is a synthetic compound used in various industrial processes and is not a common contaminant in groundwater.

* **Aral Sea:** The Aral Sea was once one of the world's largest inland lakes, but due to extensive water diversion for irrigation projects, particularly from the Amu Darya and Syr Darya rivers, it has shrunk by over 75% since the 1960s. This has led to severe environmental, economic, and social consequences, including increased salinity, loss of marine life, and desertification.  * **Black Sea:** While the Black Sea faces environmental challenges such as pollution, biodiversity loss, and reduced oxygen levels in deep waters, it has not experienced significant drying like the Aral Sea. * **Lake Baikal:** Lake Baikal in Russia is the deepest and oldest freshwater lake in the world, containing about 20% of the world's unfrozen freshwater supply. Although it experiences natural fluctuations in water levels, it has not undergone severe drying like the Aral Sea. Thus, the correct answer is option (d).

* **Statement 1 is correct:** **Role of Biofilters in Recirculating Aquaculture System (RAS):** Biofilters provide waste treatment by removing uneaten fish feed. Biofilters in a RAS help in waste treatment by removing uneaten fish feed and other organic matter from the water. The biofilter media provide a surface for beneficial bacteria to grow, which help in breaking down and removing these wastes. * **Statement 2 is correct:** Biofilters convert ammonia present in fish waste to nitrate. One of the primary functions of biofilters in a RAS is to convert toxic ammonia (produced from fish waste) into less harmful nitrate. Beneficial bacteria in the biofilter media perform nitrification, a process where ammonia is first converted to nitrite and then further converted to nitrate. * **Statement 3 is incorrect:** Biofilters do not increase phosphorus as a nutrient for fish in the water. Their primary role is to remove wastes and convert ammonia to nitrate. Phosphorus levels in the water are managed through other means, such as fish feed formulation and water quality management. **Therefore, option (B) is the correct answer.**

* Membrane Bioreactors are often discussed in the context of: D. Wastewater treatment technologies. * Membrane Bioreactors (MBRs) combine a membrane process like microfiltration or ultrafiltration with a biological wastewater treatment process. * They are used to improve the quality of treated wastewater and are known for their efficiency in removing contaminants.

**Activated carbon is widely used to clean pollutants because of its unique properties and sustainable production methods.** ✅ **Statement I: Correct.** Activated carbon effectively removes pollutants from industrial effluents due to its high adsorption capacity. It’s commonly used in water treatment and pollution control. ✅**Statement II: Correct.** This is because activated carbon has an extremely large surface area, created by its porous structure, allowing it to trap heavy metals and other contaminants efficiently. ✅ **Statement III: Correct.** Moreover, activated carbon can be produced easily and cost-effectively from carbon-rich wastes like coconut shells, rice husks, and other agricultural residues, making it an eco-friendly option. **Thus, both Statement II and Statement III are correct, and together they explain why Statement I is true.**