Microbial fuel cells are considered a source of sustainable energy. Why? 1) They use living organisms as catalysts to generate electricity from certain substrates. 2) They use a variety of inorganic materials as substrates. 3) They can be installed in waste water treatment plants to cleanse water and produce electricity. Which of the Statements given above is/are correct?

The correct answer is C, statements 1 and 3. A microbial fuel cell uses bacteria as biocatalysts. The bacteria oxidise organic matter and release electrons to an anode, which pass through an external circuit to the cathode and produce current. It is seen as sustainable because the fuel is waste or biomass rather than fossil fuel.

• Statement 1 is correct. Living microorganisms act as the catalyst for the oxidation step, so no costly chemical catalyst is needed.
• Statement 2 is wrong. MFCs run on organic substrates such as acetate, glucose, sewage, agricultural residue and food waste. Inorganic materials are not the microbial fuel.
• Statement 3 is correct. Because they treat organic-rich sewage, MFCs can be fitted into wastewater plants, cutting treatment energy costs while generating power.

Core concept: bioelectrochemical systems that link waste management with renewable energy. For Mains, it fits GS-III on energy, environment, waste-to-energy and the circular economy. Power output is still low and electrode costs are high, so MFCs remain largely at pilot scale.

Trap: candidates pick D because the whole idea sounds green. Statement 2 looks plausible because "variety of materials" sounds broad, but the substrate must be biodegradable organic matter.