Question

The major constituent of biogas is:

• A. Carbon dioxide
• B. Methane
• C. Hydrogen
• D. Nitrogen dioxide

Hint:

Recall which gas methanogenic bacteria produce as the final step of anaerobic digestion.

Answer 1

The Correct Option is B

Step 1: Biogas is produced when organic matter such as cattle dung, plant waste, and sewage sludge is broken down by anaerobic bacteria in the absence of oxygen, a process called anaerobic digestion.
Step 2: The bacterial breakdown happens in stages, first complex organic molecules are hydrolysed into simpler sugars and acids, then acid forming bacteria convert these into short chain fatty acids, and finally methanogenic (methane producing) bacteria convert these acids into methane and carbon dioxide. Because methanogenesis is the dominant final step, the gas mixture that collects is mostly methane, typically about 55 to 65 percent by volume, with carbon dioxide making up most of the rest, roughly 30 to 40 percent, along with small traces of hydrogen sulfide, nitrogen and moisture.
Step 3: Carbon dioxide is indeed present but only as the second largest component, not the major one. Hydrogen is not a significant product of this bacterial pathway, it may appear only as a fleeting intermediate, not as a stored end product. Nitrogen dioxide has no role at all in anaerobic digestion chemistry, it is not a fermentation product.
Step 4: Since methane dominates the gas mixture and is also the component that makes biogas flammable and useful as a fuel, option B, methane, is correct.