In a village biogas plant, a slurry of cattle dung is fed into the concrete tank and a floating cover is placed over the slurry. Over the next few days the cover is seen to rise steadily. Which of the following best explains the rise of the cover?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Methane, along with CO2 and H2, is being produced by methanogens growing anaerobically on the cellulosic material in the dung slurry.
Cattle dung (gobar) is rich in methanogens such as Methanobacterium, which also live in the rumen of cattle. Inside the closed tank these bacteria grow without oxygen on the cellulosic material in the slurry and give out biogas, which is mostly methane. As the gas collects, it pushes the floating cover up.
Option (1) Water from the slurry is turning into vapour under the cover due to the heat given out by the bacteria. — Incorrect. The cover is lifted by the gas made by microbial activity. Water vapour is not what fills the tank.
Option (2) Methane, along with CO2 and H2, is being produced by methanogens growing anaerobically on the cellulosic material in the dung slurry. — Correct. This gas collects under the cover, and the cover keeps rising as more gas is produced. The gas is then piped to nearby houses as fuel.
Option (3) Oxygen is being released by aerobic microbes that grow on the surface of the slurry. — Incorrect. Aerobic microbes use up oxygen; they do not release it. Biogas is made by bacteria growing anaerobically.
Option (4) Ethanol vapour is being produced by yeast fermenting the sugars present in the bio-wastes. — Incorrect. Yeast fermentation is used for beverages and commercial ethanol. Biogas is a mixture of gases made mainly of methane by methanogens.
NCERT Extract:
Microbes in Production of Biogas – “… Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel. … However, certain bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO2 and H2. These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium.”
Microbes in Production of Biogas – “… The biogas plant consists of a concrete tank (10-15 feet deep) in which bio-wastes are collected and a slurry of dung is fed. A floating cover is placed over the slurry, which keeps on rising as the gas is produced in the tank due to the microbial activity. The biogas plant has an outlet, which is connected to a pipe to supply biogas to nearby houses.”