Which of the following statements is incorrect?

The correct answer is Option (1) → Methanogens are commonly found in the aeration tanks during sewage treatment.

Methanogens are bacteria that grow anaerobically (without oxygen) on cellulosic material and produce a large amount of methane along with CO₂ and H₂. So they are found only where oxygen is absent, such as the anaerobic sludge digesters of a sewage plant and the rumen of cattle.

Option (1) Methanogens are commonly found in the aeration tanks during sewage treatment. — An incorrect statement. In sewage treatment, air is pumped into the aeration tanks so that aerobic microbes grow into flocs. Methanogens are found in the anaerobic sludge, not in these aerated tanks.

Option (2) Methanobacterium is a type of methanogenic bacteria that can produce methane gas. — A correct statement. Methanobacterium is a common methanogen and produces methane gas.

Option (3) Methanogens play a significant role in the digestion and nutrition of cattle. — A correct statement. Methanogens live in the rumen of cattle, where they help break down cellulose and play an important role in the nutrition of cattle.

Option (4) The excreta (dung) of cattle, commonly called gobar, is rich in Methanobacterium. — A correct statement. Since methanogens live in the rumen, cattle dung (gobar) is rich in them. This is why gobar is used to produce biogas.

NCERT Extract:
Microbes in Production of Biogas – “… 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. These bacteria are commonly found in the anaerobic sludge during sewage treatment. These bacteria are also present in the rumen (a part of stomach) of cattle. A lot of cellulosic material present in the food of cattle is also present in the rumen. In rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle.”