Which among the following statement is correct?

Activated sludge sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria.

The correct answer is Option (3) → Activated sludge sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria.

Explanation:

Activated sludge refers to the flocculent biomass produced during the aerobic treatment of sewage in wastewater treatment plants. In this process, microorganisms, primarily aerobic bacteria, are introduced into aeration tanks where they metabolize organic pollutants present in the wastewater. These bacteria form aggregates or flocs that settle in the settlement tanks, creating what is known as activated sludge. This sludge is not only a byproduct but also a rich source of aerobic bacteria, which can be recycled back into the aeration tanks to maintain the microbial population necessary for efficient treatment .

Other options :

• Methanobacterium is an aerobic bacterium found in rumen of cattle.
  This statement is incorrect. Methanobacterium is a genus of strictly anaerobic archaea found in the rumen of cattle. These microorganisms aid in digestion by producing methane through the anaerobic breakdown of organic matter .
• Biogas is pure methane.
  This is inaccurate. Biogas is a mixture of gases produced by the anaerobic decomposition of organic matter. While it contains a significant proportion of methane (CH₄), it also includes carbon dioxide (CO₂) and trace amounts of other gases like hydrogen sulfide (H₂S).
• Biogas is produced by activity of aerobic bacteria on animal waste.
  This statement is false. Biogas production is facilitated by anaerobic bacteria, which thrive in oxygen-free environments. These bacteria decompose organic material, such as animal waste, to produce biogas.

Final Answer: Activated sludge sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria.