Methanogenic bacteria are not found in :

Activated sludge .

The correct answer is Option (4) – Activated sludge .

Methanogenic bacteria are anaerobic microorganisms that produce methane during the decomposition of organic matter.

They are typically found in:

• Gobar gas plants (anaerobic digesters)
• Rumen of cattle (helps in digestion of cellulose)
• Anaerobic sludge (during the anaerobic phase of sewage treatment)

Activated sludge is part of the aerobic phase of sewage treatment, where aerobic bacteria are active — methanogens cannot survive in this oxygen-rich environment.