Which among the following statement is correct?

Methanobacterium is an aerobic bacterium found in rumen of cattle. Biogas is pure methane. Activated sludge sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria. Biogas is produced by activity of aerobic bacteria on animal waste.

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

The effluent from aeration tank is passed into a settling tank where the aerobic bacterial ‘flocs’ are allowed to sediment and the sediment is called activated sludge.       2. Biogas is produced by the activity of anaerobic bacteria on activated sludge.         3. Methano bacterium is an anaerobic bacteria found in rumen of cattle.         4. Biogas is a mixture of gases such as methane, hydrogen sulphide and carbon dioxide.