Biogas is produced by the activity of aerobic bacteria on animal waste
Methanobacterium is an aerobic bacterium found in rumen of cattle
Biogas, commonly called gobar gas, is pure methane
Activated sludge-sediment in settlement tanks of sewage treatment plant is a right source of aerobic bacteria
Solution
Activated sludge is a process dealing with the treatment of sewage and industrial wastewaters. Atmospheric air or pure oxygen is introduced to a mixture of primary treated or screened sewage (or industrial waste water) combined with organisms to develop a biological flock, which reduces the organic content of the sewage. Sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria because small amounts are used as inoculum in secondary treatment or biological treatment stage of sewage treatment.