Methanogenic bacteria are not found in
1. rumen of cattle
2. gobar gas plant
3. bottom of water-logged paddy fields
4. activated sludge
microbes present in the activated sludge are aerobic bacteria that grow rapidly and form flocs.
During secondary treatment of wastewater itis in the digestors where other kinds of bacteria
which grow anaerobically, digest the bacteria and the fungi in the sludge producing mixture of
gases such as methane, hydrogen sulphide and CO which form the biogas.