What kind of process is biogas production?

Anaerobic process.

Biogas production is an anaerobic process. Certain bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO₂ and H₂ .

These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium. These bacteria are commonly found in the anaerobic sludge during sewage treatment.