Augustine Ifelebuegu Sir, thank you for sharing this important information.

Some corrections are needed here, related to question and its answer:

Nevertheless, sure you can add cow manure or get inoculum / digestate from any biogas plant/reactor which has stable biogas production.

More importantly with your question, it is clear that you don't have a biology background and you don't have much experience in anaerobic digestion, for that please read the following book. It's very easy to read and understand.

Book Microbiology of the biogas process

In my previous study, we collected fecal sludge and stored it in a jar without air supply condition. After a few days, we observed biogas generation from the jar.

Faecal sludge may contain methanogens needed for aerobic respiration.

Les boues fécales peuvent certainement contenir des batteries methanogene car après une période de fermentation, il ya des nutriments dans ces dèchets qui favorisent la croissance et la prolifération des bactéries.

I know that faeces contains such microb, but addition of cow dung will facilitate the production of methane.

Thanks Tagne Tiegam Rufis Fregue Kate Ochiagha . I am considering your suggestion to improve my study.