Usually identification of anaerobic microbes is via molecular technique PCR using primer pairs for archaea. Thus 20 ml of digestate/slurry sample is mixed with 20 ml of 99% ethanol and keep frozen at - 20 degree celcius prior to DNA extraction. The rest is following protocol in extraction kit provided by the supplier. As example you can use Fast DNA Spin Kit for Soil (MP Biomedicals Heidelberg, Germany).

Good luck!

p.s. Those method mentioned are from thesis of Nuruljannah Khairuddin

https://www.nature.com/articles/ismej201159

https://doi.org/10.1016/j.msec.2003.11.005

https://link.springer.com/article/10.1007/s13233-016-4100-y

https://doi.org/10.5713/ajas.15.0507

Good luck.

Hamid Reza Farhang Ahmad Zainuri Thank you brothers

Dear Dr Halayit Abrha . See the following useful RG link: Chapter The microbiome driving anaerobic digestion and microbial analysis

Kindly see also the following useful RG link: Article Microbial Ecology of Anaerobic Digesters: The Key Players of...

Also check please the following very good RG link: Article Critical Analysis of Methods Adopted for Evaluation of Mixin...

The following RG link is also very good: Article Critical Analysis of Methods Adopted for Evaluation of Mixin...