How does cellulose digestion occurs?

In animals like Termites, the cellulose is digested by microbes present in the gut or stomach. In herbivores, there is a different compartment in the stomach for the digestion of cellulose which is known as rumen, and the animals are called ruminants.

The ingested food containing cellulose is stored in the rumen temporarily. The bacteria and enzymes present in the rumen conduct anaerobic bacterial digestion of cellulose.

Later the partially digested food is regurgitated to chew their cud. Cellulose is not digestible by the human digestive system. However, this fibrous component helps in the smooth functioning of the intestinal tract.  

[Extra information:

Herbivores use microbial fermentation to break down cellulose. Herbivores with only one stomach that can break down cellulose almost as well as ruminants are called hindgut fermenters.

Ruminants, on the other hand, are called foregut fermenters. These are further divided into two groups based on how big the digestive organs are compared to the rest of the system.

Colonic fermenters, like horses and rhinos, tend to be bigger animals, while cecal fermenters, like rabbits and rodents, tend to be smaller. Great apes get a lot of phytanic acid from the fermentation of plant matter in their hindguts. The fiber molecule cellulose can't be broken down as well by monogastric as it can by ruminants, though the ability of different species to do so varies.]