I recently started collecting used coffee grounds from several local Starbucks locations. Some stores save the grounds, some don't. A few offered to save them if I would pick them up every day (much as I would like to I can't make the 20 mile round trip every day). After a month I ended up with more than I can use at one time. So I started putting the grounds in buckets, mixed with some shredded newspaper and wood pellet rabbit litter pan waste. I drilled some small holes around the tops of the buckets, just under the rim, so excess moisture can evaporate. I was surprised to find that this mixture gets pretty warm. After one week I can see where the material is breaking down. I water it occasionally because when it heats it dries the mix out quite a bit.
Right now I've got 2 buckets in the process of decomposing. Another with coffee grounds, wood pellet litter and kitchen waste. A couple more buckets are awaiting use when I get a chance to repot my citrus trees.