I posted a while back on some trouble I'd had using coffee grounds in my bin - specifically an ammonia smell that was getting very unpleasant. I got the bugs worked out of that so I thought I'd post to share what I've learned.

Coffee grounds have between 1.5 and 2% nitrogen by weight, and a C/N ratio of about 20. This puts them about midrange on the list of "greens" for a compost bin. They have other properties though that make those numbers a little misleading.

Naturally they're very finely ground, giving them a particle size approaching medium sand or even sawdust. This gives them a HUGE surface area relative to their mass, and surface area is one of the main things that determines how fast microbes can attack them. This is why composters have long observed that adding coffee to a compost pile can heat it up fast: they decompose much faster than you might expect from their nitrogen content.

In my worm bin, I found that this led to a sharp drop in the C/N ratio when I added coffee grounds. It was worse for me than it might have been for others, because I use compost from my hot bin as bedding in my worm bin, meaning that the C/N ratio is already a lot lower than it might be if I was using peat or shredded paper for my bedding like many wormers do. The low C/N led to ammonia offgassing, just like it would in a regular compost pile.

The solution was also the same as it would be for a regular compost pile: add in carbon to raise the C/N ratio back where it should be. In my case this meant finely shredded paper mixed thoroughly into the bedding. I may also start using sawdust, since it has an even finer particle size and a higher C/N, so I can use less of it.

I've also found that the usual "pocket" method of feeding doesn't work so well with coffee. The usual way of feeding worms is to dig a hole in the bedding, add food, and cover it up. But with coffee grounds this leads to compaction and a local drop in the C/N ratio as the coffee grounds take up the carbon in their immediate area. I find I get much better results by scattering the grounds over a wider area of the worm bin and stirring up the bedding a little to mix them in.

Since I started doing these things the ammonia smell has vanished, and I've even been able to increase the amount I feed them without a problem. So I pass it on to others who might want to use coffee grounds, which are freely available at local coffee shops.