I read somwhere or heard somewhere or dreamed that coffee grounds and eggshells are the only organic substances that will not bio-degrade. I am therefore reluctant to put these down my disposal, for fear of messing up my system ( I'm on septic, as I live in back of beyone) Any truth to the coffee grounds/eggshell quandry? Harold McGee? Anyone??
I see both coffee grounds and egg shells recommended for composting so I think you can safely assume that they are quite bio-degradeable.
A mixture of coffee grounds and eggshells is a great fertilizer for roses, so if you don't want to put in your disposal use them on your roses. And they do bio-degrade.
I know you're not supposed to put them down a garbage disposal to get ground up...apparently eggshells can mess with the blades somehow? They were on the no-no list an old landlord gave me along with stuff like potato skins and plastic forks (duh...). It doesn't seem like they'd cause much of a problem, though.
That mixture of grounds and shells is also good to keep cats away from your vegetable garden.