I don't know what it will do, if any. My grandmother put coffee grounds down her sink for over 20 years!
A smattering of remnant grinds washing down the sink won't matter, but the whole load can easily bind to greasy substances stuck to the pipe walls, and over time, that could lead to clogs and backed up sinks.
Now a garbage disposal I have no knowledge of.
Personally I compost mine, religiously.
I have heard that they are good for plants...put them into your plants!
PG you are right, though some of us country bumpkins have septic systems and treatment and processing is a different animal.
That said, it is not good for these private systems to have unnecessary products (most destructive being man made items such as tampons or condoms and bleach products) tossed down the drain either.
I read in a women's magazine that you should scrub your butt and thighs with the leftover grounds during your next shower. The tannins and caffeine in the grounds helps improve the look of cellulite and loose skin. I don't drink coffee, though, so I have never tried it.
I save coffee grounds for plants and also if you put them on ant hills they are suppose to stop them or something so they go away.
I'll have to try that shower trick some time.
oh also we had to have a professional plumber come clean our pipes and he said he loves garbage disposals because that's the way he keeps getting business. He said if everyone stopped using the disposals he'd lose most of his business - so I no longer use my disposal at all.
Per this websiteInsect repellant: Coffee grounds are good ant repellent, just sprinkle some coffee grounds near ant hills or at borders where you want ants to be around. This will drive away ants to a different area. Also it helps to repel snails and slugs. Earl Proulx's "Yankee Home Hints" recommends sprinkling coffee grounds around your garden to eliminate cutworms and ants. He also says that "to get rid of ants in the lawn, mix about 1 pound of coffee grounds in 1 quart of hot water and pour this on anthills. Red ants, which make big hills, will require a big dousing."
There are some other really cool ideas -- including flea dip for dogs and they have the recipe for cellulite creme- too cool I'm never throwing away coffee grounds again!!!
I grew up throwing them in the disposal -- until it had to be repaired one day -- then we started putting them in the compost....