It goes with the other sewage to a sewage treatment plant, where it's gradually filtered out, along with the paper, the tampons, and all the other stuff people flush, and separated from the waste water. Different municipalities do different things with the collected sewage, which is often dried and ground--and, for that matter, they do different things with the waste water, too. Often the treated, dried sewage is used in land fills since it's largely biodegradable (but very nasty). Call your city and ask them what they do with it in your area. Ideally, they do something environmentally sound.

Now you know. . . .