In a much larger boat (this one to be precise), the, um, remnants from the toilet get pumped out to sea when you’re out far enough. I would assume no chemicals are added (or only all-natural ones) because we were told that the fish and other marine life consume it. (Gross, I know.)
All I know is that while on the boat, there were times were our tanks were almost full, and we were told, “If it’s yellow let it mellow, if it’s brown flush it down.” It was sort of disturbing.