Sodium thiosulfate is used in water purification plants for town water. Don't worry about it. The silver in it will become silver sulfide (no shortage of available sulfur compounds in the sewer!) which is stable and won't bother any living thing (so stable that sulfided prints last longest).
Recover the silver if it's convenient, of course.
I got in quite a row saying pretty much this same thing on here before. You are, of course, absolutely right.