call your local water/sewer authority, instead of getting advice on an internet message board.
on a message board you are going to get a variety of answers from " it deadly poisonous " to " none of it is toxic
they are all a bunch of dopes who say it is "
by calling your local authority you will get the actual answer to your question ..
some places have strict laws some don't.
where i live it is very strict, where i used to live they would fine people thousands of dollars a day for being non-compliant with
the rules and regulations governing photochemical waste.
if you find out that you have to dispose of your fixer, and you want to take some of the silver out of it first, feel free to contact me
i sell silver recovery systems that are inexpensive, and efficient.
good luck !