It's probably a good idea to obey, to the letter, local disposal regulations. Not because they are chemically or biologically correct but - more practically - to avoid fines, imprisonment and having your darkroom confiscated.

If you can reclaim the silver then that is a very good plan. In any case the majority of the container consists of water, and this evaporates very nicely leaving minimal residue. Obviously, arrange things so that animals cannot drink the stuff (or drown in it) while you are evaporating the water.