Are you talking about sodium thiosulfate? It's less acutely toxic (oral LD 50) than table salt in animals, has no known chronic health risks, doesn't persist in the environment, and is non-flammable.
"We found that silver nanoparticles are extremely toxic. The nanoparticles destroy the benign species of bacteria that are used for wastewater treatment. It basically halts the reproduction activity of the good bacteria."
I photograph onto paper, develop with caffenol, digitize after the stop bath (quickly) and discard the paper so as to avoid using fixer. Yes, there is silver on that paper but I'm hoping that silver in an incinerator or even landfill is better than silver in the wastewater.
Back in the day when analog photography was just called photography, there were inexpensive silver recovery filters available. Now, if you want to use fixer, try to find a local film lab that will take your "spent" fixer.