I used to take my spent fixer to a local photo lab that would then put it through their silver-recovery unit. That was a great solution. Since then, the lab is gone and there are no other options locally with silver recovery.
So, I tried the hazmat route: I took gallons of accumulated used fixer to the local hazmat facility (Bend, OR). I explained to them it was used photographic fixer and should go to silver recovery. The goons working their just labeled it "used photo chemicals" and sent it God knows where. I'm fairly sure it didn't get to any kind of silver-recovery unit. Who knows what they do with it. I doubt it ends up being any better than just pouring it down the drain.
So, why bother...
My darkroom is connected to a septic tank system, but where I live is on the municipal sewer system. I now just take my used fix and dump it at home into the municipal sewer. I hope the 10 gallons or so I dump every six months is not a problem.
If I ever find a lab that will take it and recover the silver from it, I'll go back to doing that.