Last summer, I tried something new, I evaporated it. I poured about a gallon of used fixer into a flat pan and stuck that outside so it would be exposed to the hot, summer sun. It took a few weeks, but the liquid finally evaporated leaving behind a white residue. I scraped that up, stuck it in a bag, and deposited in my trash. From there it went to a landfill. For me, problem solved.