“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
So I wonder how much silver was in that 5 liter jug of fixer that I recently threw out. I developed more than 25 rolls of 120 film in it!
25 rolls is only 25% of the capacity of 5L of (rapid) fixer at working (1+4) dilution; no need to throw it out just yet!
It can get up to about 7g/L (starts to slow right down) so yours probably had about 1g/L in there, or $3.75 worth at $24/oz. Plus or minus a factor of at least two depending on your choice of film and how heavily it was exposed.
So, silver comes from the ground but can't go back in... ?
There's a pretty big difference between metallic silver, silver ions in concentrated solution and silver salts physically mixed with rocks. If you don't believe me, dump some metallic sodium on your dinner next time instead of table salt...
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.