Yes, I think that evaporation is about the only "sensible" way.
Is this really an effluent control question? In the 1980s and 1990s the photo industries did most of its research on how to make photoprocessing waste "acceptable" with respect to all of the regulations that came up about that time. The main issues with the photographic effluent, at least the mainstream processes, were "oxygen demand" (aka BOD and COD), the tendency to consume oxygen out of the water as various chemical components were degraded, and the silver content. The first one is readily handled by sewage treatment plants; the second is preferably done, to some arbitrary degree, by the photofinisher. I say arbitrary because once you get some silver in solution, via fixing, you pretty much cannot prevent some of this from getting into your wash water.
So one question is, how much silver is allowed to "escape" at the end of the line? What is basically happening is that the film emulsion is wet with fixer when you move it into the first wash bath, then it is wet with that when you move it into the next wash bath, etc. It's not unlike using a sponge to mop up some spilled coffee, then rinsing the sponge in a series of trays; the sponge (and water) gradually get more and more clean. At some point you finally say, well, I guess it's clean enough now.
Some of the important methods used by the photoprocessing business are based on both minimizing the total amount of waste, and in keeping it as concentrated in one place as much as possible. This is done by replenishing chemicals (you don't discard them if they are only "lightly used") and using multi-tank "counter-current flow" replenishment (this is loosely what you do with a 2-bath fixing system).
Too much info? Or is it going in the wrong direction?
