Unfortunately the local authorities find it easier to just say no rather than understand the issue. When I had a darkroom in the last city I lived in, I actually got to talk to one of the plant engineers who actually understood the process. He told me no issues with dumping developer down the drain and didn't have a problem with the quantity of fixer I would be dumping. After I moved I didn't start up a darkroom again but figured I'd just do film and scan. Now I'm on a septic system but figured I could save the spent chems and dump them at one of my daughters house. I called again to see if anything had changed. I explained what I was doing and kept getting the can't dump any chemicals answer. I tried to elaborate what some of the stuff was and still got the oh no, can't dump any of those. I figured I was talking to someone who really didn't know. My last question was, could I dump some sodium chloride down the drain. Want to guess what the answer was.
I have called some of the local labs and none would take spent fixer.
This is now mute as the local hazardous waste location will take them in the quantities I will bring. I'm just trying to decide which is the safest in terms of handling to take them, separate or all together ( my thinking is still that all together gives a more neutral mix ). If it doesn't really matter, it is more convenient to just mix them. Keeping things separate for silver recover is not a concern because they do not do that. I may look into doing it myself but haven't gotten there yet.