We have septic, too. So far, I am keeping my used fixer & looking for a safe recycling method. I do dump my developer & stop bath.
With the film developer, I mix it with around a gallon of water, and rinse that all down the drain with the water from the rinses between development & fix.
With the paper chemicals, when I am done, I mix the developer, stop bath & the 2 1/2 gallons of water I used for my primary rinse. I dump that down the drain and follow it with the rinse water from cleaning the trays.
We have sort of an advanced septic system that is required by law when your septic decides to take a holiday and not do its job. It's called a mound system. My joke here is that in a 1000 years, when archeologists find it, they are going to wonder what the heck we worshipped here! haha
Our well is here, too, but with this mound system, there is little chance of contamination. I was told that the same is true of traditional septic systems...if we were concerned about contamination from the small amount of chemical that will get dumped, shouldn't we be more concerned about the 'normal' things that go in there?? haha
You do not say what area of the country (this or another) you are in, so I am hoping that this will help anyway!