Brian,

I lived for 7 years in an isolated farmhouse where we had a septic tank and all my black and white chemicals were just poured away into the system without any problems or affect on the efficency of the tank. I was carrying out workshops and doing a lot of personal printing so I did have to dispose of a little more than I think you are dealing with.
I would be hesitant to speak to the authorities about these matters for in my experience in the UK it tends to encourage a rather hysterical over reaction; for example the authorities in one area that I used to live in told me that I would be required to entomb any waste chemistry in one cubic yard of concrete and have it transported to the nearest waste disposal site.