My darkroom is a freestanding shed 20ft by 8ft and although I don't now have a septic tank I once lived in a farmhouse that did and the darkroom drained into it. I was there for more than 10 years and we experienced no problems with the septic tank. The only chemical that I didn't pour away to the tank was used selenium toner, I prepared a small gravel pit in a corner of the garden and poured the selenium into that. If you would like me to post some photographs of the outside and inside of the darkroom please let me know.