Kevin, I use a Delta 1 air vent for darkrooms and it really moves the air out - available from Calumet (see link at top of page).

As to the septic system, I am on one also and have had no problems (mine is actually an aerobic system, like a mini-process plant). Developer, stop and wash water all go down the drain. Fixer into a bucket with steel wool, the the yuck is tossed out. Getting ready to switch from stop bath to water for stop to remove that (it really is just like strong vinegar though).

How big will the space be? Hope you have enough room for post print prep (mounting,etc), that's the one thing I don't have at all.

Good Luck.