Brian, I also live in a rural area and have a well and septic system. From the information I have been able to gather from Kodak and different manufacturers your septic system will handle up to 25% of it's total volume. There's no way that a normal home darkroom would produce that much volume in a session so I was told not to worry about it. What I do just to be a little safer is to dispose of the fixer separately. Our local recycling centre will accept small amounts of chemicals from home darkrooms without any hassle (as well as paint, oil, tires etc.). It's not just the septic system that I'm worried about it's my drinking water also.