Patrick, I have been using my darkroom out here in the country for 10 years now without any problems to the septic system. I was told by a Kodak rep that a normal septic system will handle 25% of it's total volume in darkroom waste without problems. I do watch that I am not putting anything exotic into it (I don't do Pyro at home), and I do not put any fix into the septic. I have a place locally that will accept small amounts of fix from home darkrooms for proper disposal.
Most developers are fairly dilute when used, and I will sometimes put a bit of stop into it before washing it down the drain. There are probably worse items under your kitchen sink or in the bathroom (cleaners) that will end up down the drain.