[JOSarff]

A lot of variables. If the OP is sending all his other "common" contaminants (bleach from washing machine and dishwasher, antibacterial soap) out as grey water, the impact of the photo chemicals may be less significant. But if his septic tank is small (reflecting the separate grey water system) there is less dilution.

The system is maintained properly and is biologicially active. The major problem in most home darkrooms/septic systems (and it has been discussed elsewhere) is the volume of wash water damaging the biological activity of the tank, not the chemicals. Putting 5 to 600 gallons of waste water from a household a month is not a problem, 200 gallons a day of wash water from the print washer is.