Neil, Some of the concerns would be what type of contrast agent you are using. If you use Hydrogen Peroxide there would be nothing more hazardous than the developer only. If you are using dichromates in your developer and regularly dispose of it, you may be adding a hazardous substance to the environment.
Hypo Clearing agent shouldn't be much of an issue.
And unless your volume is many many gallons a day, your small amount of affluent shouldn't make much of statement down stream.
I'd be more concerned about the harzards in the darkroom for yourself than the consequences from a small discharge. The powders of PT can be linked to asthma, etc. Practice good darkroom safety and things will be fine.