This might help: http://www.sunsetscavenger.com/hazar...teFacility.htm
I would talk to them as well.
And I bet Matt (tjaded here on APUG) would have some perspective on this.
By the way, it would be great if threads like this had a location in the title, because the rules on this are location specific. For example, there are no restrictions imposed in my area on down the drain disposal of small volumes (home darkroom size) of standard darkroom chemicals.
For clarity, I would include selenium toner among the chemicals that should be handled carefully.