Regarding the hazardous issue, the EPA states that pressure treated wood is not “hazardous” under federal law and “can be disposed of with regular municipal trash (i.e., municipal solid waste, not yard waste) in many areas,” so long as the municipality doesn’t have environmental laws that are stricter than federal law. http://www.epa.gov/oppad001/reregistration/cca/cca_consumer_safety.htm
And regarding Ft. Totten transfer station, I’ve been permitted to dump building debris from my house (including scrap lumber, masonry debris, sheetrock, etc.). As long as the waste materials are from your house and not a job site, and you’re a DC resident (bring ID), they seem to allow the materials to be dumped. Maybe, to be safe, you can take a small amount and see if they let you dump it. Or you could just drive out there during open-to-the-public hours and ask. I’ve never been turned away with anything I’ve brought to dump.