Dehumidifiers remove moisture from the air. They do this by cooling the air that is drawn into the unit, causing condensation. The condensed moisture is stored in a holding tank that you must empty on occasion. To cool the air within the unit, a dehumidifier uses Freon (chloroflourocarbon gas). Because it contains Freon, you just can't throw out your dehumidifier or other appliances that contain Freon--such as refrigerators and air conditioners--with your regular trash.
Call your local household hazardous waste facility to see if they accept dehumidifiers. If so, dispose of it there. If not, which is likely, proceed to the following steps.
Call your local department of sanitation office to schedule an appointment for the dehumidifier to be picked up or to have the Freon removed. You may have to pay for this service.
Place your dehumidifier by the curb the night before the scheduled appointment. It will either be picked up by your local government facility or it will be drained of the Freon. If it's drained, they will tag it as drained.
Leave the tagged humidifier by the curbside for regular trash or recycling pick-up (depending on the community). You may also be able to throw it away as you do your normal household trash, such as disposing of it in the dumpster; however, some communities may mandate recycling. You can also take it to a public recycling center, but call first to be sure they accept small appliances like empty dehumidifiers.