We have a grease jar and when it's filled it goes out in the garbage. For a tray of water, pour it down the drain. If there's fat in the water skim it off and put it in the grease jar. The proper way would be to run it down to a hazardous waste depot, most communities should have one.
I save drippings and add a little bit to my dog's food so he can enjoy the Traeger too. I'm talking more about when there is a little water/fat mixture....not pure fat that will solidify at room temps.