At lunch at work today, an irreligious Jewish coworker asked a religious one why we don't need separate meat and dairy trash receptacles. After all, we separate meat and dairy otherwise. The religious coworker replied that we don't eat from the trash. But the truth is that besides the ban on eating meat with dairy, there's a ban on cooking meat with dairy. And, as far as I know (from laws of Shabas, I guess), that includes allowing yad-soledes-bo meat to come into contact with dairy (or vice versa), at least if it's hot enough to make the surface of the latter yad soledes bo also. So there would seem to be a problem with dropping hot meat directly on dairy in the garbage (or vice versa). Is there indeed such a problem? If not, why not?
There are 3 issues with meat and dairy: cooking it, eating it and getting pleasure from a cooked meat and milk mixture (such as feeding an animal a meat/milk combination). Cooking may be an issue if it is yad soledes bo. So boiling meat sauce should not be poured on top of dairy in your trash can as it can "cook" it according to Halacha. So some hold it is an issue so you need to wait for it to cool off before discarding (it might melt your garbage bag anyways so it might be a good idea to cool it down irregardless). (See first article I quoted).
However, some hold that cooking only applies as a preparation for eating and therefore something that could not be eaten would not be an issue. (Kessef Mishneh (Hilkhos Tumas Meis 1:2)) (see the second article I quoted).
Maybe this can help: