How to safely dispose of latex paint?
The municipalities just don't want it liquid or with the lid on.  Imagine a paint can full of paint when the compactor squishes down on it.  Paint everywhere! You can put whatever you want in the paint to make it more solid. Kitty litter, the store bought stuff drying agent, sand.  I have used some old mortar that I had on hand.    You just want to make sure that it's a solid or semi solid, and you can put it out on the curb for pickup without worry of it spilling with no lid. It would take too long for it to air dry.  Also, a film might form on the top, preventing the bottom from drying.