To dispose of water-based paint, mix it with something that causes it to congeal, and put it in the trash. Another option is to leave the can open so that the paint dries up and then can be safely disposed of.
Mixing cat litter with a water-based paint prevents it from leaking out of the can. Mix equal parts of litter and paint by either pouring the paint into a container with litter or mixing the cat litter into the paint can. Mix this together till the paint reaches a state where it cannot flow. Let the mixture sit for one hour, and then put it in the trash can with the lid taken off. Commercial paint hardeners can also be used to harden the paint.
If the quantity of paint is small, just allow it to dry in the can by leaving the lid open. Keep it in a safe place in the open air, away from the reach of children and pets. After it has dried completely, put it in the trash with the lid still off so that the trash collector knows what is inside. Another disposal method is to recycle the paint by giving it away. Clubs and youth groups and even schools and neighbours may be able to use small quantities of paint.