You don't need to discard paint thinner that has been used for cleaning paint brushes. Put used paint thinner into a glass or metal container with a tight fitting lid. The paint particles will sink to the bottom leaving you with clear paint thinner towards the top. Just pour off the clear paint thinner and discard the paint particles. Before disposing of any paint, even paint particles, check witih your local waste management to find out the correct way to dispose of your paint.