I seriously question the validity of that statement. Please provide a reference that corroborates your claim. In my experience, raising the temperature of water tends to allow more impurities to go into solution, which would preclude separation by settling. I know of very few substances that precipitate out of solution as the temperature increases, unless you are referring to evaporation, in which case you would be losing clean water to the atmosphere, and concentrating the solution to the point that it exceeds the saturation point.