Decantation is a fast method for separating a mixture of a heavier solid and a liquid.
For separating the mixture of water & sand, first, we should let the sand to settle on the bottom of the container (sedimentation). Then we pour off the water at the top into the other container.
This method cannot be used to separate the mixture of a liquid & a light sold, like chalk in water. The particles of chalk are suspended in water. They are so light which they do not sink down to bottom for a long time.