Why does a layer of dirt build up on top of the sand in water treatment?

Dirt might be dissolved into the water while sand can not. However, not all dirt will dissolve. The dirt can be separated out from the sand as the sand would be heavier and would form a lower layer than the dirt when the suspension separates after shaking or stirring. Then the water with any suspended dirt particles could be decanted (poured) off from the top. This process could be repeated until the water on top was clear. The water could be evaporated off.

Sand is non-polar while water is polar, therefore, sand will not dissolve. You can remember this by using a saying "like dissolves like." Polarized substances will dissolve polarized substances and vice versa.