Silica Gravel is a hard, different-colored, and round shaped.  

Gravel is mined, washed, and sieved to minimum / maximum grain sizes that meet or exceed international standards such as AWWA B100-1 or EN12904.

Gravel is used as a support layer to prevent migration of filter media through filter floor. Gravel does not contribute to removal of suspended particles.

Gravel is arranged in hour-glass configuration where the retaining layer is placed in the middle of the support gravel layers. The grain size of support gravel largest size prevents filter floor blinding. The same arrangement is placed in reverse order above and below the retaining layer to prevent upset of support gravel and hence cause migration of filter media.

Retention layer prevents fine filter media from migrating through support gravel and hence filter floor. It is sized to be 2.5 of the smallest media size.

Gravel is sieved to maximum and minimum size. Due to its complex shape, it is misleading to use uniformity coefficient (UC), especially in large sizes.