Company with over 20 Years of experience in the field of water treatment equipment for human consumption. The GRAVEL rock is at least two millimeters in size and no more than 75 millimeters. Rock of less than this size is called sand. In deep bed filters gravel is used as the basis for the minerals used as a means gravel filtrante. Para these systems should be siliceous rock, with smaller, hard angles, and high abrasion resistance.