The oldest and most popular method of filtration is sand. Sand filters share two things in common: 1) When in the filtration mode, water always flows from top to bottom; 2) They all have some sort of lateral or under drain with slots to hold back sand while allowing clean, filtered water to pass through. High-rate sand filters use a special filter sand, normally .45 to .55mm (also known as pool-grade #20 silica sand), because it has sharp edges that serve to separate particles, allowing filtration to take place. They operate on the basis of 'depth' filtration; dirt is driven through the sand bed and trapped in the minute spaces between the particle of sand. Initially, a clean sand bed will remove larger particles, and then, as the bed starts to load up, it will remove finer particles. Cleaning of the media, or sand bed, is accomplished through reversing the flow through the filter, to the 'waste' line. This is known as backwashing.