In-ground Pool Filters

Sand is the oldest and most popular method of filtration. 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 underdrain 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 .55 mm (also known as pool grade #20 silica sand), because it has sharp edges that separate particles, allowing filtration to take place. They operate on the basis of "depth" filtration where dirt is driven through the sand bed and trapped in the spaces between the sand particles. Initially, a clean sand bed removes larger particles, and then, as the bed starts to load up with debris, the filter removes finer particles. The sand can be cleaned by backwashing which involves reversing water flow through the filter to the "waste" line. Sand Filters trap debris as small as 20 to 40 microns.