Sand, Swimming Pool Filters by Hayward, Pentair, Sta-Rite
Sand is the oldest and most popular
method of filtration. All sand filters share two things in common: When in the filtration mode, water
flows from top to bottom, 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 dirt, the filter removes finer particles. The sand can be
cleaned by backwashing which involves reversing water flow through the filter to the "waste" line. Most sand filters have Top or Side Mount valves that make pool maintenance and filter cleaning easy and convenient.
In this method, impurities are extracted from the water utilizing sand to collect
and attract the debris. Sand filters must be backwashed (running it in reverse) to unload the water waste. This is a manual
process that must be performed every few weeks. Sand filters trap debris as small as 20 to 40 microns.