Filtering swimming pool water through a layer of fine sand is one of the oldest forms of filtration. Dirt particles in the water are trapped and held in the very small openings between the grains of sand. In addition, gelatinous substances and oils cling to the fine grains of the sand. This process of coagulation enhances the filtering efficiency of sand in capturing very small dirt particles.

The most modern style of sand filter being sold today, whose performance is beyond the best, is the high-rate sand filter. With this type of a pool filter the water enters the filter near the top of the filter tank. It is then disbursed evenly throughout the filter tank by the diffuser or baffle. This evens out the influent distribution of water to avert sand migration and channeling.

The water is pushed, under pressure, toward the bottom of the filter. As the water travels from the diffuser toward the bottom of the filter it is forced through the sand filter media, where the dirt is strained from the water.
High-rate sand filters have a filter rate of 15 to 20 gallons per minute per square foot of filter area. They also work on the basis of depth filtration. The dirt removed from the water is not collected on top of the sand bed, but is driven into the sand bed by the high velocity of the water. In a typical pool filter, the primary dirt load is confined to the top 2-4” of the sand bed. In a high-rate sand filter the dirt is collected through the total depth of the sand bed.