Water pumps from skimmer or tank must be cleaned. The leaves and trash are well filtered by the pump . Then, the water is pumped into the filter it sprayed to the top of the sand layer. Water is forced through sand, dirt, algae, leaves and other material grains are retained sand. The charger goes through the long sand layer, which is filtered cleanly out of the filter at the bottom of the filter. The braces have a good gap that futher then water returns to the pool. 
 
As the sand becomes dirty, the pressure inside the filter increases. This pressure is read on the pressure gauge of the filter. The normal pressure varies with the filter and how long the sand is in the filter. New sand will usually give a reading of 12 to 14 psi.When the pressure rises 8-10 psi on normal reading, it is time to clean up the sand . When the purge valve is placed at the inverted washing position, the water is driven backwards from upward through the sand and the dirty water and debris that is in and on top of the sand are pumped out of the filter’s waste port. Backwashing the filters after your weekly cleaning usually keeps the sand filter clean and operating at the highest performance.
Vinapool  recommends sand changes in your filter every two to three years. While non-technical sand “worsens” the particles being eroded and losing the ability to trap dirt and debris. Chemical accumulation can also inhibit proper filtration. Pool water that is not properly filtered uses more chemicals and cleans the pool more time each week.