Dirty pool water is pumped through your pool's skimmer into the pump. The pump strainer basket catches some debris that passes through the skimmer basket. The water is then pumped into the filter tank where it sprays onto the top of the sand bed. The water is forced through the sand, which catches dirt, algae, leaves, and other fine particulate matter. The clean water is then sucked into the laterals at the bottom of the filter. The laterals have fine slits that futher filter the water as it passes through them and back up the center pipe where it returns to the pool.
When the sand becomes dirty, the pressure inside the tank rises. This pressure is read on the filter's pressure gauge. Normal pressure varies by filter and by how long the sand has been in the filter. New sand will usually give a reading of 12 to 14 psi. When the pressure rises 8 to 10 psi above the normal reading, it is time to backwash. When the filter valve is placed in the backwash position, water is forced backward up through the sand and the dirty water and debris that is in and on top of the sand is pumped out of the filter's waste port. Backwashing the filter after your weekly cleaning usually keeps the filter sand clean and operating at peak efficiency.
Barbi-Lin Pools recommends changing the sand in your filter every two to three years. While sand does not technically "go bad" the grains erode and lose the ability to trap dirt and debris. Chemical buildup can also inhibit proper filtration. Pool water that is not properly filtered uses more chemicals and makes cleaning the pool more time-consuming each week.