When it comes to the types of pool filters you can install in your pool, there are enough choices to make your head spin! The sand in a pool filter has been ground to about .
45 to .
55 mm in diameter and is very rough when you first start using it.
Over time the sand loses its roughness and needs to be changed.
The sand typically needs to be changed every five years; some have been known to go as long as 20 years without being changed although they may not be as efficient as they should be at that time.
As the sand loses its rough texture the filter has to run more frequently to complete its cleaning task.
Tips on when to change your sand:

1. Remove the old sand from the swimming pool filter and to do this you will need to pull the drain plug and let the water drain out.
   
2. Use a small cup to dig out the sand so you can expose the laterals and move the stand pipe out of the way.
   
3. Use caution when digging the sand out so you don't break the laterals of the filters' underdrain.
   They are fragile and are easily broken.
   
4. Once the sand is completely removed, clean and examine the laterals.
   Most laterals will unscrew allowing for easy removal from the tank.
   
5. If you need to remove impacted dirt from the laterals, soak them in a mix of water and muriatic acid; rinse thoroughly.
   
6. Rinse out the tank and re-install the clean laterals.
   
7. Replace the underdrain assembly then add water until the tank is half full to cushion the laterals when you start to add the sand.
   
8. Add a bag of sand, making certain you level the sand in the sand bed before you add the next bag of sand.
   The amount of sand you need to add is determined by the manufacturer's instructions.
   If you can't find the instructions contact a swimming pool professional for guidance.
   
9. Once the proper amount of sand has been added, reassemble the filter tank and multiport valve.
   
10. It's important to start your filter system in the backwash mode to completely flush the dust from the sand and to allow the sand to settle completely around the laterals.
    

Because cleaning a sand filter is not something that is done with any regularity, you may find it easier to contact a swimming pool professional when the time comes to replace the sand in your filter.
  How the sand filter system works: A swimming pool sand filter separates backwash contaminates from the backwash water and then they're discarded separately.
The backwash filtering system has an inlet and an outlet and the filter and the backwash valve connects the pump outlet and sand filter inlet, and connects the sand filter outlet and pool return.
When in the backwash position, the backwash valve connects the pump outlet and sand filter outlet, and connects the sand filter inlet and backwash filtering system inlet.