9. What is the frequency to replace the silica sand of a sand filter?

----It is best to replace the silica sand of sand filters every 1-2 years for the following reasons: a During backwashing, the filtration bed volume will expand upwards by 1.2 to 1.3 times. The water flow will drive the filtration layer to flow up and down, causing the filtration material particles to collide with each other. Backwash will let the small sand evacuate. b.Some silica sands will be welded when the acid and alkali are changed due to insufficient purity; Therefore, the longer the time, the worse the filtering effect of the sand tank. Since backwashing will consume and take away some sand, we need to replace the sand in the sand tank within the specified period.