Silex

• The commonest filter media used in sand-filters is Silex - it's cheap, available and familiar

• (It's also not the best thing to put in a sand-filter; more on that subject here)

• The vast majority of sand in the world, when examined under magnification, can be seen to be rounded

• This is because it has been formed by rocks banging into each other in water in rivers, on beaches, etc., which tends to knock the sharp edges off

• It's pretty - but it's absolutely no use at all in a sand-filter

• The rounded grains do not pack together very tightly (imagine a box full of basketballs - and how much empty space there is in between them)

Silex

Pool-filter sand, on the other hand, is produced by crushing and sieving silica rock, leaving the sharp edges intact

• These sharp edges help the sand to 'pack down' tightly, leaving tiny gaps and passages between the grains

• It is the size of these gaps that determines how small a particle the filter can trap

• When newly installed to a filter Silex will remove particles down to around 15 microns but, over time, the passage of water (bearing particles smaller than 15 microns) will erode the sharp edges and therefore the spaces between the grains will become larger

• The particles that are knocked off the individual grains during this erosive process get flushed into the pool and are unlikely to be trapped by the filter during vacuuming, so they will be returned to the pool and make the water cloudy

• After 4 years or so the sand will be unable to trap particles smaller than 25 microns and the water will never appear completely clear again until you change the media

• A micron is one-millionth of a meter and it is impossible for humans to see anything smaller than about 35-40 microns but, in huge numbers they can impart a cloudiness to the pool

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Ken Walker - MyPoolGuru©