Hi, bear,
Yeah, the issue is interesting to me, too. Let me say, first of all, that I've got a big sand filter and my water is pristine....I couldn't imagine it could be improved.....naturally, it follows I am a big advocate of sand.
I'm not sure I agree with this but the thinking of others on the forum is that, as the sand captures smaller particles, it captures smaller and smaller particles as they accumulate. If so, then it would seem backwashing would eliminate any temporary advantage.
But if your backpressure is too high you will not get effecient water flow through the medium so cleaning the filter medium is essential.
And it is well known among aquarists that a freshly cleaned mechanical filter medium does not filter as effeciently as one that has been in service for a while.
Your thought of the "rounding off" of the sand particle might likewise have some merit. However, might not the rounded particles "fit" tighter together allowing them to capture smaller particles?
From my understaning it is the sharp edges on the sand grains that provide the mechanical filtration. The particles get caught up in the junctures between the sand grains. Once they 'round off" there is less places for them to get caught and the sand grains act like tiny ball bearings. It is true that as a filter becomes slightly dirty it will tend to filter better becuase the spaces between the grains (or pores in the cartridge matierial in a cartridge filter) become smaller due to the trapped particles. The reason many add a bit of DE to both sand and cartridge filters, in fact.
My thinking about channeling is that it's caused by the filter being undersized causing the water to pass through the filter at an excessive velocity.
My take on it is the rounded grains provide less resistance to the force of the water, having fewer edges and sides to deflect the water in different directions. This allows the water to pass through the medium at a faster rate which can then lead to channeling. I have actually observed this in a slow sand bed filter that I built for an aquarium. I actually got a faster flow rate through the filter after it had been in use for 3 years with the same sand.
So, since I can't come to a logical conclusion that changing the sand will provide a benefit I can see or understand, I plan on leaving mine in there.
I have no idea if my thought process is correct or if any of this matters a hill of beans to anyone but it's too dark to go outside and mess with the pool and there's never anything decent on TV.
Dave S.
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