What's not to understand? Sand filters do filter down to 25 microns, and you can't see anything under about 40 microns. Sand filters clean water so clear that you can't see particles in the water x about 2.You appeared to be suggesting that all filter types can filter particles smaller than the limits of human visibility, and that therefore there will be no visible difference in water quality between filter types.
I’ve heard that argument several times, but it doesn’t make sense to me, as a large number of smaller particles could get thru the filter, and be visible in larger numbers, regardless of whether individual particles are too small to see.
I see what your problem is. You stop thinking after "filters down to the limits of human visibility". But then you forget the process repeats, then repeats again, and again etc. Which means there will never be enough particles to collect into bigger, and bigger particles that you can see.
I can see atoms with the naked eye you know, and by atoms I mean water, waves, bubbles, fumes, fire, smoke, blood cells, and even bacteria.
Let me ask this, can you see algae coming out of your jets right now? Oh wait you might have a DE filter. Then can you see the algae floating in the air or across the pool surface? Because it's there even in the pools with DE filters. Can you see it floating in new or old pool water before or after chlorination? Can you see floating bacteria in spring water?
Have you ever seen a bunch of human skin cells floating around in pool water? How about white blood cells, or red blood cells?
Can you see singal particals of pollen floating in your water? You can't (about 30 microns), but you have seen pollen on your car, or on tree leafs etc. You have also seen blood, skin, algae, pollen, dust, smoke, carbon dust, and yes even bacteria somewhere right? I have.
Can you see any of that under water in a clean clear blue pool looking at the other side? Yeah sure, maybe, maybe not. Depends how much of each and what particle is in there with you, and how far across your field of view is.
You can't see singular smoke particles in the air, but you have seen smoke. Yeah sure most of us have not seen smoke in a pool or (fog), but if enough of those smoke particles are in the air together, I'd believe you if you told me you can see smoke. Because I can too.
So can you see anything at 25 microns by the pool light? No you can't.
But can you see anything under 25 microns grouped together in large mass like blood cells, algae, pollen, or even smoke across a long enough field of view, with enough mass holding it. Or maybe in front of a pool light? Yeah I guess we all can. But not if they are filtered, again, and again, like pool filters do, so that they do not become big enough clumbs of Crud that you can see. Instead that stuff gets filtered, and the only things passing through the fitler is the small singal particals of Crud you can't see going back to the pool.
I stand by my statements, sand filters, filter enough small particles out of the pool water that you can not see the difference between the quality of other filters. And you knew that was what I meant, and have added nothing to this thread.
If it's big it get filtered.
If it's small it passes through to the pool.
Until small collects enough together to become big.
When it does the big gets filtered.
At no time is the sand filter putting out anything that can't be filtered down to the limits of human visibility. Got it?
edit: After all that small stuff collects in the pool body after being repeated through the filter, what do we use to finish the process and oxidize everything left over in the water?